materials Article Recycled Mortars with Ceramic Aggregates. Pore Network Transmutation and Its Relationship with Physical and Mechanical Properties Francisca Guadalupe Cabrera-Covarrubias 1 , José Manuel Gómez-Soberón 2, * , Carlos Antonio Rosas-Casarez 1 , Jorge Luis Almaral-Sánchez 1, * and Jesús Manuel Bernal-Camacho 3 1 Faculty of Engineering Mochis, Autonomous University of Sinaloa, Fuente de Poseidón y Ángel Flores s/n, Col. Jiquilpan, Module B2, Los Mochis, Sinaloa 81210, Mexico; guadalupe.cabrera@uas.edu.mx (F.G.C.-C.); carlos.arc@uas.edu.mx (C.A.R.-C.) 2 Barcelona School of Building Construction, Polytechnic University of Catalonia, Av. Doctor Marañón 44-50, 08028 Barcelona, Spain 3 School of Engineering Mazatlan, Autonomous University of Sinaloa, Ave. Ejército Mexicano esq. Ave. Universidad s/n., Ciudad Universitaria, Fracc. Antiguo Aeropuerto, Mazatlán, Sinaloa 82017, Mexico; jmbernalc@uas.edu.mx * Correspondence: josemanuel.gomez@upc.edu (J.M.G.-S.); jalmaral@uas.edu.mx (J.L.A.-S.); Tel.: +34-93-401-6242 (J.M.G.-S.)  Abstract: The porosity of mortars with recycled ceramic aggregates (10, 20, 30, 50, and 100% as a  replacement of natural aggregate) was evaluated and analyzed using three different techniques. The Citation: Cabrera-Covarrubias, F.G.; results of gas adsorption (N2 ), Scanning Electron Microscopy (SEM) image analysis and open porosity Gómez-Soberón, J.M.; Rosas-Casarez, allowed establishing the relationship between the recycled aggregate content and the porosity of these C.A.; Almaral-Sánchez, J.L.; mortars, as well as the relationship between porosity and the physical and mechanical properties of Bernal-Camacho, J.M. Recycled the mortars: absorption, density, compressive strength, modulus of elasticity, and drying shrinkage. Mortars with Ceramic Aggregates. Using the R2 coefficient and the equation typology as criteria, additional data such as Brunauer, Pore Network Transmutation and Its Relationship with Physical and Emmett, and Teller (BET) surface area (N2 adsorption) established significant correlations with the Mechanical Properties. Materials 2021, mentioned properties; with SEM image analysis, no explanatory relationships could be established; 14, 1543. https://doi.org/10.3390/ and with open porosity, revealing relationships were established (R2 > 0.9). With the three techniques, ma14061543 it was confirmed that the increase in porosity is related to the increase in the amount of ceramic aggregate; in particular with gas adsorption (N2 ) and open porosity. It was concluded that the Academic Editor: Miguel Bravo; Rui open porosity technique can explain the behavior of these recycled mortars with more reliable data, Vasco Silva; António P.C. Duarte in a simple and direct way, linked to its establishment with a more representative sample of the mortar matrix. Received: 18 February 2021 Accepted: 15 March 2021 Keywords: recycled mortars; ceramic wastes; recycled aggregates; porosity; adsorption N2 ; image Published: 21 March 2021 analysis; open porosity Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- 1. Introduction iations. In recent years, some of the most commonly used terms in the construction sector include the life cycle of materials, waste management, economic sustainability, sustainable development, environmental care, closed-cycle materials, etc.; these terms are related to the increase in the consumption of materials and energy, as well as the currently used Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. incorrect and unsustainable waste management system. This has influenced governments, This article is an open access article industries, and researchers to seek solutions for the implementation of new alternatives to distributed under the terms and achieve responsible solutions related to resource recovery and waste management [1–6]. conditions of the Creative Commons It is known that the construction sector generates a large amount of Construction Attribution (CC BY) license (https:// and Demolition (C&D) waste, which has further increased the interest of researchers creativecommons.org/licenses/by/ to find methods to reduce, reuse or recycle it [7]. In the specific case of recycling, this 4.0/). is feasible through selection, grinding, crushing and screening processes, to obtain as Materials 2021, 14, 1543. https://doi.org/10.3390/ma14061543 https://www.mdpi.com/journal/materials Materials 2021, 14, 1543 2 of 23 a result aggregates of different sizes and properties, which can be used as substitutes for sand or gravel in mortars and concretes [8]. Although previous studies reported that the use of such waste in concrete is detrimental to its mechanical performance and durability—up to certain replacement percentages, its use in mortars (either in the form of fine aggregate as a substitute for natural sand, or in the form of powder as a partial substitute for cement—with an impact on pozzolanic reaction properties) has also been considered to be an alternative [7–13]. For all of the above, the need to use 100% of all resources again as new resources is evident, integrating the paradigm of a sustainable city; and at the same time, there is a need to significantly increase the efficiency of the use of resources, energy, and materials [4]. In the specific case of studies concerning recycled mortars, one of the materials of interest as separated waste is ceramics; for example, in China (2017), 1800 million tons of construction and demolition waste (CDW) were produced, of which 87% are concrete and clay bricks [14]. In general, ceramic waste has its origin from masonry rubble waste, and is represented by clay bricks (solid, hollow, or domed bricks that have been sorted and crushed) [1,3,7,14–17], tiles [1], rubble partition walls [5], roofing tiles [3], or from the ceramic industry for not meeting quality standards (broken, distorted, burned, etc.) [1,18]; these wastes also include ceramic sanitary wastes [6]. At present, numerous investigations have been carried out on recycled ceramic mortars (CRM), within which different variables were studied for their use; one of the main ones has been to discern the role of the recycled aggregate within the mortar matrix, using it as a partial or total replacement of the natural aggregate, or as a partial replacement of the cement. CRM studies using aggregate replacement established that the particle size of the ceramic aggregate is another important factor in mortar properties (particle size is a function of the crushing method). These researchers established that fine ceramic aggregate produced by rotating ball mill (finer particles) produces more porous mortars than those crushed by jaw crusher (coarser particles) [17]; however, excellent results in terms of improvement of the mortar-brick interfacial transition zone (ITZ) were achieved with the use of the finer ceramic aggregates (even replacing 100% natural aggregate). This improved adhesion of the ITZ was identified as the contribution of the fine powder in the fresh rheology of the CRM, allowing better penetration into the brick surface [16,17]. In another investigation, using brick residues in ranges of particle sizes (0.15–0.3, 0.3–0.6 and 0.6–4.75 mm), they obtained those mortars with finer sand (0.15–0.3 mm) and reduced their workability with respect to the control mortar, due to the fact that they tend to absorb water faster (higher specific surface area); on the other hand, the compressive strength was notably higher, and their elastic modulus and shrinkage were similar to the control mortar [7]. On the other hand, using recycled ceramic aggregate with sizes similar to those of natural sand (10% and 20% replacements), do not cause negative effects on the compressive strength of CRMs, but they do affect the consistency of the mortar; as for shrinkage, it is reduced when 20% of the aggregate is replaced and increased when 10% is replaced (in both cases with respect to the control mix) [1]. Another trend of researchers has been to perform studies on different replacement percentages—including total aggregates. In one study, replacements of 0, 5, 10, 20, and 40% were carried out, concluding that the use of up to 40% of ceramic aggregate does not significantly affect the properties of the CRM, with the only exception being the density (fresh) and its workability [5]. In another investigation that replaced 20, 35, 50, 50, 70, and 100% by ceramic recycled aggregate (ratio 1:6), improvements in flexural behavior and increase in compressive strength were observed; however, the water absorption coefficient was reduced, and the density decreased as the substitution increased. As for shrinkage, it was considered acceptable up to 50% replacements [19]. Continuing with the shrinkage property, in another study, 30, 60, and 100% replace- ments were performed, and it was reported that the drying shrinkage improves when the recycled sand content increases (due to the release of water previously stored in the porous Materials 2021, 14, 1543 3 of 23 aggregate); however, the compressive and flexural strength decrease with increasing aggre- gate replacement [14]. For extreme cases of replacement (100% of the ceramic aggregate), it was shown that CRM does not perform well with respect to the reference mortar; however, with lower replacement percentages (20 and 50%) it does [3]. In another study, it was concluded that the use of 10% of ceramic aggregate (this time in powder form), leads in general terms, to improve its performance in comparison with its reference mortar (contribution of hydraulic capacity) [15]. In any of the previous ways in which the use of ceramic waste has been investigated, the property of porosity in CRMs (conferred by the porosity of the ceramic waste itself), concomitate as one of the significant causes of variable mechanical behavior and dura- bility [16], and as an example, brick can have a porosity of up to 40% [1]. The use of such replacement materials interferes with the porosity and pore size distribution of the mortars that include them, and consequently with properties such as compressive strength, shrinkage, and durability (e.g., chloride and sulfate diffusion) [18]. Previous research on porosity in CRM [17], using crushed bricks with two particle size distributions (fine and coarse) and total replacement of natural sand, reported results of total open porosity, and pore size distribution by Mercury Intrusion Porosimetry (MIP), concluding that these aggregates influenced the microstructure of CRMs; specifically, those that used the fine fraction of ceramic aggregates presented higher pore content in general, as well as higher percentage of micropores and lower percentage of macropores. Similar results were established in another research [20], in which four types of ground clay bricks were used to be used as partial replacement of cement (0, 10, 20, and 30%). In this work, MIP was also used, and the results indicated that the pore volume increases with increasing replacement percentage—in turn decreases with increasing curing period; concluding that the resistance increases with the decrease of the total pore volume and with the increase in the proportion of small pores, mentioning that the production of C-S-H gel additional to the reaction of the ground brick with CH, refines the pore size distribution of the CRMs, which ends up producing the increase in compressive strength. In contrast, reduction in porosity is what was exposed in another study [21], in which ceramic aggregate (product of electrical insulators) was used in powder form to replace cement or as an addition (percentages of 10, 20 and 30%); the MIP tests performed indicated that the pore volume of each variable varied from 0.03 to 1.0 m pore diameter (mortars with brick powder), and this being lower than the one that did not contain ceramic aggregate. Using histograms of accumulated pore volume (variability from 0.05 to 2.0 µm), it was established that the samples with ceramic powder presented porosity reductions of 30, 28, and 43%, respectively, with respect to the control mortar, relating this to the gain in compressive strength obtained. In another study that studied the performance of CRMs in aggressive environments (chloride and sulfate attack) and their relationship with porosity [18], researchers used ceramic aggregate from tile waste to replace the aggregates and also as a cement substi- tute (40%); with the Scanning Electron Microscopy (SEM) technique, they identified the existence of a greater number of voids in the reference mortar than in the CRMs. With the X-Ray Diffraction (EDX) technique, they confirmed that CRMs may have better durability behavior, since they have a denser and less porous microstructure matrix, as the ceramic powder is the one that densifies the matrix when filling the cavities and reduces the amount and size of the pores. Conclusions that were also supported by a different study [22]. Taking as a reference what has been reported in the literature on research related to the properties of CRMs and their incidence with their porosity, the information is shown as scarce and also sometimes controversial. Moreover, these studies largely focused on the use of ceramic wastes as a cement substitute. For this reason, this research proposes the approach of using ceramic aggregates as a replacement for natural aggregates (10, 20, 30, 50, and 100%), and validating the relationship between their mechanical properties and the porosity of CRMs (using open porosity tests, N2 adsorption and SEM image analysis); seeking to validate viable alternatives in the production of mortars. Materials 2021, 14, 1543 4 of 23 2. Materials and Methods 2.1. Materials Recycled ceramic aggregate (CA) from defective tiles (size and/or geometry) was used; it was acquired from a local waste treatment plant (Reciclàrids SL, located in Sant Joan de Vilatorrada, Barcelona, Spain) certified for its processing. The initial aggregate had a size of 0–5 mm, so the coarse fraction was separated by sieve No. 4 (4.75 mm) and thus only the fine fraction was used. Natural aggregate (NA) purchased from a local construction aggregate company (Arids Anton SL, located in Molins de Rei, Barcelona, Spain) was used; the sand size was 0–4 mm, and of the siliceous type. Portland cement CEM I 42.5 N/SR (Cementos Portland Valderrivas SA, Vallcarca–Sitges, Spain), according to EN 197-1 [23], was used as binder for the manufacture of mortars; and tap water was used as water. 2.2. Properties of Study Aggregates To obtain the granulometry of the study aggregates, the material was passed through the sieves specified by ASTM C144 [24]; once the procedure was completed, the corre- sponding curves were plotted, as well as the limits established by the standard. As a result, non-compliance with the established limits was observed; therefore, it was necessary to make a granulometric adjustment to achieve similar profiles between the two aggregates, thus avoiding possible attributable variables in the behavior of the CRMs. For the adjust- ment, it was proposed to separate each aggregate into two fractions by means of sieve No. 30 (0.59 mm); then, combinations were made between them (each type of aggregate separately) substituting size fractions with progressive increments of 10% of one of the fractions. The common criterion that established the fractions in both types of aggregate was their compactness (combination of fractions that presented the maximum value); which is affected by the shape of the aggregates (sometimes linked to the size of the aggregate). It is worth mentioning that the shape of the aggregate possibly influences aspects such as the adherence of the aggregates with the cement, and foreseeably, it will also influence aspects of workability, fluidity of the mixture, etc. [3,15]. This will be reflected in the density, absorption and finally the porosity of the hardened mortar. The ideal combination for this study was: for CA, 60% of the material retained on the No. 30 sieve, and 40% of the material passing, and for NA, 50% of the material both retained and passing through the No. 30 sieve; Figure 1 shows the particle size curves without adjustment (before), as well as adjusted (after) of the two aggregates. Physical properties of the aggregates are presented in Table 1; in this table it can be observed in general that the density [25] and bulk density [26] both in oven-dry condition (OD) and in saturation-surface-dry condition (SSD) of the CA is lower than that of the NA; on the contrary, the NA has a lower percentage of voids, a notable difference in absorption, a lower amount of fines [27] and both have similar fineness modulus [28]. Table 1. Physical properties of aggregates [29]. Recycled Ceramic Natural Property Unit Aggregate (CA) Aggregate (NA) Density (OD) 1820.9 2581.6 Density (SSD) 2155.4 2623.6 Kg/cm3 Bulk density (OD) 1182.0 1735.1 Bulk density (SSD) 1399.1 1860.8 Void content 35.3 32.9 Water absorption coefficient % 18.4 1.6 Particles < 75 µm (sieve No. 200) 8.2 2.9 Fineness modulus materials - 2.8 2.4 Materials 2021, 14, 1543 5 of 23 Materials 2021, 14, x FOR PEER REVIEW 5 of 24 CA (before) CA (after) 100 NA (before) NA (after) Upper limit Lower limit 80 60 % Passing 40 20 0 0.01 0.1 1 10 Sieve size (mm) Figure Figure1.1.Recycled RecycledCeramic Aggregate Ceramic (CA)(CA) Aggregate and Natural Aggregate and Natural (NA) granulometries. Aggregate (NA) granulometries. 2.3. Mortar Table Mixes 1. Physical properties of aggregates [29]. CRM specimens were manufactured with mixtures with different Recycled Ceramic percentages of CA Natural Property Unit (10, 20, 30, 50 and 100%) replacing the NA, these were identified Aggregate (CA) asAggregate CRMXX (XX (NA)represents the percentage of CA). Density (OD) In addition, a usual mortar (UM) with 100% NA2581.6 1820.9 was manufactured to be used asDensity a reference (SSD)mortar. All mortars were designed 2155.4 with a cement:aggregate 2623.6 ratio Kg/cm3 (c:a) = 1:4, Bulk as well as an density initial water:cement ratio (w/c) (OD) = 0.5. Due to the 1182.0 high absorption 1735.1 of the CABulk (see density Table 1), the amount of water needed 1399.1 (SSD) for mixing increased1860.8to achieve the target workability for all the mixtures (110 ± 5%)—in accordance Void content 35.3 with ASTM 32.9 C109 [30] by means of the flow table test ASTM C230 [31]. Water absorption coefficient % 18.4 1.6 The mixing Particles procedure < 75 μm (sieve No.consisted 200) of a previous saturation 8.2 of the aggregates 2.9 in a period of oneFineness minutemodulus with thematerials initial amount of water - (result of2.8the w/c ratio),2.4 in order to avoid water mobility necessary for the hydration process; then, the cement was added to the 2.3. mixerMortar Mixes (Mod. E93, Matest brand) which contained the aggregates to start the container mixingCRM specimens sequence at were medium manufactured speed forwith 60 s,mixtures with different then continued percentages at high speed forof30CA s, then the (10, 20, 30,was mixture 50 and left100%) to restreplacing for 90 s,the andNA, thesethe finally were identified high speed as CRMXX was (XXfor selected represents 60 s more. Once the thepercentage process was of CA). In addition, completed, theaflow usualtable mortar (UM) test waswith 100% NAinwas continued, manufactured which low consistencies to be used as a reference mortar. All mortars were designed with a cement:aggregate were presented due to the lack of water; for this reason, small quantities were ratiogradually (c:a) = 1:4, as well as an initial water:cement ratio (w/c) = 0.5. Due to the high absorption added until the aforementioned workability was reached. Table 2 shows the quantities of of the CA (see Table 1), the amount of water needed for mixing increased to achieve the each of the materials to manufacture one dm3 of the different CRMs. target workability for all the mixtures (110 ± 5%)—in accordance with ASTM C109 [30] by means of the flow table test ASTM C230 [31]. 2.4. Physical and Mechanical Properties Tests The mixing procedure consisted of a previous saturation of the aggregates in a period of oneTo compare minute withthe thebehavior of theofCRMs initial amount studied water (result of with respect the w/c ratio),to intheir orderporosity, to avoid physical properties water (density, mobility absorption, necessary for theand open porosity), hydration and the process; then, mechanical cement was properties added to(compressive the strength mixer [fm], modulus container (Mod. E93,of elasticity Matest brand) [E], which drying shrinkage) andcontained weretodetermined; the aggregates start the mix-since the ing sequence objective at medium of this speed for work focuses on 60 thes,incidence then continued at highthese of porosity, speed for 30 s, then properties the are presented in mixture was3.left the Section Theto specimens rest for 90 s, andin used finally thesethe high tests werespeed 4 × was 4 × selected 16 cm, andfor for 60 sdrying more.shrinkage, Once 2.5 ×the2.5process × 28.5wascm.completed, the flow The standards tablewere used test was thecontinued, Spanish UNE in which low consist- (UNE-EN 1015-10 [32], UNE-EN 1936 [33], and ASTM standards, respectively. More details of the tests for these properties can be found in previous publications by the authors [29,34]. Materials 2021, 14, 1543 6 of 23 Table 2. Dosage of the study mortars. Mixtures Used for Mortars Materials (g) UM CRM10 CRM20 CRM30 CRM50 CRM100 Cement 400 433 381 372 348 323 NA 1 800 780 610 521 348 0 NA 2 800 780 610 521 348 0 CA 1 0 70 122 178 278 517 CA 2 0 104 183 268 417 775 Water 334 390 355 373 397 476 w/c 0.84 0.90 0.93 1.00 1.14 1.48 1 Size passing through sieve No. 30. 2 Size retained on sieve No. 30. NA = Natural Aggregate. CA = Recycled Ceramic Aggregate. UM = Usual Mortar. CRM = Recycled Ceramic Mortars (where 10, 20, 30, 50 y 100, represents the percentage of CA). 2.5. Nitrogen (N2 ) Adsorption Porosimetry Mortar specimens of 4 × 4 × 16 cm were cured in water for 90 days after manufacture. Then, using a concrete disk cutter, four cuts were made, selecting the central part of the specimen resulting from the cuts (volume of approximately 36 cm3 ) and discarding the outer faces (discarding the border effect); then, the central part of each specimen was placed in an oven at 60 ◦ C for drying. Once the central parts of the samples were dried, they were ground in an agate mortar (trying to provoke a diversity of particle sizes), so that each sample under study contained sizes representative of the sieves used in the particle size established by ASTM C144 (with the exception of mesh No. 4 (4.75 mm), since the opening of the sample holder of the porosimetry equipment is 4 mm). Due to the different densities of the CRMs studied, and as a standardization procedure proposed to be performed on all the samples, it was decided to introduce in the sample holder flask for the test the same amount (in weight) of material corresponding to each sieve size (used in aggregate granulometry), resulting finally in 3.5 g samples to be tested for each study variable. Once the samples were obtained, they were prepared using a VacPrep 061 micromerit- ics degasser (Micromeritics Instrument Corporation, Norcross, GA, USA), which offers two methods to remove contaminants such as water vapor and adsorbed gases from samples (avoiding interference with surface area measurements). The N2 adsorption test was per- formed on a micromeritics TriStar—surface area and porosity analyzer, which uses TriStar 3000 V6.04 A software (Micromeritics Instrument Corporation, Norcross, GA, USA). This equipment is an automated gas adsorption analyzer that contains three ports and uses the principles of physical adsorption and capillary condensation to obtain information on the surface area and porosity of a solid material—it can measure surface areas down to 0.01 m2 /g using nitrogen as adsorbate (Figure 2). The gas adsorption procedure involves bringing the study sample (cleaned by the degassing method) to a constant temperature (by means of an external bath). Then, the gas (adsorbate) is introduced into the pipette containing the sample in small doses, causing the gas molecules to adhere to the surface of the solid (adsorbent) and form a thin layer that coats the surface of the adsorbent [35]. According to the theory by Brunauer, Emmett, and Teller (BET) [36], it is feasible to determine the number of molecules to form a monolayer of adsorbed gas, which allows the surface area of the sample to be established. As the gas molecules continue to adsorb on the solid (after the first monolayer), multilayers are formed (deposited sequentially one on top of the other); this together with capillary condensation (a process feasible to establish using the Kelvin equation [36], which establishes the ratio of the equilibrium vapor pressure of a curved surface, such as that of a liquid in a capillary or pore, to the equilibrium pressure of the same liquid on a flat surface [37]). In order for the equilibrium pressure of the gas to approach saturation, the pores are filled mainly with adsorbate; therefore, knowing the density of the adsorbate, the volume it occupies and, consequently, the total pore volume can be determined. This adsorption process establishes as a result the so-called “isotherms” (experimental curves of adsorbed gas Materials 2021, 14, 1543 7 of 23 volumes with respect to relative pressures), which by performing the process inversely (removing known amounts of gas) will establish the desorption isotherms; both curves form a hysteresis that provides information about the different pore shapes [37,38]. The Barrett, Joyner, and Halenda (BJH) calculation method allows determining the cumulative or differential pore size distribution by taking the isotherms as data [38]; such distributions are represented by the plot of pore volume versus pore size, usually7 ofused Materials 2021, 14, x FOR PEER REVIEW 24 to describe the porosity of materials [39]. Figure 2. Gas adsorption test equipment. Figure 2. Gas adsorption test equipment. The gas adsorption procedure involves bringing the study sample (cleaned by the 2.6. Porosity by SEM and Microanalysis of Chemical Elements with an Energy Dispersive degassing method) to a constant temperature (by means of an external bath). Then, the Spectrometer (SEM-EDS) gas (adsorbate) is introduced into the pipette containing the sample in small doses, caus- From ing the the 4 × 4to× gas molecules 16 cm adhere to specimens, the surface of and after(adsorbent) the solid curing inand water formfor a period of 90 days, a thin layer that small coats the samples surface were of the taken from adsorbent [35]. According the central part of theto the theory by Brunauer, specimens—a cube of approximately 1Emmett, and Teller (BET) [36], it is feasible to determine the number of molecules to form cm3 , taking care that the transversal face of the specimen was the one that was exposed a monolayer of adsorbed gas, which allows the surface area of the sample to be estab- for obtaining lished. As the gasthe imagescontinue molecules in the to microscope. adsorb on theFour samples solid (after of monolayer), the first the mortars were included inside multilayers are formed (deposited sequentially one on top of the other); this together(EpoFix a circular mold, then covered with transparent epoxy resin with resin, Struers, Hørsholm, Denmark). capillary condensation The mold (a process feasiblewith the samples to establish using thewas placed Kelvin [36], an oven at 40 ◦ C inside equation which for establishes a period ofthe 40ratio minof(hardening the equilibrium ofvapor the pressure of a curved resin causing thesurface, samples such toas be encapsulated that of a liquid in a capillary or pore, to the equilibrium pressure of the same liquid on a in the resin). The circular sample was then “sliced” to achieve a flat face and with the flat surface [37]). In order for the equilibrium pressure of the gas to approach saturation, samples the pores areexposed outside filled mainly with the resintherefore, adsorbate; (a precisionknowingcutter was used); the density next, the exposed face of the adsorb- was subjected ate, the volume itto a progressive occupies roughing-polishing and, consequently, process the total pore volume canwith different sandpaper (until be determined. Thisgloss the adsorption process establishes was achieved on the as a result study the so-called face). Once the “isotherms” tablet with (experimental the different samples was curves of adsorbed gas volumes with respect to relative pressures), which by performing ready (Figure 3), they were analyzed in a Scanning Electron Microscope (SEM), visually the process inversely (removing known amounts of gas) will establish the desorption iso- selecting therms; both the possible curves form aareas of interest hysteresis that information that provides would allow aboutdetecting thepore the different natural and ceramic aggregates, shapes [37][38]. respectively. The Barrett, Joyner, and Halenda (BJH) calculation method allows deter- mining A the JEOL JSM-6510 cumulative microscope or differential pore(Jeol size Ltd., Tokyo,byJapan) distribution wasisotherms taking the used forasimage acquisition, data [38]; such distributions are represented by the plot obtaining them in two different ways: (a) high resolution secondary of pore volume versus poreelectron size, detector “SEI” usually used to describe the porosity of materials [39]. (secondary electron image), and (b) backscattered electron detector “BEI” (backscattered electron image); 2.6. Porosity by SEM the latter, with ofhigher and Microanalysis Chemicalresolution than Elements with the former, an Energy but with greater contrast Dispersive to examine(SEM-EDS) Spectrometer the surface topography of the samples. Two different “zooms” were used to obtain Fromthetheimages: cm × 4 × 4 × 16200 in the areas specimens, of interest and after (identification curing in water for a period of 90 the types of aggregates, days, small samples were taken from the central part of the specimens—a cube interfaces between them and the ITZ), and 500× for the mapping; the latter consisted of approximately 1 cm3, taking care that the transversal face of the specimen was the one that was exposed of determining the chemical composition of the samples, detecting by microanalysis the for obtaining the images in the microscope. Four samples of the mortars were included basic inside aelements circular mold,thatthen make it up. covered with On this occasion, transparent epoxy resinthe equipment (EpoFix used was an energy resin, Struers, dispersive spectrometer Hørsholm, Denmark). The mold(EDS) with(Inca 200, Oxford the samples Instruments, was placed inside an oven Abingdon, at 40 °C for United Kingdom) a period of 40tomin connected the(hardening SEM, which of theseparates resin causing thethecharacteristic samples to be encapsulated in the X-rays of different elements into an energy spectrum, and by means of the EDS system software, the energy spectrum is analyzed in order to determine the abundance of specific elements. By means of scans on Materials 2021, 14, 1543 8 of 23 Materials 2021, 14, x FOR PEER REVIEW 8 of 24 resin). The circular sample was then “sliced” to achieve a flat face and with the samples the sample, exposed outsidethe the different basic cutter resin (a precision chemical elements was used); requested next, the arewas exposed face obtained, sub- which taking into consideration jected the chemistry ofprocess to a progressive roughing-polishing the usual reactions with different of a mortar sandpaper and its constituent (until the gloss was achieved materials, on the study the following face).established: were Once the tablet Si,with Ca,the Fe,different Al, Mg, samples wasN, and K. Table 3 Na, Cl, ready shows (Figure 3), they were analyzed a representative in a Scanning relationship of eachElectron Microscope of the elements (SEM), visually in the CRM samples, which selecting the possible areas of interest that would allow detecting the natural and ceramic were obtained through EDS analysis. aggregates, respectively. Figure 3. Recycled Ceramic Mortars (CRM) samples used for SEM analysis: (a) UM, (b) CRM10, Figure 3. Recycled Ceramic Mortars (CRM) samples used for SEM analysis: (a) UM, (b) CRM10, (c) CRM20, (d) CRM30, (e) CRM50 and (f) CRM100. (c) CRM20, (d) CRM30, (e) CRM50 and (f) CRM100. A JEOL JSM-6510 microscope (Jeol Ltd., Tokyo, Japan) was used for image acquisi- tion, Table obtaining them in 3. Content of two eachdifferent elementways: in (a) thehigh resolution Recycled secondary Ceramic electron Mortars detector (CRM) samples obtained by “SEI” (secondary electron image), and (b) backscattered electron detector “BEI” (backscat- Energy Dispersive Spectrometer (EDS) analysis. tered electron image); the latter, with higher resolution than the former, but with greater contrast to examine the surface topography of the samples. Percentage Amount of the Elements (%) Two different “zooms” were used to obtain the images: 200× in the areas of interest Variable (identification of Si the types Caof aggregates, Fe interfaces AlbetweenMg them and Nathe ITZ), andCl500× N K for the mapping; the latter consisted of determining the chemical composition of the sam- ples,UM detecting by2.78 0.00 the basic microanalysis 2.08 elements 0.69that make1.39it up. On2.08 65.28the 25.00 this occasion, 0.69 CRM10 28.04 50.15 4.59 7.47 4.80 0.18 equipment used was an energy dispersive spectrometer (EDS) (Inca 200, Oxford Instru- 0.38 0.00 4.39 CRM20 ments, Abingdon,2.78 United0.00Kingdom) 2.08 connected 0.69 to the SEM,1.39 2.08 the65.28 which separates charac- 25.00 0.69 CRM30 teristic 47.77 X-rays of different34.03 5.47an energy elements into 7.94spectrum,1.86and by 0.51 means of the0.13EDS 0.00 2.29 CRM50 system 23.10 software, the energy40.82 spectrum8.36 7.95 is analyzed 2.30 in order to determine0.00 0.81 of 14.55 the abundance 2.11 CRM100 specific 3.90 elements. By means-of scans 2.60 1.30the different on the sample, 1.95 basic 3.25 87.01 chemical elements 0.00 - requested are obtained, which taking into consideration the chemistry of the usual reac- tions of a mortar and its constituent materials, the following were established: Si, Ca, Fe, 2.7.Mg, Al, Image Analysis Na, Cl, N, and K. (Obtained with aScanning Table 3 shows Electron representative Microscope—SEM), relationship Using NI of each of the ele- VisioninAssistant ments the CRM samples, which were obtained through EDS analysis. The analysis carried out consisted of examining an image by separating the com- ponents, and by means of a specific procedure calculating the porosity. For the analysis of the images obtained from the SEM, NI Vision Assistant 2018 software from National Instrument (LabWindows/CVI Version 7.1, Austin Texas, TX, USA) was used. This allows a series of integrated operations with which a “script” (script file, containing processing functions, and relevant parameters, to be used on individual images or in a batch to analyze a collection of images) is created, which allows separating, highlighting, processing, and analyzing the images where the paste and ceramic aggregates of the CRMs under study are shown [40]. Finally, the images selected for porosity analysis were processed from the conjunction of preliminary images obtained in two ways: the one obtained through the main Scanning Electron Microscope-Backscattered Electron Image (SEM-BEI) image of each sample, and the one established through the images corresponding to the mapping of chemical elements. Once entered into the software, the main images were treated by applying a “threshold Materials 2021, 14, 1543 9 of 23 filter” (pixel color ranges between 0 and 75), to then obtain their histogram (equivalent porosity values in the dark areas). Next, the mapping images were used to establish the elements corresponding to the aggregates (silicon, magnesium, and aluminum), which were processed with the “advances morphology” and “basic morphology” commands. The final result of each image process was stored in different buffers and then the “add” operation was applied for all the aggregates, and again the “add” operation was applied to incorporate the porosity values of the main SEM-BEI image. Finally, after completing the procedure, the percentages of aggregates (white pixels), paste (red pixels) and porosity Materials 2021, 14, x FOR PEER REVIEW (black pixels) of each processed image were quantified using the “histogram”10command. of 24 Figure 4 shows graphically the above-mentioned procedure on the image analysis. FigureFigure 4. Example 4. Example of image of image analysis analysis procedure;CRM10 procedure; CRM10 sample. sample.The Thesteps stepsthatthathave a sequence have a sequence are linked with yellow are linked with yellow arrows, and the steps that present a parallel process (i.e., the same, but for each of the different elements), are linked with arrows, and the steps that present a parallel process (i.e., the same, but for each of the different elements), are linked with arrows with yellow stripes. The image is obtained by SEM (A); “Threshold filter" is applied to the original image, and then arrows itswith yellow histogram is stripes. obtainedThe(B); image is obtained The image by SEM corresponding (A);Silicon to the “Threshold mapping filter” is applied is processed (C);tothe the original same image, for the imageandof then its histogram is obtained the Magnesium (D) and(B);Aluminum The image (E)corresponding mappings. The to the Silicon threshold filter mapping is applied is toprocessed the mappings (C);tothe same obtain thefor the image image of therepresenting Magnesiumthe (D)aggregates in the sample and Aluminum (C-1). The image (E) mappings. of the element The threshold filterisisprocessed applied using to thethe "advancestomorphology" mappings obtain the image and "basic representing themorphology" aggregates commands in the sample (C-2 (C-1). and C-3); TheThe sameof image steps theare performed element in the Magnesium is processed using theand Aluminum “advances map- morphology” pings (D-1, D-2, D-3, E-1, E-2 and E-3). The "add" operation was applied to the processed images of the elements (images and “basic morphology” C-3, D-3 and E-3); andcommands the image (F) (C-2,C-3); The same was obtained. steps The "add" are performed operation is applied inagain, the Magnesium to incorporate andtheAluminum mappings porosity values (D-1,D-2,D-3,E-1,E-2,E-3). The “add” of the original image; image operation (G) [result of (F) was + (B)]applied to theAprocessed is obtained. histogram images of the is obtained elementsthe to quantify (images C-3,D-3,E-3); percentage of and theaggregate, image (F)paste wasand pores (H).The "add" operation is applied again, to incorporate the porosity values of the original obtained. image; image (G) [result of (F) + (B)] is obtained. A histogram is obtained to quantify the percentage of aggregate, paste and pores (H). 3. Results and Discussion The following sections present the data on the physical and mechanical properties of the CRMs studied, followed by the porosity results obtained (gas adsorption test (N2) and by means of the SEM image analysis process), exploring for each case their correlation, linkage, or general incidence. Materials 2021, 14, 1543 10 of 23 3. Results and Discussion The following sections present the data on the physical and mechanical properties of the CRMs studied, followed by the porosity results obtained (gas adsorption test (N2 ) and by means of the SEM image analysis process), exploring for each case their correlation, linkage, or general incidence. 3.1. Physical and Mechanical Properties of CRMs In Table 4 the results corresponding to the physical and mechanical properties of the CRM and UM studied, respectively, are shown. These results (open porosity and absorption [29]; density, fm, E and drying shrinkage [34]) were analyzed in previous works. The objective of the current work is to establish their relationship with porosity. Table 4. Mechanical y physical properties of mortars. Physical Properties Mechanical Properties Density Open Absorption Shrinkage Variable fm (MPa) E (MPa) (g/cm3 ) Porosity (%) (%) (%) 60 días 90 días UM 1.95 19.09 11.87 28.77 26,252 −0.0793 CRM10 1.95 22.68 14.18 30.58 26,515 −0.0704 CRM20 1.89 24.96 15.57 30.95 26,251 −0.0847 CRM30 1.86 27.04 16.90 30.10 24,065 −0.0709 CRM50 1.80 32.00 19.91 26.76 21,731 −0.0986 CRM100 1.53 41.17 25.56 20.56 14,194 −0.1141 A simplified and general synthesis of the behavior of these CRMs could be established as follows: density decreases with the increase of replacement, reaching a maximum difference of 0.42 g/cm3 (CRM100 w/r UM); open porosity and absorption present similar behavior, since the values increase with the increase in the percentage of CA, with a difference between CRM100 w/r UM of 22.08 and 13.69%, respectively; fm presents close values between UM and mortars with replacement of up to 30%, and for replacements of 50 and 100% the values decrease notably (10. 39 MPa between CRM20 w/r CRM100); E presents similar behavior to fm (decrease with the increase of CA replacement), with difference between the highest and lowest value of 12,321 MPa; drying shrinkage established an opposite behavior to fm and E, by increasing with the increase of CA in mortars, particularly with CRM50 and CRM100 (0.0437% in difference between the lowest and highest value of shrinkage). 3.2. Porosimetry by N2 Adsorption Gas adsorption (N2 ) is an appropriate technique to characterize pore sizes and distri- bution mainly in the range of mesopores (from 2 to 200 nm) [37], gel pores (<10 nm), and capillary pores (10 to 50 nm) [41,42]; Therefore, for an investigation in mortars, this can be considered the most approximate, accurate, and adequate. Gas adsorption measurements have been extensively used to determine the surface area and pore size distribution of an important variety of solid materials—among which construction materials are also included [36]. On the other hand, total pore volume and pore size distribution are also common techniques used to describe the porosity of materials; the latter being a distribu- tion of pore volume with respect to pore size [39]. The results of the different variables (besides porosity) obtained by this technique are presented below. The N2 adsorption isotherms for the samples studied (ratio of the amount of adsorbed molecules with respect to the pressure at constant temperature) are shown in Figure 5 (the letter “A” at the end of the name corresponds to the adsorption phase; and the letter “D” to desorption, respectively)—sample CRM30 could not be included due to contamination of the sample. It can be seen that CRM100 reports the highest adsorbed amount for Materials 2021, 14, 1543 11 of 23 all pressures with respect to the rest of the samples, and in the same figure (upper left x FOR PEER REVIEW enlargement) it can also be appreciated that for low pressures (beginning of the 12 of 24 curves) and in general, the adsorbed amount presents an increase related to the highest amount of CA in the CRMs, where the UM (0% CA) is the one that presents the lowest values. With regard to high pressures (end of the curves), which is observed in the upper right cm3/g); for the rest of the mortars, enlargement adsorption of the same figure, thecaused amountsimilar adsorbedamounts continues (values from to be higher for 32.35 CRM100 3 to 35.84 cm /g)—the 3 (65.9UMcm being /g); forthe the one with rest of the highest the mortars, amount. adsorption According caused to the (values similar amounts way the from 32.35 to 35.84 cm 3 /g)—the UM being the one with the highest amount. According to the hysteresis was presented, it can be said that to remove the gas from the sample, lower way the hysteresis was presented, it can be said that to remove the gas from the sample, pressure was necessary; however, for relative pressures of 0.5 (approx.) in all samples, the lower pressure was necessary; however, for relative pressures of 0.5 (approx.) in all samples, amounts of gasesthe decreased, amounts of so desorption gases decreased, sobecomes desorptionslower. becomes From slower.the data From theprovided data providedin in the isotherms, thethesurface area isotherms, theof the solid, surface area ofthe size and the solid, its and the size descriptive pore its descriptive parameters, pore parameters, its distribution, its distribution, etc. etc. were calculated. were calculated. Figure Figure 5. Adsorption-desorption 5. Adsorption-desorption isotherms ofisotherms CRMs: (a)oftheCRMs: (a) of beginning thethebeginning of the adsorption-desorp- adsorption-desorption isotherm curves is tion shown; isothermthe specifically, curves amountisofshown; molecules specifically, adsorbed andthe amount desorbed of molecules at low adsorbed pressures (blue and box a’); (b) thedesorbed atthe final part of isotherm low pressures (blue box a’); (b) the final part of the isotherm curves is shown; specifically, theb’). curves is shown; specifically, the amount of adsorbed and desorbed molecules at high pressures (green box amount of adsorbed and desorbed molecules at high pressures (green box b’). With this technique, it is feasible to measure the surface area of solids; and for which the data in Figure 5 [43] are used, in particular those of the low relative pressure region With this technique, (P/Po from it0istofeasible 0.30). Theto measureisthe calculation surface carried areathe out using ofBET solids; andsince method, for itwhich is a used the data in Figureand5 [43] are used, established in particular approach to determinethose of thearea the surface low of relative pressure solid materials, region especially those with different open porosity [37]. (P/Po from 0 to 0.30). The calculation is carried out using the BET method, since it is a used The results obtained are shown in Figure 6. In this figure it can be observed that and established approach to determine the surface area of solid materials, especially those the surface area values present, in general terms, an increase related to the increase of with different open porosity [37]. The results obtained are shown in Figure 6. In this figure it can be observed that the surface area values present, in general terms, an increase related to the increase of the percentage of CA in the CRMs. CRM100 has the highest surface area with respect to UM, Materials 2021, 14, 1543 12 of 23 the percentage of CA in the CRMs. CRM100 has the highest surface area with respect to UM, with a difference of 5.05 m2 /g (73% higher). According to the above, and due to its greater surface area, it can be affirmed that CRMs have greater porosity, which increases as a function of the CA content [36]. In a previous study, similar behavior was observed (case of concrete with recycled concrete aggregates); in which the surface area increased in rials 2021, 14, x FOR PEER REVIEW 13 of 24 relation to the increase in the percentage of recycled aggregate in the mixes (and higher than the reference concrete) [41]. 14 13 BET Surface area (m2 /g) 12 11 10 9 8 7 6 5 4 0 10 20 30 40 50 60 70 80 90 100 CA Content (%) Figure 6. BETFigure 6. area surface BET surface area according toaccording to the the Recycled Recycled Ceramic Ceramic Aggregate Aggregate (CA) (CA) content in content in the Recy- the Recycled Ceramic Mor- tars (RCM). cled Ceramic Mortars (RCM). Total pore volume andvolume Total pore pore size anddistribution are two common pore size distribution are twotechniques also used also used to common techniques to describe the porosity describe theofporosity materials of[39]. Figure[39]. materials 7 shows Figurethe7pore shows sizethe distribution curves, pore size distribution curves, which indicate the indicate which accumulated pore volumepore the accumulated as avolume functionasofa the average function poreaverage of the diameter pore diameter of the of the different different CRMs CRMs studied andstudied and the the reference UM reference UM (adsorption (adsorption and desorption andphase, desorption phase, respectively). respectively). Figure 7a showsFigure that7aCRM100 shows that CRM100 is the is the one that one that contains contains significantly significantly higher higher pore volume, pore withvolume, with an increase an accelerated accelerated increase of pore of pore volume in thevolume poresinwith the diameters pores with diameters between between 20 and 20 and 150 nm; 150 the while nm;restwhile of the rest of thecontain mortars mortarsabout contain48% about lower48% lower volume; the volume; generaloftrend the general trend poreof pore volume volume increaseincrease is a function is a function of the of percentage the percentage of CA of CA in in thethe CRMs. In CRMs. In thethe desorption desorption phase phase (Figure (Figure 7b), 7b), thethe same same order order inin the the behaviorofofthe behavior theCRMs CRMsisis presented; however, the presented; however, the decrease in in volume volumeisisreflected reflectedinin smaller smaller pore diameters pore diameters included in- between 10 and 70 nm. cluded between 10 and 70 nm. The above graphs provide the porosity of the CRMs, which is expressed as cumulative 0.12 pore volume (cm3 /g)0.12 and are presented in Table 5. The above porosity values have a UM UM CRM10 similar trend to the values obtainedCRM10 by means of the open porosity test (Table 4). By the N2 adsorption technique,0.10 the average CRM20 superior difference of CRM100 with respect to the rest Cumulative pore volume (cm3 /g) 0.1 CRM20 Cumulative pore volume (cm3 /g) CRM50 of the mortars is 48% (both inCRM50 the adsorption and desorption phase), while that obtained CRM100 CRM100 0.08 by open porosity is 40%. 0.08 0.06 Table 5. Porosity measured 0.06 in pore volume (N2 adsorption and desorption phases). 0.04 Mortar 0.04 Vads (cm3 /g) Vdes (cm3 /g) UM 0.054321 0.054842 0.02 CRM10 0.02 0.048443 0.049154 CRM20 0.050341 0.050890 0 CRM50 0.00 0.052853 0.053522 100 10 CRM100 1 100 0.099844 10 0.101067 1 Average diameter (nm) Average diameter (nm) (a) (b) Figure 7. Pore size distribution with respect to cumulative pore volume: (a) adsorption phase; (b) desorption phase. The above graphs provide the porosity of the CRMs, which is expressed as cumula- respectively). Figure 7a shows that CRM100 is the one that contains significantly higher pore volume, with an accelerated increase of pore volume in the pores with diameters between 20 and 150 nm; while the rest of the mortars contain about 48% lower volume; the general trend of pore volume increase is a function of the percentage of CA in the Materials 2021, 14, 1543 CRMs. In the desorption phase (Figure 7b), the same order in the behavior of the CRMs is 13 of 23 presented; however, the decrease in volume is reflected in smaller pore diameters in- cluded between 10 and 70 nm. 0.12 0.12 UM UM Materials 2021, 14, x FOR PEER REVIEW 14 of 24 CRM10 CRM10 Cumulative pore volume (cm3 /g) 0.1 CRM20 CRM20 Cumulative pore volume (cm3 /g) 0.10 CRM50 CRM50 CRM100 CRM100 0.08 rest of the mortars is 48% (both in0.08 the adsorption and desorption phase), while that ob- tained by open porosity is 40%. 0.06 0.06 Table 5. Porosity measured in pore volume (N2 adsorption and desorption phases). 0.04 0.04 Mortar Vads (cm3/g) Vdes (cm3/g) 0.02 UM 0.02 0.054321 0.054842 CRM10 0.048443 0.049154 0 0.00 100 10 CRM20 1 0.050341 100 10 0.050890 1 Average diameter (nm) Average diameter (nm) CRM50 0.052853 0.053522 (a) CRM100 0.099844 (b) 0.101067 Figure7.7.Pore Figure Poresize sizedistribution distributionwith withrespect respectto tocumulative cumulativepore porevolume: volume:(a) (a)adsorption adsorptionphase; phase;(b) (b)desorption desorptionphase. phase. Regarding the size distribution (diameter) of pores defined by IUPAC (International Union of Pure The aboveand Regarding the Applied graphs Chemistry), size provide distribution poresofcan (diameter) the porosity of be thepores classified CRMs, defined which into: by micropores is IUPAC expressed as(pore (International <2 cumula- nm), Union mesopores of Pure tive pore volume (between and Applied (cm /g) and 3 2–50 nm) Chemistry), and macropores pores can are presented (pore be classified in Table 5. Theinto:> 50 above nm) [36,37]. micropores porosity (pore Figure values< have 8aa 2 nm), shows similarthat mesopores thetohighest (between trend the 2–50 porosity valuesnm) andpercentages by meansof macropores obtained ofallthe (pore mortars > 50 are found nm)porosity open [36,37]. testin(Table Figure the 8a macropore shows 4). Bythat the zone, the with porosity highest N2 adsorptionan technique, average difference percentages of of the averageall 25%, mortars with superior are respect found in difference toofthe mesopores macropore CRM100 withand 62%with zone, respect oftomi- an the cropores; in whichof average difference the restwith 25%, of the gas was respect introduced. to mesopores However, and in the desorption 62% of micropores; in which phase the (Figure 8b), rest of the gasit was is indicated introduced.that However, the highest in porosity the desorptionis represented by mesopores, phase (Figure with an 8b), it is indicated that the highest average porosity difference withis represented by mesopores, respect to macropores ofwith27%,anandaverage 64%difference with respect with respect to mi- to macropores cropores. of 27%, and 64% with respect to micropores. 100 0.11 0.19 0.23 0.23 0.09 100 0.00 0.00 0.08 0.03 0.00 90 90 37.13 36.06 32.49 80 38.19 80 44.09 70 70 60.15 59.89 61.93 69.96 65.85 Porosity (%) Porosity (%) 60 60 50 50 40 40 62.64 63.72 67.42 30 61.70 30 55.72 20 20 39.85 40.04 38.04 30.04 34.15 10 10 0 0 UM CRM10 CRM20 CRM50 CRM100 UM CRM10 CRM20 CRM50 CRM100 Macropores Mesopores Micropores Macropores Mesopores Micropores (a) (b) Figure Figure 8. 8. Pore Pore size size distribution distribution according to IUPAC: according to IUPAC: (a) (a) adsorption adsorption phase; phase; (b) (b) desorption desorption phase. phase. Research onon two twotypes typesofofmortars mortarswith with100% 100% CA CA(with finefine (with aggregate aggregateandand coarse ag- coarse gregate) aggregate)reported that reported the that mortar the mortarwithwithfine fineaggregate aggregate(similar (similarparticle particlesizes sizes to to this this study), study), showed higher porosity in general than the mortar with coarse aggregate (particles larger than 150 µm). μm). Regarding Regarding the distribution distribution of pore sizes, the mortar with fine aggregate aggregate presented presented aa lower loweramount amountof ofmacropores macroporesand anda ahigher higheramount amount ofof mesopores mesopores withwith respect to respect thethe to reference mortar. reference TheThe mortar. above waswas above attributed to the attributed toown porosity the own of theofCAs porosity the (these influ- CAs (these ence the microstructure influence of the of the microstructure mortar when itwhen the mortar is finely it isground) [17]. Therefore, finely ground) this research [17]. Therefore, this presented similar behaviors research presented to those obtained similar behaviors to those here—in obtainedparticular here—in with desorption particular data. with desorp- From complementary information obtained from the gas adsorption technique, and in tion data. particular Fromfrom the pore sizeinformation complementary distribution obtained curves (Figure from 7), theseveral categoriestechnique, gas adsorption of pore radius and in particular from the pore size distribution curves (Figure 7), several categories of pore radius can be identified. In Table 6 (adsorption and desorption phase, respectively), the results of the maximum radius (rmax), minimum radius (rmin), average radius (rave), medium radius (rmed) and critical radius (rcri) are provided; a description of each is given below. Materials 2021, 14, 1543 14 of 23 can be identified. In Table 6 (adsorption and desorption phase, respectively), the results of the maximum radius (rmax ), minimum radius (rmin ), average radius (rave ), medium radius (rmed ) and critical radius (rcri ) are provided; a description of each is given below. Table 6. Pore radius categories (N2 adsorption and desorption phases). Mortar rmax (nm) rmin (nm) rave (nm) rmed (nm) rcri (nm) Adsorption de N2 UM 103.57 0.91 14.98 33.84 70.50 CRM10 87.80 0.91 12.28 29.22 61.71 CRM20 87.74 0.92 12.52 36.31 87.74 CRM50 91.64 0.92 12.69 37.61 63.89 CRM100 102.64 0.92 16.75 37.89 69.79 Desorption de N2 UM 122.47 1.32 11.69 21.52 24.41 CRM10 101.48 1.32 9.14 16.88 23.61 CRM20 91.92 0.92 9.92 21.09 24.08 CRM50 97.82 0.91 9.58 20.18 22.78 CRM100 113.23 1.63 11.52 20.32 24.26 The rmax and rmin establish the maximum and minimum radius corresponding to each variable; these were obtained directly from the reports provided by the test equipment. The rmax values are in the range of 87.74 to 103.57 nm (adsorption phase), and 91.92 to 122.47 nm (desorption phase); while those of rmin , established radius in the range of 0.91 to 0.92 nm (adsorption phase), and 0.91 to 1.63 nm (desorption phase). As can be seen in the table, these did not show any relationship with respect to the CA content. The rave was obtained by dividing by two the average diameter value calculated with the equation 4V/A (where V = cumulative pore volume, and A = cumulative pore area). The data are obtained with the BJH method (diameters between 17 and 3000 Å). The values obtained show similar behavior to the porosity calculated by N2 adsorption, which is an increase with the percentage of CA (without considering those of the UM sample), and with similar values in mortars with CA substitutions of up to 50%, as well as a notable increase in the mortar with total replacement of the NA. The rmed corresponds to the radius determined by a Lagrange interpolation of the closest points corresponding to 50% of the total volume of each sample [44,45]; obtaining these from the pore size distribution of the test. Table 6 shows that the rmed varies from 29.22 to 37.89 nm, and from 16.88 to 21.52 nm (adsorption and desorption phase, respectively) for the different study variables. The rcri , or critical pore, is the term given to the corresponding radius that causes the onset of the maximum slope in the curve of pore radius versus the pore volume differential (dV/dlogD). This pore radius is usually an indicator of the microstructure of the material and is used to detect a variety of materials [44]. The method for its determination consists of detecting the maximum peak in the pore size versus dV/dlogD curve [14,45,46]. This also indicates the minimum radius of continuous pores within the material [47]; that is, it establishes the space capable of being filled without forming any other adsorption pathway [45,46]. Figure 9 shows the graphs—for adsorption and desorption stages—in which the maximum peak of the curve (with respect to the ordinate axis) is observed; when crossed with the abscissa axis, it establishes the rcri . As an example, that of the variable MCR100 is shown in Figure 9a). Using the data of the mechanical properties of the CRMs (Table 4), a correlation analysis and equation fitting were performed in order to establish the dependencies of these with respect to the different porosity variables obtained by the N2 adsorption technique (Table 6). The criteria established to define the adjusted equation were as follows: (a) A coefficient R2 as close to 1 (R2 ∼ = 1), (b) The type of equation selected should generate a curve plot coincident to the points of the related variables. Materials Materials 2021, 2021, 14, 14, x1543 FOR PEER REVIEW 16 15 of of 24 23 (a) (b) re 9. Pore radius with respect to pore volume differential: (a) adsorption phase; (b) desorption phase. (a) (b) Figure Figure 9. 9. Pore For the Pore radius radius with properties respect withof respect fm, Eto pore pore volume toand densitydifferential: volume differential: of the CRMs,(a) (a) adsorption it wasphase; adsorption (b) desorption established thatphase. the best fit was with the variable of Forrave For the porosityof the properties properties of(in fm,the fm, EE and andadsorption density of density of thephase); the CRMs, itthe CRMs, it was wastype of fitting established established thatequa- that the best the best fit fit tion was that of a second-degree was was with the polynomial variable of rave equation porosity (in (see the Figure adsorption 10). phase);In this the equation, type variable of rave porosity (in the adsorption phase); the type of fitting equa- of fitting the equation R2 values and the equations was was tion of for that that a of fitting second-degreethe data a second-degree for theequation polynomial polynomial three CRM equation properties (see(see Figure In are 10).10). Figure this indicated, equation, In this equation, thetheR2 Rvalues 2 values with a range of validity and ofand thetheequations equations application for forfitting fittingthe established the data datafor between forthe the three three CRM CRM10 andproperties CRM properties CRM100. areIn are indicated, indicated, the with aa range with range of of validity validity ofof application application established established between between CRM10 CRM10 and and CRM100. CRM100. In In the the three equations, thethree UMequations, sample is isolated the UM sample from is the analysis, isolated from the as it establishes analysis, as it inconclusive establishes inconclusive three equations, the UM sample is isolated from the analysis, as it establishes inconclusive values for the study, which values values for may for the the have study, study, their which which may may origin have in aspects have their their origin in origin inherent in to the test aspects inherent aspects inherent to the to theortest toor test itsto or to its its degree of precisiondegree (nm).of degree of precision precision (nm). (nm). 38 38 2.0 2.0 33 1.8 33 1.8 fm and E (x10 3) (MPa) 28 1.6 y = 0.0637x2 - 1.9435x + 16.225 fm and E (x10 3) (MPa) Density (g/cm2) 28 2 R² = 0.9911 1.6 y = 0.0637x UM - 1.9435x + 16.225 23 R² = 0.9911 Density (g/cm2) 1.4 CRM10 UM y = 1.4678x2 - 45.001x + 362.52 23 R² = 0.9312 1.4 CRM20 CRM10 18 y = 1.4678x2 - 45.001x + 362.52 1.2 CRM50 R² = 0.9312 2 CRM20 y = 1.9295x - 58.914x + 459.68 18 R² = 0.9531 1.2 13 1.0 CRM50 fm E y= 1.9295x2 - 58.914x + 459.68 CRM100 Density R² = 0.9531 13 8 1.0 0.8 fm 12.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5 16.0 16.5 17.0 E rave (nm) CRM100 Density 8 Figure 10. Correlation between fm, E and density, with respect to rave. 0.8 12.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5 16.0 16.5 17.0 rave (nm) The rave, as an evident representative parameter (central tendency) of the porosity of the CRMs, meets the expectations of its capacity to link the physical-mechanical behavior Figure Figure 10. Correlation 10. Correlation between fm, Ewith and density, with rrespect to rave . of between the mortars fm,under E and density, study to their respect porosity.toTherefore, ave. an increase in rave is related to lossesThe in fm and density, with CA causing the increase rave , as an evident representative parameter (central in rave,tendency) which in of turn theproduces porosity ofa The rave, as anweaker evident CRM representative matrix parameter structure. On the (central other hand, tendency) the nature ofof thethe the CRMs, meets the expectations of its capacity to link the physical-mechanical behavior oftype porosity of equationof es- the CRMs, meets the tablished (second under order theexpectations mortars polynomial) of itsto study capacity tomay their porosity.link suggest the existence the physical-mechanical Therefore, an increase in of raveanother variable to lossesnot behavior is related in included fm and in this density, study, with CA which causingcould of the mortars under study to their porosity. Therefore, ave the be the increase cause in r of , the which nonlinearity in turn produces an increase in rave is related to in thea equation. weaker Fi- CRM nally, matrixthe existence ofthe a significant change in the of plotting of of theequation curves between the CRM50 losses in fm and density, with CA causing the increase in rthe structure. On other hand, the nature type ave, which in turn produces a established (second order polynomial) may suggest the existence of another variable not included in this study, weaker CRM matrix structure. On the other hand, the nature of the type of equation es- tablished (second order polynomial) may suggest the existence of another variable not included in this study, which could be the cause of the nonlinearity in the equation. Fi- nally, the existence of a significant change in the plotting of the curves between the CRM50 , x FOR PEER REVIEW Materials 2021, 14, 1543 17 16 of of 2423 which could be the cause of the nonlinearity in the equation. Finally, the existence of a and CRM100 variables is notorious, and is worthy of future studies to establish the conti- significant change in the plotting of the curves between the CRM50 and CRM100 variables nuity of behavioris for the percentages notorious, and is worthyofofCA between future them. studies to establish the continuity of behavior for the As for the adsorption percentages of andCAdrying betweenshrinkage them. properties, these established a better link- As for the age with the rmax porosity adsorption variable and drying shrinkage (adsorption phase),properties, obtaining these establishedRa2better significant withlinkage sec- with the r porosity variable (adsorption phase), obtaining significant R 2 with second- ond-degree polynomial type equation curves as shown in Figure 11. As in the previous max degree polynomial type equation curves as shown in Figure 11. As in the previous case, case, their application is valid for the range between CRM10 to CRM100, as well as the their application is valid for the range between CRM10 to CRM100, as well as the assump- assumption of another variable tion of another not included variable not includedinin the thestudy. study. Figure 11. Correlation between absorption and drying shrinkage with respect to r . Figure 11. Correlation between absorption and drying shrinkage with respect tomax rmax. In this case, it seems that the existence of large pores in the CRM matrix is what best In this case, it seems explains thethat the existence behavior of these twoof large pores in properties—both the CRMinvolve, intrinsically matrixinistheir what best respective behavioral explains the behavior phenomena, of these the ability of water intrinsically two properties—both mobility between the exterior involve, and interior in their respec-of the matrix; therefore, the existence of these large pores seems to provide the conditions tive behavioral phenomena, the ability of water mobility between the exterior and interior that promote these behaviors, the increases in the CA in the CRM being what favors the of the matrix; therefore, increase inthe existence their size. of these large pores seems to provide the conditions that promote these behaviors, the increases From the results in the CA of the BET surface area in the CRM property with being respectwhat to the favors physicalthe and mechanical properties of CRMs, density, E, and absorption obtained R 2 > 0.9; while for fm increase in their size. and drying From the results shrinkage of the were 0.89area BET surface and 0.79, respectively. property All the fitting with respect to theequations physicalwereandof the second-degree polynomial type. On this occasion they are 2valid for all the research mechanical properties of CRMs, density, E, and absorption obtained R > 0.9; while for fm variables—dispersion of variables according to expectations (Figure 12). and drying shrinkage were 0.89 and 0.79, respectively. All the fitting equations were of the 3.3. Porosimetry second-degree polynomial by SEM type. On Image Analysis they are valid for all the research varia- this occasion The resultsaccording bles—dispersion of variables of the porosity determined by to expectations SEM image (Figure 12). analysis are presented in Figure 13; in this, two important aspects are highlighted: (a) a significant increase in the percentage of porosity of the CRMs with respect to the reference UM—especially with the CRM100 and (b) for the CRM10, CRM20, CRM30, and CRM50 variables, with values close to or without evident order—attributable to the complex analysis process (prompted by aspects such as the selection of the capture image by the microscope, or by the quality of the image to be treated). that promote these behaviors, the increases in the CA in the CRM being what favors the increase in their size. From the results of the BET surface area property with respect to the physical and mechanical properties of CRMs, density, E, and absorption obtained R2 > 0.9; while for fm Materials 2021, 14, 1543 and drying shrinkage were 0.89 and 0.79, respectively. All the fitting equations were 17 ofofthe 23 second-degree polynomial type. On this occasion they are valid for all the research varia- bles—dispersion of variables according to expectations (Figure 12). Materials 2021, 14, x FOR PEER REVIEW 18 of 24 3.3. Porosimetry by SEM Image Analysis The results of the porosity determined by SEM image analysis are presented in Figure 13; in this, two important aspects are highlighted: (a) a significant increase in the percent- age of porosity of the CRMs with respect to the reference UM—especially with the CRM100 and (b) for the CRM10, CRM20, CRM30, and CRM50 variables, with values close (a) to or without evident order—attributable to the complex (b) analysis process (prompted by aspects Figure 12. Correlation such as the selection of the capture image by the microscope, or by the quality of Correlation of BET surface area and: (a) fm, E and density; (b) absorption and drying shrinkage. the image to be treated). 35 Total porosity by SEM image 30 25 analysis (%) 20 15 10 5 0 UM CRM10 CRM20 CRM30 CRM50 CRM100 Mortars Figure 13. Percentage Figure 13. Percentage of of total total porosity porosity of ofCRMs CRMsby bySEM SEMimage imageanalysis. analysis. With With respect respecttotothe theporosity porosityvalues valuesobtained obtainedby bythis thistechnique, technique,aacorrelation correlationanalysis analysis was was also also performed performed to to compare compare thethephysical physical and andmechanical mechanical properties properties of ofthe theCRMs. CRMs. InIn Figure 14 we present the results of fm, E and density with respect to the Figure 14 we present the results of fm, E and density with respect to the total porosity total porosity obtained obtained by bySEM SEMimage imageanalysis. analysis.TheThebest-fit best-fitregressions regressionswere werewith withaasecond-degree second-degreepoly-pol- nomial ynomial trend, with application to all the study variables—including CRM30;of trend, with application to all the study variables—including CRM30; ofthese, these,the the relationship relationship withwith respect respect toto density density is is the the most most outstanding. outstanding. Comparatively, Comparatively, the the adjust- adjust- ments ments achieved achieved with with this this technique technique to to establish establish the the porosity porosity with with respect respect to tothe thephysical physical and and mechanical properties of CRMs are less satisfactory than those obtained with N2 N mechanical properties of CRMs are less satisfactory than those obtained with 2 ad- adsorption—not because of its calculation accuracy, but because of the degree sorption—not because of its calculation accuracy, but because of the degree of represent- of represen- tativeness ativeness of of the the test test sample. sample. 3.4. Total Open Porosity The results of open porosity presented in this study were determined in previous work [29]; however, a brief abstract of their determination would be the use of dry, sub- merged, and saturated state weights, applying the formulation of the standards [33]. With the data obtained for the CRMs under study, an analysis was performed to establish the relationship with the physical and mechanical properties. Regarding the properties of fm, E and density, R2 values higher than 0.9 were obtained, with a second-degree polynomial trend (see Figure 15a), in the case of drying shrinkage it was 0.8101 (see Figure 15b); finally, (a) (b) UM CRM10 CRM20 CRM30 CRM50 CRM100 Mortars Figure 13. Percentage of total porosity of CRMs by SEM image analysis. Materials 2021, 14, 1543 With respect to the porosity values obtained by this technique, a correlation analysis 18 of 23 was also performed to compare the physical and mechanical properties of the CRMs. In Figure 14 we present the results of fm, E and density with respect to the total porosity obtained by SEM image analysis. The best-fit regressions were with a second-degree pol- absorption was not ynomial trend, withincluded application in to theallstudy of correlations the study because its variables—including determination CRM30; used of these, the data identicalwith relationship to those used respect to establish to density is the open most porosity (numerical outstanding. redundancy Comparatively, that pro- the adjust- duces mentsidentity). In any achieved with oftechnique this the properties correlated to establish with open the porosity withporosity, it the respect to is evident physicalthat open and Materials 2021, 14, x FOR PEER REVIEW porosity mechanical establishes properties aofsatisfactory CRMs are relationship, less satisfactorywith than an ordered, those equidistant, obtained with 19 and N2 ad- of 24 sorption—not continuous because distribution dispersion of its calculation of theaccuracy, studiedbut because of the degree of represent- variables. ativeness of the test sample. 3.4. Total Open Porosity The results of open porosity presented in this study were determined in previous work [29]; however, a brief abstract of their determination would be the use of dry, sub- merged, and saturated state weights, applying the formulation of the standards [33]. With the data obtained for the CRMs under study, an analysis was performed to establish the relationship with the physical and mechanical properties. Regarding the properties of fm, E and density, R2 values higher than 0.9 were obtained, with a second-degree polynomial trend (see Figure 15a), in the case of drying shrinkage it was 0.8101 (see Figure 15b); finally, absorption was not included in the study of correlations because its determination used data identical to those used to establish open porosity (numerical redundancy that pro- duces (a) identity). In any of the properties correlated with(b) open porosity, it is evident that Figure open porosity establishes a satisfactory relationship, with an ordered, equidistant, and Figure 14.14.Correlation CorrelationofofPorosity Porosityby by SEM SEM and: and: (a) (a) fm, fm,EEand anddensity; density;(b) (b)absorption absorptionand drying and shrinkage. drying shrinkage. continuous dispersion distribution of the studied variables. (a) (b) Figure 15. Correlation Open porosity and: (a) fm, E and density; (b) drying shrinkage. 3.5. 3.5. Correlations Correlations between between Porosimetry Porosimetry Techniques Techniques With With the results obtained fromthe the results obtained from thethree threedifferent different porosimetry porosimetry techniques, techniques, it was de- it was termined determined which which of of these theseoffered offereda abetter bettercorrelation correlationwith withthe the physical physical and mechanical and mechanical properties of of the theCRMs. CRMs.FromFromthe theprevious previousresults, results, it it waswas observed observed that that thethe most most signifi- significant cant correlations correlations betweenbetween the different the different porosityporosity values values were were presented presented with thewithdata the obtained data ob- tained from the from theporosity—the open open porosity—the simplest, simplest, most method most direct direct method and theandonethe thatone that estab- establishes a lishes a more representative sample of the matrix of a mortar. The results more representative sample of the matrix of a mortar. The results of total porosity—the of total poros- ity—the only commononly common result comparable result comparable among theamong the study techniques study techniques for the three fortechniques—are the three tech- niques—are presented inpresented Figure 16in(porosity Figure 16by (porosity by N2 is included N2 is included in the adsorption in the adsorption and de- and desorption sorption phase—both phase—both very similar). very similar). In this In this graph, graph, it hasitbeen has been chosen chosen to present to present the the results results in in percentage percentage to simplify to simplify thethe comparison; comparison; thus, thus, allowing allowing making making visible visible the same the same behav- behavior of porosity ior increase of porosity between increase the study between variables the study (CA content) variables and the (CA content) andtechniques the techniques used.used. To numerically validate the previous results of total porosity of the CRMs, taking into account the different experimental techniques of their determination, the correlations with the data obtained with the same were analyzed. It was observed that the comparison between open porosity and N2 porosity (desorption phase) resulted in a coefficient R2 = 0.9881 (second-degree polynomial) (Figure 17); which shows the affinity between both techniques, and also corroborates the incidence of the use of CA in the behavior of the CRMs. In accordance with the above, and with the objective of establishing the optimal tech- nique to use—or in its case to establish the hierarchical order of importance in the selection of the technique when porosity is to be established, it is determined that the open porosity physical behavior of CRMs); although it is true that its requirements for laboratory equip- ment and its representativeness of the samples are more important and complex. Finally, as a third option, porosity by SEM image analysis would be the last of the three tech- Materials 2021, 14, x FOR PEER REVIEW 20 of 24 niques; this limitation of choice is due to the fact that its numerical determination may Materials 2021, 14, 1543 involve factors such as: the choice of the image for its study, its representativeness19ofofthe 23 complete mortar matrix, its study scale factor, its requirements of specialized laboratory used as a second equipment alternative and its (verification own complex sample of the first one, or with the contribution of special preparation. and different porosity parameters, which can help to better explain the mechanical and physical behavior of CRMs); although it is true that its requirements for laboratory equip- ment and its representativeness of the samples are more important and complex. Finally, as a third option, porosity by SEM image analysis would be the last of the three tech- niques; this limitation of choice is due to the fact that its numerical determination may involve factors such as: the choice of the image for its study, its representativeness of the complete mortar matrix, its study scale factor, its requirements of specialized laboratory equipment and its own complex sample preparation. Figure 16. Figure 16. Percentages Percentages of of total total porosity porosity obtained obtained by by the the techniques techniques used. used. To numerically validate the previous results of total porosity of the CRMs, taking into account the different experimental techniques of their determination, the correlations with the data obtained with the same were analyzed. It was observed that the compari- son between open porosity and N2 porosity (desorption phase) resulted in a coefficient R2 = 0.9881 (second-degree polynomial) (Figure 17); which shows the affinity between both techniques, and also corroborates the incidence of the use of CA in the behavior of the CRMs. Figure 16. Percentages of total porosity obtained by the techniques used. Figure 17. Correlation between open porosity and N2 porosity (desorption phase). As additional information in this study, it is suggested that the optimum percentage of CA to be used in CRMs is 20%. Taking into account the porosity results obtained by the three techniques, it was observed that for each of them, the CRMs presented similar be- havior —increased porosity, with increased CA content, particularly with the open poros- ity and N2 porosity techniques; however, although this information is relevant, it prevents choosing the adequate percentage of CA. Therefore, with knowledge of the physical and mechanical Figure Figure 17. properties 17. Correlation Correlation of CRMs, between between open the open fm results porosity porosity and N and N22(property considered porosity (desorption porosity (desorption important or repre- phase). phase). sentative in mortars and concretes) were taken as an indicator for such decision. Accord- In ing to accordance Asthe additional informationwith the above, information presented in in thisand withit the study, the present objective isstudy, suggested of establishing that the optimum it is considered the that the optimal percentage use of up technique of CA to betoused use—or in CRMsin itsiscase 20%.toTaking establish intothe hierarchical account orderresults the porosity of importance obtained in bythe the selection of the technique three techniques, when porosity it was observed is to that for be established, each of them, theitCRMs is determined presentedthat the open similar be- porosity technique isporosity, havior —increased the one that withprovides increased more reliable data CA content, (in addition particularly withtothe doing opensoporos- with aity minimum processing and N2 porosity of the samples). techniques; however, Then, the N2this although adsorption information analysis technique is relevant, should it prevents be used as a second alternative (verification of the first one, or choosing the adequate percentage of CA. Therefore, with knowledge of the physical andwith the contribution of special and different mechanical propertiesporosity of CRMs,parameters, which(property the fm results can help to better explain considered the mechanical important or repre- and physical sentative behavior in mortars andofconcretes) CRMs); although were takenit isastrue an that its requirements indicator for laboratory for such decision. Accord- equipment and its representativeness of the samples are more important ing to the information presented in the present study, it is considered that the use of up and complex. Finally, as a third option, porosity by SEM image analysis would be the last of the three techniques; this limitation of choice is due to the fact that its numerical determination may Materials 2021, 14, 1543 20 of 23 involve factors such as: the choice of the image for its study, its representativeness of the complete mortar matrix, its study scale factor, its requirements of specialized laboratory equipment and its own complex sample preparation. As additional information in this study, it is suggested that the optimum percentage of CA to be used in CRMs is 20%. Taking into account the porosity results obtained by the three techniques, it was observed that for each of them, the CRMs presented similar behavior —increased porosity, with increased CA content, particularly with the open porosity and N2 porosity techniques; however, although this information is relevant, it prevents choosing the adequate percentage of CA. Therefore, with knowledge of the physical and mechanical properties of CRMs, the fm results (property considered important or representative in mortars and concretes) were taken as an indicator for such decision. According to the information presented in the present study, it is considered that the use of up to 20% of CA in CRMs will guarantee a similar behavior to UMs, having as an added value the environmental benefits that the use of CA entails (as previously mentioned in the introduction). 4. Conclusions In this study, an experimental investigation was carried out to present the results referring to the porosity of CRMs, correlating these with the physical and mechanical properties of the mortars, as well as between the techniques used to study porosity. Based on the results obtained, the following conclusions can be drawn: 1. The incremental variability of the porosity in the CRMs (increase directly correlated with the increase in CA percentage content), is validated by the different test tech- niques used; in particular with that of open porosity and that of N2 adsorption. 2. The results obtained from the open porosity calculation correlate best with the behav- ior of the CRMs—percentage of CA addition as well as with the results obtained from the physical and mechanical properties of the CRMs. 3. The N2 adsorption test provides information related to the amount of adsorbed gas, pore size distribution (diameter/radius), BET surface area, diversity of pore radius types, etc., which can be used to analyze or compare with respect to the physical and mechanical properties of CRMs. The technique can be considered with a contribution of important diversity of parameters that can be used to understand the effect of the porous network of mortars. 4. The properties such as fm, E and density of the CRMs, presented the best fit with the variable of rave porosity, (in adsorption phase); while absorption and drying shrinkage, with respect to rmax . The former because it is a variable of general representation of the porous network with incidence on properties on which the whole mortar matrix has an impact; and the latter, because it is a parameter on which the related properties have an impact on the capacity of water mobility. 5. The BET surface area with respect to physical and mechanical properties, established acceptable coefficients R2 (R2 ∼ = 1), as well as equations that generate a trace of its curve coincident to the points of the related variables. 6. Porosity results obtained with SEM image analysis do not show an evident correlation with respect to physical and mechanical properties of CRMs; they are so for the extreme variables (UM and CRM100), but without evident order for the rest. This is attributed to the image analysis process itself. 7. It is determined that the open porosity technique is the one that provides the most reliable data, which can help to better explain the physical and mechanical behavior of the CRMs, followed by the N2 adsorption analysis technique, and finally the SEM image analysis technique. 8. It is suggested that the optimal percentage of CA to be used in CRMs is 20%, which will guarantee similar properties to UMs. Materials 2021, 14, 1543 21 of 23 Author Contributions: Conceptualization, J.M.G.-S. and F.G.C.-C.; methodology, J.M.G.-S., F.G.C.-C. and C.A.R.-C.; software, J.M.G.-S. and C.A.R.-C.; validation, J.M.G.-S., F.G.C.-C. and J.L.A.-S.; formal analysis, F.G.C.-C. and J.M.G.-S.; investigation, F.G.C.-C. and J.M.G.-S.; resources, J.M.G.-S., J.L.A.-S. and J.M.B.-C.; data curation, J.M.G.-S., F.G.C.-C. and C.A.R.-C.; writing—original draft preparation, F.G.C.-C.; writing—review and editing, J.M.G.-S. and F.G.C.-C.; visualization, J.M.G.-S.; supervision, J.M.G.-S.; project administration, J.M.G.-S.; funding acquisition, J.M.G.-S., F.G.C.-C. and J.M.B.-C. Authorship must be limited to those who have contributed substantially to the work reported. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the large amount and variety of data that were processed. Acknowledgments: The authors would like to thank to the Barcelona School of Building Construction- UPC, the Department of Architecture Technology-EPSEB-UPC, Faculty of Engineering Mochis-UAS, to CONACYT and Research and Postgraduate Coordination-School of Engineering Mazatlán-UAS. Conflicts of Interest: The authors declare no conflict of interest. Abbreviations The following abbreviations are used in this manuscript: rave Average radius BEI Backscattered Electron Image BJH Barrett, Joyner and Halenda BET Brunauer, Emmett and Teller c:a Cement:aggregate ratio R2 Coefficient of determination R2 fm Compressive strength C&D Construction and Demolition CDW Construction and Demolition Waste rcri Critical radius A Cumulative pore area V Cumulative pore volume EDS Energy Dispersive Spectrometer ITZ Interfacial Transition Zone IUPAC International Union of Pure and Applied Chemistry rmax Maximum radius rmed Medium radius MIP Mercury Intrusion Porosimetry rmin Minimum radius E Modulus of Elasticity NA Natural Aggregate N2 Nitrogen OD Oven-Dry condition CA Recycled Ceramic Aggregate CRM Recycled Ceramic Mortars SSD Saturation-Surface-Dry condition SEM Scanning Electron Microscopy SEI Secondary Electron Image UM Usual Mortar Vads Volume adsorption phase Vdes Volume desorption phase w/c Water:cement ratio EDX X-Ray Diffraction Materials 2021, 14, 1543 22 of 23 References 1. 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