materials Article SEM Image Analysis in Permeable Recycled Concretes with Silica Fume. A Quantitative Comparison of Porosity and the ITZ Manuel J. Chinchillas-Chinchillas 1 , Carlos A. Rosas-Casarez 1 , Susana P. Arredondo-Rea 1 , José M. Gómez-Soberón 2, * and Ramón Corral-Higuera 1 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 2 Barcelona School of Building Construction, Polytechnic University of Catalonia, Av. Doctor Marañón 44-50, 08028 Barcelona, Spain * Correspondence: josemanuel.gomez@upc.edu; Tel.: +34-934016242  Received: 4 June 2019; Accepted: 3 July 2019; Published: 8 July 2019  Abstract: Recycled aggregates (RA) from construction and demolition can be used in permeable concretes (PC), improving the environment. PCs have a significant porous network, their cement paste and the interaction between the paste and the RA establishing their strength. Therefore, it is important to evaluate the porosity in the interfacial transition zones. The porosity of the cement paste, the aggregate and the interfacial transitional zones (ITZ) of a PC with recycled coarse aggregates (RCA) and silica fume (SF) is measured by means of image analysis–scanning electron microscope (IA)-(SEM) and by mapping the chemical elements with an SEM-EDS (energy dispersive spectrometer) detector microanalysis linked to the SEM and, as a contrast, the mercury intrusion porosimetry technique (MIP). In the IA process, a “mask” was created for the aggregate and another for the paste, which determined the porosity percentage (for the anhydrous material and the products of hydration). The results showed that using SF caused a reduction (32%) in the cement paste porosity in comparison with the PC with RA. The use of RA in the PC led to a significant increase (190%) in the porosity at different thicknesses of ITZ compared with the reference PC. Finally, the MIP study shows that the use of SF caused a decrease in the micropores, mesopores and macropores. Keywords: porosity; recycled aggregates; permeable concrete; interfacial transition zone; image analysis; porosimetry mercury intrusion 1. Introduction PC is a special concrete with a high gap content and high permeable capacity when compared with conventional concrete [1,2]. This PC offers several environmental benefits, among which are rainwater runoff control, replenishment of underground water supplies, an improvement in water quality and a reduction in water and subsoil contamination [3]. Therefore its use is habitual in parks, reduced traffic areas, tennis courts, etc. [4]. This PC contains interconnected macropores of sizes that oscillate from 2 to 8 mm and with gap content of between 18% and 35%; therefore, its mechanical strength is closely linked to the strength of the cementing matrix itself and the interaction of the matrix with the aggregates that form it. Consequently, it is necessary to determine and study the porosity of these areas of interest [5]. The use of natural aggregates is currently widespread in the manufacture of PC [6], but few studies have used RA [7]. As important quantities of construction and demolition waste are contaminating many ecosystems worldwide, it is imperative to recycle them and produce new building materials, Materials 2019, 12, 2201; doi:10.3390/ma12132201 www.mdpi.com/journal/materials Materials 2019, 12, 2201 2 of 19 recycled concrete [8]. The production of RA from construction and demolition waste has been studied widely in the last two decades and the studies agree that these aggregates are more heterogeneous, less dense and more porous than the natural aggregates [9,10]. Those studies which have analyzed the microstructure of the RA show that there are two ITZ a) between the old natural aggregates and the old matrix of the cement and b) between the RA and the new cement matrix [11]. The ITZ is considered by many researchers to be the weak link in the concrete [12,13], in which the normal concrete fails, significantly compromising both mechanical properties and permeability; this is also characterized by being an area of high porosity [14]. This differentiated zone can form due to the “wall” effect caused by the looser packing of the cement granules against the surface of the aggregate, or because of the accumulation of free water around the surface of the aggregate [15]. Some models of the ITZ have established that it is a space with ‘uniform’ porosity, located 5–30 µm from the aggregate surface; in other models, it has been found up to 50 µm from the aggregate [16]. Porosity in the ITZ is one of the basic factors influencing the strength and durability of concrete; the more porosity in this zone, the less mechanical strength and the greater the vulnerability to environmental degradation [17]. Additionally, the porosity in the cementing matrix can be determined as the sum of the volume of the capillary pores, the ice pores, the macropores formed by the presence of air and macropores caused by compacting [18]. This porosity can also be considered as the spaces not filled by the solid components of the hydrated cement paste, which depend to a great extent on the ratio of water/cement (w/c) and the degree of hydration reached by the cement [18]. Nowadays several alternatives that allow a reduction in the porosity percentage in concrete have been established, such as the inclusion of additives, the setting of low w/c ratios, the inclusion of fibers or the application of pozzolans [19,20]. Regarding the last alternative, SF is the most used pozzolan for improving the properties of concrete and reducing the porosity [21] when the ITZ is densified; this can react with the cement hydration products (calcium hydroxide—CH–) to form calcium silicate hydrate (CSH), which thereby improves the mechanical strength. Similarly, this SF also acts as a filler which reduces porosity, as the granule size of this material is between 0.1 and 0.2 µm [22]. Having seen the importance of porosity in the binding matrix of concrete and in its ITZ, several studies have used various methods to try to establish and calculate the percentage of porosity. Among them are computerized tomography X-ray scans [23], mercury intrusion porosimetry technique (MIP) [24,25], mathematical models of finite elements [26] and X-ray microtomography [9]. However, the IA technique using software is noteworthy, as the treatment of images obtained from IA-SEM is a non-destructive, precise and low-cost method [27]. This method breaks the image down to a scale of gray tones to obtain a binary image of the area of interest; it is also applicable when the aggregate or cement paste is made up of various phases, making it possible to identify the hydration products such as CSH, CH or ettringite [28]. The IA makes it possible to make a mask—a selection filter—of the aggregate or the cement paste, allowing these elements to be distinguished, then establishing the porosity with exactitude [29]. The importance of this technique is reflected in those studies where the porosity of the ITZ is calculated at different thicknesses of the aggregate [28], quantifying the percentage of pores and micro-cracks in the concrete [30]. This leads to the identification and quantifying of the hydration products and the anhydrous materials of the concrete [31,32]. However, the majority of these studies have focused on conventional concretes and there is no evidence of studies of image analysis in PCs. Therefore, this study aimed to evaluate and correlate the microporosity of PC made with RA and SF using MIP and the IA of SEM; this was done through studying the porosity of the aggregates, the porosity of the cement paste, the percentage of hydration products, the percentage of anhydrous materials and the percentage of porosity of the ITZ at different zones of the aggregate. Materials 2019, 12, 2201 3 of 19 2. Materials and Methods 2.1. Materials CEMEX® brand composite Portland cement (30R) [33] was used to prepare the PCs. Local companies supplied the distilled water, the natural coarse aggregate (NCA) and the SF. The RCA was obtained by means of grinding waste paving concrete, with a strength of 23 MPa, in a jaw crusher in the sustainable materials laboratory of the Universidad Autónoma de Sinaloa (UAS), Mexico. The properties of the NCA and RCA are shown in Table 1; these were determined from the ASTM C127 and ASTM C29 [34,35] standards. It should be mentioned that the maximum size of the aggregates was 12.7 mm, obtained after sieving. Of the studied properties, the high absorption of the RCA—more than 3.5 times that of the NCA—is noteworthy. In order to determine the chemical compounds present in the cement and the SF, an X-ray fluorescence analysis was carried out with a set of ARL 8680 (ThermoFisher Scientific, Waltham, MA, USA.), using the wave dispersion method according to the ASTM C114 [36] standard (see Table 2). Both the ASTM C150 cement [33] and the SF (ASTM C1240) [37] showed typical percentages of compounds. Finally, the physical properties of the Portland cement and the SF are to be found in Table 3, establishing that both are within the specifications [33,37]. Table 1. Properties of the aggregates. Properties NCA RCA Aggregate size (mm) 12.70 12.70 Concrete strength of origin (MPa) - 23.00 Absorption (%) 1.67 6.28 Humidity (%) 1.67 1.80 Density (kg/cm3 ) 2.65 2.35 Porosity (%) 11.00 16.31 Rebound index (MPa) 33.20 29.90 Table 2. Chemical composition of Portland cement and silica fume. Oxide (%) SiO2 Al2 O3 Fe2 O3 CaO SO3 K2 O Na2 O MgO Cement 19.94 4.40 2.97 63.5 3.08 0.42 0.12 - Silica Fume 95.22 0.08 2.37 0.26 0.11 0.56 0.30 0.24 Table 3. Properties of Portland cement and silica fume. Material Density (kg/m3 ) Specific Surface (m2 /kg) Average Size (µm) Cement 3150 1400 15–25 Silica Fume 2270 19,600 0.1–0.2 2.2. Design of the Permeable Concrete Mixture Table 4 shows the components and quantities of the different materials that make up the four dosages of the PC mixtures to be used in this study. The criteria of the RCA content percentages have been established with reference to previous similar research [38,39], as well as in the case of SF (10%) [40,41]. The values shown are those needed to make a beam of 15 × 15 × 50 cm, from which the study samples will later be taken (see Figure 1). Materials 2019, 12, 2201 4 of 19 Materials 2019, 12, x FOR PEER REVIEW 4 of 20 Figure 1. General scheme of the test samples. (a) real photo of the specimen beam and (b) areas of studyFigure 1. General for analysis scheme of of scanning the test microscope electron samples. (a) (SEM) real photo andofmercury the specimen beamporosimetry intrusion and (b) areas(MIP). of study for analysis of scanning electron microscope (SEM) and mercury intrusion porosimetry (MIP). The nomenclature used is classified as follows: the first three letters (NCA or RCA) refer to the The nomenclature used is classified as follows: the first three letters (NCA or RCA) refer to the origin of the coarse aggregate; the following value shows the percentage of recycled aggregate used origin of the coarse aggregate; the following value shows the percentage of recycled aggregate used (100% or 50%); finally the last letter indicates the cement used (C = 100% Portland and SF = 90% (100% or 50%); finally the last letter indicates the cement used (C = 100% Portland and SF = 90% Portland + 10% SF). A value of 0.35 was used for the w/c ratio, as is common in this type of study [42,43]. Portland + 10% SF). A value of 0.35 was used for the w/c ratio, as is common in this type of study As this [42,43].aAs was PC, nowas this fine aggregates a PC, (FA) were(FA) no fine aggregates added weretoadded the mixture. to the mixture. 4. Previous concrete mixtures for 15 ××15 15××5050 3 TableTable 4. Previous concrete mixtures fora aspecimen specimen of of 15 cmcm (0.0113 (0.0113 m 3).m ). Materials Materials NCA NCA100100CC NCA NCA 100100 SFSF RCA 50RCA C 50 C RCA 50 SF RCA 50 SF Water (L) 1.9781 1.9781 1.9467 1.9467 Water (L) 1.9781 1.9781 1.9467 1.9467 NCA (kg) NCA (kg) 17.5568 17.5568 17.5571 17.5571 8.77858.7785 8.7785 8.7785 RCA (kg) RCA (kg) -- - - 8.77858.7785 8.7785 8.7785 Cement (kg) (kg) Cement 5.5621 5.5621 5.0059 5.0059 5.56215.5621 5.0059 5.0059 Silica fume Silica (kg) fume (kg) -- 0.5562 0.5562 - - 0.5562 0.5562 The mixing process of the PC followed the methods laid down by previously reported studies [44], as wellprocess The mixing as the recommendations of the PC followedfor the using RCA, such methods laidas the need down for pre-saturation by previously reported[4]. When [44], studies the mixture was ready, the concrete was poured into metal 15 × 15 × 50 cm beam molds, as well as the recommendations for using RCA, such as the need for pre-saturation [4]. When the in accordance with the ASTMC31 norms [45]. The beams were kept in laboratory conditions for 24 h before being mixture was ready, the concrete was poured into metal 15 × 15 × 50 cm beam molds, in accordance with removed from the molds and immersed in a curing tank (ASTM C192) [46], until being tested or the ASTMC31 norms [45]. The beams were kept in laboratory conditions for 24 h before being removed processed. from the molds and immersed in a curing tank (ASTM C192) [46], until being tested or processed. 2.3. Test Methods 2.3. Test Methods 2.3.1. Mechanical and Physical Properties of the PC 2.3.1. Mechanical and Physical Properties of the PC After 28 days of curing, the PC beams were tested for their flexural strength, in accordance with After ASTM28 days C 348 of and [47], curing, then the PC beams were the compressive tested strength for their (ASTM was evaluated flexural strength, C-349 inaddition, [48]). In accordance with the ASTM C permeability water 348 [47], and then the coefficient compressive was strength assessed (ASTM was evaluated D-2434-68 [49]) with a(ASTM variableC-349 charge[48]). In addition, the water permeability coefficient was assessed (ASTM D-2434-68 [49]) with a variable permeameter. charge permeameter. Materials 2019, 12, 2201 5 of 19 2.3.2. Mercury Intrusion Porosimetry (MIP) The porosimeter used for the MIP tests was a Micromeritics Autopore IV 9500 (Micromeritics Corporate Headquarters, Norcross, Atlanta, GA, USA.), with a precision of 33.000 psi (228 MPa) and Materials 2019, 12, x FOR PEER REVIEW 5 of 20 pore diameter detection capacity ranging from 0.006 to 175 µm. As there are no specific norms for concrete, theMercury 2.3.2. norms Intrusion of ASTMPorosimetry D4404 (for(MIP) soil and rock) [50] were followed in the test, along with the evidence from previously completed studies [51]. The test age of the specimens was 90 days; they were The porosimeter used for the MIP tests was a Micromeritics Autopore IV 9500 (Micromeritics extracted from the Corporate PC beamsNorcross, Headquarters, (approximate Atlanta, volume GA, USA.), cm3 )a using 2 with a Buehler precision Isomet of 33.000 psi 4000and (228 MPa) precision saw9500 (Buehler, pore diameterLake Bluff, detection IL, USA.), capacity rangingwith froma blade 0.006 tocutting 175 μm.thickness As there areofno 20specific µm. This prevents norms for the possibleconcrete, formation the norms of ASTM D4404 of microfissures (for soil during and the rock) [50] were extraction followed process. Theintest the test, tubes along with were the in the dried evidence permeameter from previously vacuum chambercompleted for 60 minstudies at 24[51]. ◦ C The test the before age inclusion of the specimens of thewas 90 days; mercury they The Hg (Hg). were extracted from the PC beams (approximate volume 2 cm3) using a Buehler Isomet 4000 precision inclusion process was started at low pressure so that the test team could identify the biggest pores; saw9500 (Buehler, Lake Bluff, IL, USA.), with a blade cutting thickness of 20 μm. This prevents the then thepossible pressure was increased formation so thatduring of microfissures the Hg thepenetrated all the The extraction process. simple pore cavities. test tubes were driedEach in thetest took approximately permeameter 7 h vacuum chamber for 60 min at 24 °C before the inclusion of the mercury (Hg). The Hg inclusion process was started at low pressure so that the test team could identify the biggest pores; 2.3.3. Scanning Electron then the pressure Microscope was increased so(IA-SEM) that the Hgand Microanalysis penetrated of Chemical all the simple Elements pore cavities. Each testwith took an Energy approximately Dispersive Spectrometer 7h (SEM-EDS) Small 2.3.3.representative Scanning Electronsamples Microscopeof(IA-SEM) the concrete beams were and Microanalysis taken (with of Chemical Elements anwith exposed an face of approximately 5 cm long Energy Dispersive by 5 cm(SEM-EDS) Spectrometer wide) in order to obtain the IA-SEM images of each of the PC variables in the study. These samples were then placed within circular molds and covered with Small representative samples of the concrete beams were taken (with an exposed face of transparent epoxy resin (EpoFix ◦ C (catalyzer), approximately 5 cm long byResin). They 5 cm wide) in were order then put in to obtain thean oven for IA-SEM 40 min images at 40 of each of the PC by which time the variables resin in the hadThese study. hardened samplesand sealed were then the sample placed withininside. circularThe samples molds in thewith and covered resin were transparent then subjected epoxy resin (EpoFix to progressive Resin). and smoothing They polishing were then putonintheir an oven for 40 min exposed studyat 40 °C (catalyzer), face until they shone. by which time the resin had hardened and sealed the sample inside. The The “areas of interest” were then chosen visually, to be studied later in the IA-SEM. As samples in the resin were the RCA then subjected to progressive smoothing and polishing on their exposed study face until they shone. used in the PC introduced a different component to the study matrix [52,53], they were located The “areas of interest” were then chosen visually, to be studied later in the IA-SEM. As the RCA used as (a) ITZ between in the the new PC introduced aggregate–old a different component aggregate, (b) ITZ to the study matrix between [52,53], newlocated they were paste–old paste and as (a) ITZ (c) ITZ between between the new newaggregate–new paste (see aggregate–old aggregate, Figure (b) ITZ 2); they between new were present paste–old paste in and the (c) RCA 50 SF sample. ITZ between Once thenew“areas of interest” aggregate–new pastehad (seebeen Figurelocated, the samples 2); they were present in were the RCAsubjected to drying 50 SF sample. Once thein“areas an oven at a of interest” had been located, the samples were subjected to drying in an oven at low temperature until they had attained a consistent weight; they were then metallized with graphite a low temperature powderuntil they had (obtaining attained a consistent a conductive surface) weight; [54] and they were in placed then metallized a dryer until with graphite just before powder being studied in (obtaining a conductive surface) [54] and placed in a dryer until just before being studied in the IA- the IA-SEM. SEM. Figure 2. RCA 50 SF embedded in epoxy resin with a polished surface. (a–c) “areas of interest" for study in IA-SEM. Materials 2019, 12, x FOR PEER REVIEW 6 of 20 Materials 2019, 12, 2201 Figure 2. RCA 50 SF embedded in epoxy resin with a polished surface. (a), (b) and (c) “areas of 6 of 19 interest" for study in IA-SEM. The IA-SEM used a JEOL JSM-6510 microscope (Jeol Ltd., Tokyo, Japan) for each image obtained The IA-SEM used a JEOL JSM-6510 microscope (Jeol Ltd., Tokyo, Japan) for each image obtained from the study. The images were taken with (1) high-resolution detector of secondary electrons from the study. The images were taken with (1) high-resolution detector of secondary electrons SEI SEI(secondary (secondaryelectron electronimage) image)forforhigh-resolution high-resolutionimages; images;(2)(2)backscattered backscatteredelectron electrondetector detectorBEI BEI (backscattered electron image) with lower image resolution but greater contrast (backscattered electron image) with lower image resolution but greater contrast for examining the for examining the surface topography. The zooms used for taking the images were: 70× for the general surface topography. The zooms used for taking the images were: 70× for the general composition of composition of the matrix, 200× for the matrix, 200×thefor areas of interest the areas and 500× of interest for mapping. and 500× TheThe for mapping. mapping mapping involved involved determining determining the chemical composition of the samples by detecting basic elements through microanalysis, the chemical composition of the samples by detecting basic elements through microanalysis, which which was done with andone was energy withdispersive an energyspectrometer (EDS) (Inca (EDS) dispersive spectrometer 200, Oxford Instruments, (Inca 200, Abingdon-on-Thames, Oxford Instruments, Abingdon- UK)on-Thames, connected UK) to anconnected SEM unit.toThe EDSunit. an SEM studyTherequired EDS study14 complete required 14scans for each complete image scans in BEI for each mode image in BEI mode to determine theto chemical determineelements the chemical elements present in thepresent in the samples samples chosen: Si, chosen: Al, Ca, Si, Mg, Al,K,Ca, C, Mg, K, C, Fe and Na Fe and Na (typical in the chemistry of concrete [55,56]). Figure 3 shows the locations (typical in the chemistry of concrete [55,56]). Figure 3 shows the locations of the chemical elements of the chemical elements present present in one of theinmicrographs. one of the micrographs. Si Al Ca 300 µm 300 µm 300 µm Mg K C 300 µm 300 µm 300 µm Fe Na 300 µm 300 µm Figure 3. Elemental Figure mapping 3. Elemental ofofthe mapping theRCA RCA50 50SF SFsample sample between thenatural between the naturalcoarse coarseaggregate aggregate (NCA) (NCA) and the new paste. and the new paste. 2.3.4. Image 2.3.4. Analysis Image Using Analysis UsingNINIVision VisionAssistant Assistant TheTheimage analysis image process analysis processwaswas carried outout carried using National using Instrument’s National Instrument’sNI NI Vision Assistant Vision Assistant2018 software (LabWindows/CVI 2018 software Version (LabWindows/CVI 7.1, Austin Version Texas, 7.1, Austin TX, TX, Texas, USA.), USA.),as this has as this proven has provenitself itselfto to be a useful image a useful processing image tooltool processing in industry in industryand among and amongthe thescientific scientific community [57].This community [57]. Thissoftware’s software’s fullfull set set of processing functions of processing functionsand anddigital digitalimage imagecapture capture improves projectefficiency improves project efficiencybybyreducing reducing the the programming effort and producing better results in less time [58]. Among this software’s functions are the study of particles, pattern recognition, detection of edges and barely visible colors, calculus of segmentation and frequency histograms [59]. Image analysis involves examining an image in detail, Materials 2019, 12, 2201 7 of 19 separating the components in order to know their characteristics and qualities and obtain results. As this software allows separate images of the PC components to be captured, it was possible to analyze Materials 2019, 12, x FOR PEER REVIEW 7 of 20 the porosity and calculate the amount of anhydrous material and hydration products contained in the PC. Using operations programming integrated effort into the and producing software better (addition, results in subtraction, less time [58]. Among this erosion, aggregation, software’s functions etc.) it was possible to create the SCRIP (batch processing file or command file), which was used are the study of particles, pattern recognition, detection of edges and barely visible colors, calculus in treating of segmentation the images, creating the andmasks frequency for histograms [59]. Image the aggregate and theanalysis paste.involves examining an image in detail, separating the components in order to know their characteristics and qualities and obtain results. As Making this the software Masks allows separate images of the PC components to be captured, it was possible to analyze the porosity and calculate the amount of anhydrous material and hydration products contained in Thethe procedure PC. Using of carrying operations out an image integrated into theanalysis is the result software (addition, of applying subtraction, complex erosion, numerical aggregation, algorithms etc.)that process it was possiblethe to incidences in the create the SCRIP pixels (batch of an image. processing This wasfile), file or command performed which wasbyused the software, in treating theeach which controlled images, creating the numerical masks forin operation the anaggregate and the paste. exact, balanced and precise manner for the specific area of interest of each image. Making the Masks The applied image analysis procedure began with the calibration of the image’s dimensions and The procedure of carrying out an image analysis is the result of applying complex numerical the relationship between this and its pixels. Then, as the first step in the analysis, the aggregate mask algorithms that process the incidences in the pixels of an image. This was performed by the software, was made. As this was composed mainly of Si, K, Na and Al (Figure 3), the “operating instructions” of which controlled each numerical operation in an exact, balanced and precise manner for the specific NI Vision area of interest(addition, Assistant of each image. subtraction, multiplication and division, etc.) (Figure 4a) were used to form an outline image. The applied imageSubsequently, analysis procedure morphological began with the filtering calibrationwas of theapplied. This is a and image’s dimensions numerical the relationship process based between this on the definition of and the its pixels. Then,of connectivity aspixels. the first Each step inpixel the analysis, may bethe aggregate horizontally, connected mask verticallywasormade. As this was diagonally withcomposed mainly (these its neighbors of Si, K,morphological Na and Al (Figure 3), themay filters “operating be basedinstructions” on a matrix of of NI Vision Assistant (addition, subtraction, multiplication and division, etc.) (Figure 4a) were used pixels with a pre-established structure, e.g., hexagonal, square, etc.). Therefore, in a morphological filter, to form an outline image. Subsequently, morphological filtering was applied. This is a numerical it is possible process tobased defineon several versions the definition of the in order to of connectivity surround pixels. Eachthepixel pixelmay tobebeconnected studiedhorizontally, [29]. This means that the vertically process or has diagonally with its neighbors (these morphological filters may be based on abetter sufficient ‘sensitivity’ to allow the applied procedure to adjust matrix to of reality, thereby pixels preventing with a any significantstructure, pre-established alteration e.g.,tohexagonal, the original image. square, etc.). Therefore, in a morphological filter, it is possible In accordance with the to define resultseveral of the versions operating in order to surround instructions, thethe pixel to be studied morphological filters[29]. This to the applied means that the process has sufficient ‘sensitivity’ to allow the applied images were as follows: (a) filtering (noise eradication), (b) opening of objects (eroding and dilating procedure to adjust better to the reality, thereby preventing any significant alteration to the original image. image—always with the same structural element and permitting the image to keep its original size), In accordance with the result of the operating instructions, the morphological filters applied to (c) elimination the images of small were as objects follows:(removing (a) filtering those (noise found inside larger eradication), ones)ofand (b) opening (d) (eroding objects filling and andclosing objects (joining dilating theandimage—always harmonizingwith elements). the same The image structural of theand element aggregate permitting (aggregate the image mask, to keepFigure its 4b) was established from original size), (c)the results of elimination this group of small of filters. those found inside larger ones) and (d) filling objects (removing and the Using closing sameobjects (joining analysis and harmonizing procedure elements). (operating The imageand instructions of the aggregate (aggregate morphological mask, but on filtering), Figure 4b) was established from the results of this group of filters. this occasion with the images obtained from SEM-EDS which define the basic compounds of Ca, C, Using the same analysis procedure (operating instructions and morphological filtering), but on Fe and Mg (incidences present in the paste), it was possible to obtain the masks that outline the paste, this occasion with the images obtained from SEM-EDS which define the basic compounds of Ca, C, as shown Fe in and Figure 5. Finally, Mg (incidences for the present samples in the paste), itwith RCA three was possible masks to obtain were that the masks made fromthethe outline aggregate paste, and the as paste shown in in order Figure to5.evaluate Finally, for the theporosity samples with in different RCA threeareasmasksofwerethe made ITZ (a)fromnew aggregate–new the aggregate paste, (b)andoldthe paste in order topaste aggregate–new evaluateand thec)porosity in different areas old aggregate–old of the ITZ (a) new aggregate–new paste. paste, (b) old aggregate–new paste and c) old aggregate–old paste. Figure 4. Preparation of the aggregate mask for the RCA 50 C sample (a) image after the operating instructions and (b) after the morphological filters. Materials 2019, 12, x FOR PEER REVIEW 8 of 20 Materials 2019, Figure 12, 22014. Preparation of the aggregate mask for the RCA 50 C sample (a) image after the operating 8 of 19 Materials 2019, 12, x FOR PEER REVIEW 8 of 20 instructions and (b) after the morphological filters. Figure 4. Preparation of the aggregate mask for the RCA 50 C sample (a) image after the operating instructions and (b) after the morphological filters. Elaboration Figure 5.Figure of theofmask 5. Elaboration of the the mask paste of the pasteforforthe the sample RCA sample RCA 5050 C (a) C (a) image image after after the operating the operating instructions and (b)and instructions after (b) the aftermorphological the morphological filters. filters. Figure 5. Elaboration of the mask of the paste for the sample RCA 50 C (a) image after the operating The criterion The criterion of segregation of segregation or “threshold” or morphological “threshold” was was appliedto applied to the the initial initialimages images of the study of the in order study in order to instructions and (b) after the filters. to know the values of porosity; this allowed sectioning of a specific range of tones in the gray scale— know the values of porosity; this allowed sectioning of a specific range of tones in the gray scale—which which Thecan have values criterion of between of segregation or0“threshold” and 255—while ensuringtothat was applied this therange range initial of values images of the was studydependent in order on the can have values of between 0 and 255—while ensuring that this of values was dependent on the materials. A histogram was used to determine the range of application to know the values of porosity; this allowed sectioning of a specific range of tones in the gray scale— (a pixel-frequency materials. A histogram graph vs the value was on of used the to determine grayscale), encompassingthe range of application the pixels included (a pixel-frequency in the rangewas of values fromgraph vs which can have values between 0 and 255—while ensuring that this range of values dependent the value theon on the the initial grayscale), (equal to materials. A zero) encompassing to the was histogram decrease used in the tothe pixels last ofincluded determine the thehistogram in the range of(avalues range of frequency applicationcurves. Thefrom rangethe pixel-frequency of initial pixels (equal to from 0 to 75 (threshold) was chosen for the NCA samples; from 0 to 50 graph vs the value on the grayscale), encompassing the pixels included in the range of values from pixels zero) to the decrease in the last of the histogram frequency curves. was chosen The for range the of elements from 0 to the75initial with (threshold) (equalRCA and SF. to was zero) Therefore, chosen to the allin for the decrease the NCA image the areas the containing samples; last of from 0 to histogram pixels within 50 was frequency theseThe chosen curves. ranges forrange thewereelements of considered pixels pores or gaps. Figure 6 shows the final binarized image (segmentation of pixels in only with RCA andfrom 0 to 75 (threshold) SF. Therefore, all the was image chosen areasforcontaining the NCA samples; from 0 these pixels within to 50 was rangeschosen were forconsidered the two tones: black andand white) of this process. pores orelements with gaps. Figure RCA6 shows SF. the Therefore, all the imageimage final binarized areas containing (segmentation pixels within thesein of pixels ranges onlyweretwo tones: considered pores or gaps. Figure 6 shows the final binarized image (segmentation of pixels in only black and white) of this process. two tones: black and white) of this process. Figure 6. The total porosity of the image (a) initial image and (b) segmentation and binarization of image after application of threshold segregation. The 6. Figure 6.Figure total The porosity of the total porosity image of the (a)(a)initial image initial image and(b)(b) image and segmentation segmentation and binarization and binarization of of The multiplication operation with the coarse aggregate mask and the paste mask was applied to image image after after application application of threshold of threshold segregation. segregation. the binarized image, thereby obtaining the images that allowed the porosity identified in the aggregate (Figure 7a) and The multiplication in the paste operation with (Figure the 7b) coarse to be observed aggregate mask and and quantified. the paste maskAdditionally, was applied by to Theapplying multiplication a operation threshold (range with 225–255) the the coarse elements aggregate that did mask not react and the chemically paste mask (anhydrous was applied material) to the binarized image, thereby obtaining the images that allowed the porosity identified in the the binarized image, thereby obtaining the images that allowed the porosity identified in the aggregate aggregate (Figure 7a) and in the paste (Figure 7b) to be observed and quantified. Additionally, by (Figure applying 7a) and ainthreshold the paste (Figure (range 7b)the 225–255) toelements be observed that didand quantified. not react chemicallyAdditionally, by applying (anhydrous material) a threshold (range 225–255) the elements that did not react chemically (anhydrous material) could be identified; in the (SEM-EDS) mapping of elements, they are shown as white pixels with a single basic chemical element (Figure 7c). Finally, once the porosity and the anhydrous material had been identified it was possible to obtain the percentage of hydration products; this is the result of removing the previous sum of porosity and anhydrous material from the original image. Materials 2019, 12, x FOR PEER REVIEW 9 of 20 Materials 2019, 12, x FOR PEER REVIEW 9 of 20 could be could be identified; identified; in in the the (SEM-EDS) (SEM-EDS) mapping mapping of of elements, elements, they they are are shown shown asas white white pixels pixels with with aa single basic single basic chemical chemical element element (Figure (Figure 7c). 7c). Finally, Finally, once once the the porosity porosity and and the the anhydrous anhydrous material material had had Materials 2019, 12, 2201 9 of 19 been identified it was possible to obtain the percentage of hydration products; this is the been identified it was possible to obtain the percentage of hydration products; this is the result of result of removing the previous sum of porosity and anhydrous material from the original removing the previous sum of porosity and anhydrous material from the original image. image. Figure Figure 7. (a) (a) Porosity (a)7.Porosity Porosity ofaggregate, of the the the aggregate, (b) porosity (b) porosity porosity ofpaste of the the the paste and and (c) anhydrous (c) anhydrous anhydrous material. material. Figure 7. of aggregate, (b) of paste and (c) material. After After these these procedures procedures and and the the image image calibration, calibration, thethe sector sector inin the the scope scope of of the the ITZ ITZ was was built built After these procedures and the image calibration, the sector in the scope of the ITZ was built (theoretical ITZ (theoretical ITZ thickness ITZ thickness located thickness located between located between between the the aggregate the aggregate aggregate and and paste and paste throughout paste throughout throughout all all areas). all areas). areas). To To To dodo doso, (theoretical so, so, thethe sector sector previously previously established established as as thethe filter filter mask mask was was used used to to establish establish the the (theoretical) (theoretical) ITZ ITZ the sector previously established as the filter mask was used to establish the (theoretical) ITZ thicknesses thicknessesof of10, 10,20, 20,30, 30,40 40and and50 μm[14,27]. 50µm [14, 274]. 274].TheTheimage imageprocess process used to achieve used to achieve achievethisthis was “dilation was “dilationof thicknesses of 10, 20, 30, 40 and 50 μm [14, The image process used to this was “dilation objects”, applied of objects”, objects”, appliedto the applied to original to the image the original of original image the image of aggregate of the mask the aggregate and aggregate mask establishing mask and the various and establishing establishing the thicknesses the various various of within the reach thicknesses of the within theITZ reachas “desired” of the ITZ (measured as “desired” in pixels). (measuredIf the inpixel measurements pixels). If the pixel were known it measurements thicknesses within the reach of the ITZ as “desired” (measured in pixels). If the pixel measurements was werepossible known to it establish was possiblethe thickness possible to establishwith establish the the desired distance. thickness with the Finally, this the desired desired was overlaid distance. Finally, onto the this was was were known it was to the thickness with distance. Finally, this original binarized overlaid onto onto the image. the original The results original binarized of binarized image. the previous image. The process The results results of that of the determined the previous previous process the sectors process that of ITZ that determined can determined the be the overlaid seen in Figure sectors ITZ8.can of ITZ These can be were seen ultimately in Figure Figure 8. used 8. These Theseas a mask to establishused were ultimately ultimately the percentage mask(histogram as aa mask to establish establishof the the sectors of be seen in were used as to the image) of porosity percentage (histogram in (histogram ofthe ITZ. of the the image) image) ofof porosity porosity in in the the ITZ. ITZ. percentage Figure Figure 8. Images Figure 8. 8. Images resulting from the Images resulting the sectors from the sectors of sectors of the of the ITZ the ITZ with ITZ with the with the porosities the porosities determined porosities determined at determined at different at different different theoretical theoretical thicknesses. thicknesses. theoretical thicknesses. 3. Results and Discussion 3. Results 3. Results and and Discussion Discussion 3.1. Mechanical and Physical Properties of the PC 3.1. Mechanical 3.1. Mechanical and and Physical Physical Properties Properties of of the the PC PC The results of the tests are shown in Table 5, where it can be seen that the reference concrete (NCA 100 C) had the best mechanical properties in terms of compression and flexure (21.9 and 4.9 MPa respectively). It was also observed that the inclusion of RCA in PC causes a reduction in its mechanical Materials 2019, 12, 2201 10 of 19 properties of 31% and 30% (compression and flexure respectively) with regard to NCA 100 C. Finally, it was established that the PC with SF did not generate increases in the mechanical properties. This may be because the SF does not react with the CH generated during the cement hydration, as due to the high porosity of this type of cement they were dissolved in the curing water [60]. The permeability coefficients established that there was a connection between the mechanical properties studied. This can be explained by the frequent association of permeability and porosity with the mechanical properties of concrete [61]. The PC (NCA 100 C) showed the lowest permeability coefficient in comparison with the other variables. This indicates that it has lower porosity and is, therefore, more capable of attaining better mechanical properties. Table 5. Properties of PC. Water Permeability Study Variables Compression Strength (MPa) Flexural Strength (MPa) Coefficient (m/s) NCA 100 C 21.9 4.9 8.9 × 10−03 NCA 100 SF 20.8 4.4 1.1 × 10−02 RCA 50 C 15.5 3.6 1.0 × 10−02 RCA 50 SF 15.1 3.4 1.1 × 10−02 3.2. Mechanical and Physical Properties of the PC Figure 9a shows the results obtained from the MIP tests in the enrichment phase (positive intrusion). The curves or isotherms of this graph show that their form is similar in all cases, their path being as follows: starting with slight increases from a common origin (the theoretical diameter of maximum pores ϕp max = 500,000 nm, macropores [62,63]), they continue to an abrupt inflexion point, located in the range of critical pore diameter 60 ≤ ϕpc ≤ 140 nm. Finally, they continue to an accelerated increase in the intrusion volume, which causes a steep rise until the points of maximum intrusion value are reached, minimum pore diameter ϕp min in the mesopore zone. The PC sample—NCA 100 C—establishes the most representative pore samples as 7 ≤ ϕp ≤ 110 nm, with its inflexion point located in Vp = 0.01 mL/g. This means that a lower amount of Hg is needed to fill the large diameter (100 ≤ ϕp ≤ 400,000 nm) pores. In contrast, the NCA 100 SF sample shows fewer pores, which are significant in the size range of 6 ≤ ϕp ≤ 90 nm; the reason being the insertion of SF, as this is made up of small particles and may be used as a ‘filler’ for pores in the microstructure of concretes [64]. The RCA 50 C version establishes the highest number of pores of all the samples analyzed—set at 30 ≤ ϕp ≤ 200 nm—due to the inclusion of the RA; as this material is classified as low density and high porosity [1] the Hg can pass easily through its porous structure. Finally, it can be seen that adding SF to a sample of PC with RCA (RCA 50 SF) causes a reduction in the number of pores of 6 ≤ ϕp ≤ 80 nm; the cause is similar to that of the NCA 100 SF, although on this occasion with less importance (porous RCA). To sum up, the order of the increase in porosity of PC with NCA was as follows: first NCA 100 SF and then NCA 100 C; regarding the PC with RA, first RCA 50 SF and finally RCA 50 C. The second group is affected by the RA with high original porosity. In both groups, the inclusion of SF (filling effect) is connected to the reduction of porosity. The extrusion curves in Figure 9b help to interpret the morphology of the “trapped porosity”, identified when the intrusion–extrusion gaps were compared (a hysteresis phenomenon or the ability of the material to keep to the initial intrusion curve line as the Hg pressure returns to its initial state in the test). The difference between both curves established the quantity of Hg retained in the sample, a consequence of the number of bottleneck pores in the system [65,66]. The extrusion curves shown in the test samples have similar lines and the same relative positioning on the intrusion curves. Table 6 shows the values of trapped porosity (Pa ) obtained after determining the difference in volumes between both curves. It can be seen that the samples with RCA have twice as many bottleneck pores as the NCA samples. This is understandable if we consider that the RCA has open pores which are Materials 2019, 12, 2201 11 of 19 Materials 2019, 12, x FOR PEER REVIEW 11 of 20 between both curves. It can be seen that the samples with RCA have twice as many bottleneck pores partially blocked as the NCAby the new samples. This CP cement paste is understandable and if we that that consider thethe useRCA of has C or SFpores open is not significant which are in either sample group. partially blocked by the new CP cement paste and that the use of C or SF is not significant in either sample group. Intruded Volume Vp (mL/g) 0.06 (a) 0.06 (b) Extruded volume Vp (mL/g) NCA 100 C NCA 100 C 0.05 NCA 100 SF NCA 100 SF 0.05 RCA 50 C RCA 50 C 0.04 RCA 50 SF RCA 50 SF 0.04 0.03 0.02 0.03 0.01 0.02 0.00 0.01 10 100 1000 10000 100000 1000000 10 100 1000 10000 Pore diameter (nm) Pore diameter (nm) FigureFigure 9. Mercuryintrusion 9. Mercury intrusion porosimetry, porosimetry,(a) intrusion and (b) extrusion. (a) intrusion and (b) extrusion. Table 7 and Table 7 show the values of the porous network indicators determined according to Tables 6 and 7 show the values of the porous network indicators determined according to the the previously established procedures [51,67,68]: maximum pore radius (rmax), minimum radius (rmin), previously established critical radius (rc), procedures average radius [51,67,68]: (rm), total poremaximum pore radius area (Atp), intruded (rmaxvolume and extruded ), minimum (Vi y Ve) radius (rmin ), critical radius and Pa.(rVi c ),= Volume average radiusVe(r=mVolume intruded, ), totalextruded. pore area (Atp ), intruded and extruded volume (Vi y Ve ) and Pa . Vi = Volume intruded, Ve = Volume extruded. Table 6. Porous network indicators in Hg extruded stage. Sample Table 6. Porous rmax (nm) rminnetwork (nm) rindicators c (nm) rin Hg extruded m (nm) stage. Pa (mL/g) Ve (mL/g) NCA 100 C 6708.05 3.4 273.50 90.918 0.0269 0.0154 Sample NCA 100 SF rmax6730.10 (nm) rmin3.4(nm) 121.25 rc (nm) 64.522 rm (nm) 0.0208 Ve (mL/g)0.0144Pa (mL/g) NCARCA 10050 CC 6702.55 6708.05 3.4 3.4 273.50 273.50 90.65090.918 0.0296 0.0269 0.0299 0.0154 RCA 50 SF 6637.60 3.4 273.50 150.591 0.0236 0.0246 NCA 100 SF 6730.10 3.4 121.25 64.522 0.0208 0.0144 RCA 50 C 6702.55 3.4 273.50 90.650 0.0296 0.0299 RCA 50 SF 6637.60 3.4 273.50 150.591 Table 7. Porous network indicators in Hg intruded stage. 0.0236 0.0246 Sample rmax (nm) rmin (nm) rc (nm) rm (nm) Vi (mL/g) Atp (mL/g) NCA 100 C Table 7. Porous3.400 244,514.75 network 110.700 indicators 28.333 in Hg intruded 0.0424stage. 4.15 × 1002 NCA 100 SF 246,402.35 3.400 57.600 18.522 0.0351 4.26 × 1002 Sample RCA 50 C rmax (nm) 242,670.09 rmin (nm) 3.378 rc (nm) 39.307 136.691 rm (nm) 0.0595 Vi (mL/g) A02tp (mL/g) 3.77 × 10 RCA 50 SF 245,620.64 3.400 137.000 29.588 0.0482 7.41 × 1002 02 NCA 100 C 244,514.75 3.400 110.700 28.333 0.0424 4.15 × 10 NCA Regarding 100 SF the 246,402.35 3.400 rmax in the intrusion 57.600 radius18.522 stage, the average established in 0.0351 4.26 × 1002 the study samples (244,802 RCA 50nm) C highlights the ability of3.378 242,670.09 the PC to cause large pores—up 136.691 to 40 times0.0595 39.307 3.77 × 1002 bigger than those noted in RCA 50 SF the usual recycled 245,620.64 concretes [51,69]. 3.400 However, in 137.000 the extrusion 29.588stage, the average 0.0482 (6695 nm) × 1002 7.41 is similar to the usual recycled concretes in their intrusion stage. In terms of the varieties studied, the order of radii variation between them is not considered important (less than 1% in both stages of the test). In both Regarding the stages rmax of inthe test, the and all thestage, intrusion varietiesthe studied, r min is considered average a constant (equal radius established to 3.4 in the study samples nm). This raises two points for consideration: the MIP technique may be able to establish the network (244,802 nm) highlights the ability of the PC to cause large pores—up to of pores in the micropore zone, and this minimum value probably has more in common with the 40 times bigger than those noted in the usual recycled concretes [51,69]. However, in the extrusion limits or parameters of experimental techniques than with the varieties of this research. stage, the average (6695 nm) The average values attained for the varieties studied in the case of r were 110.5 is similar to the usual recycled concretes in their intrusion stage. In terms of the varieties studied, c and 235.4 nm (intrusion and extrusion respectively). However, unlike the previous indicators (in the intrusion stage) the order of radii variation between them is not considered important (less than 1% in both stages of the test). In both stages of the test, and all the varieties studied, rmin is considered a constant (equal to 3.4 nm). This raises two points for consideration: the MIP technique may be able to establish the network of pores in the micropore zone, and this minimum value probably has more in common with the limits or parameters of experimental techniques than with the varieties of this research. The average values attained for the varieties studied in the case of rc were 110.5 and 235.4 nm (intrusion and extrusion respectively). However, unlike the previous indicators (in the intrusion stage) the NCA 100 SF sample established an rc some 50% less than that of the NCA 100 C, which shows that the use of SF is able to shift the rc toward lower rc values (closing the porous network). With regard to the RCA 50 C and RCA 50 SF versions (intrusion stage), the rc variation between them is not important (a percentage of 50% SF is not important, the change of effect in rc requires a high percentage of SF to Materials 2019, 12, 2201 12 of 19 be noticeable). Finally, the effect of the type of aggregate establishes that the use of RCA regarding NCA (intrusion stage) causes the former to establish a greater rc than the latter (about 60% more), a result of the high porosity of the RCA itself. Concerning the extrusion stage, the previous trends have the same application, although the values established are greater than in the intrusion stage. Regarding the indicators of rm , Vi,e , and Atp, the values shown in both stages of the study have, in percentage terms, similar interpretations to those established for rc ; it can be seen that these are the most appropriate and sensitive for establishing the correlation between the study varieties and the porous network produced in those PCs. 3.3. Porosity, Anhydrous Material and Hydration Products in Cement Paste by mMeans of IA-SEM After performing an IA-SEM on the study samples, it was possible to obtain the percentages of porosity, anhydrous material and hydration products (PH) (see Figure 10). Three specifically different zones of interest were also evaluated for the RCA 50 C and RCA 50 SF samples: (a) ITZ between the new aggregate and the new paste, (b) ITZ between the old aggregate and the old paste and (c) ITZ between the old paste and the new paste. As for porosity, and by comparing the samples that analyze the ITZ, it can be seen that this is higher in the samples containing 100% CP (possible paralysis of the hydration process); in the case of SF it is prolonged with age reducing the porosity, or is favored by the filling effect of pores which it causes. Among the zones of interest, b) (ITZ between old aggregate and old paste) establishes the greatest porosity (the variety linked to the less dense and more porous RCA concrete). If the use of RCA in the PC is eliminated (NCA 100 C and NCA 100 SF), it can be seen that there are minor variations of porosity between them, with the former being the best (in this case the filling effect of pores by SF is not favored, as the use of NCA does not include them). With regard to the anhydrous material and the PH, the use of SF establishes low percentages of anhydrous material and high ones of PH in the samples that contain RCA (the SF reacts with the CH present in the cement hydration to form more SCH). The sample RCA 50 SF is in the zone of interest b), the best of all (possibly due to the prolonged hydration over time of the old concrete from which the RCA originated). For the samples that use NCA, the use of C in the PC increases the formation of PH (the use of SF does not favor the creation of more PH, due to the possible effects of a blockage in hydration caused by a lack of pores or spaces wherein crystals may form). Materials 2019, 12, x FOR PEER REVIEW 13 of 20 100 % of pores % of anhydrous 80 % PH Percentage (%) 60 40 20 0 NCA 100 C NCA 100 SF RCA 50 C RCA 50 C RCA 50 C RCA 50 SF RCA 50 SF RCA 50 SF (a) (b) (c) (a) (b) (c) Samples Figure 10.Figure 10. Porosity, Porosity, anhydrous anhydrous material and pH material and pHof of the the cement paste in cement the PC. paste in the PC. 3.4. Evaluation of Porosity at Different ITZ Thicknesses by Means of IA of SEM Figure 11 shows the number of pores (unitary accumulated porosity, µ m) determined in the study samples. It also considers the different theoretically possible thicknesses of the aggregate ITZ (ITZ of 10, 20, 30, 40 and 50 μm), which allows the transition from the porous network of the PC to be verified. It can be seen that there is a direct connection in most of the samples between the increase in the percentage of porosity and the increase in the theoretical ITZ thickness. The theoretical thickness is 10 μm, which establishes the best percentage of porosity compared with the other thicknesses evaluated; this confirms that the percentage of porosity of the ITZ decreases as its theoretical thickness increases. Another way to understand the curves is that the ITZ has finished Materials 2019, 12, 2201 13 of 19 3.4. Evaluation of Porosity at Different ITZ Thicknesses by Means of IA of SEM Figure 11 shows the number of pores (unitary accumulated porosity, µm) determined in the study samples. It also considers the different theoretically possible thicknesses of the aggregate ITZ (ITZ of 10, 20, 30, 40 and 50 µm), which allows the transition from the porous network of the PC to be verified. It can be seen that there is a direct connection in most of the samples between the increase in the percentage of porosity and the increase in the theoretical ITZ thickness. The theoretical thickness is 10 µm, which establishes the best percentage of porosity compared with the other thicknesses evaluated; this confirms that the percentage of porosity of the ITZ decreases as its theoretical thickness increases. Another way to understand the curves is that the ITZ has finished when the curves of the samples tend toward a horizontal asymptote, with a constant—or non-increasing—percentage of porosity equal to that of the paste zone. Significantly, the three evaluated samples which had the lowest porosity in the ITZ are those with 50% of RCA and SF; this confirms that the use of SF leads to the filling effect in the ITZ porosity, making it denser and stronger. The RCA 50 C (a) sample shows the highest percentage of unitary porosity when the theoretical thickness is set at 10 µm (0.0216%/µm); at 50 µm reaches 0.032%/µm (the SEM image analyzed showed that this sample contained considerable porosity). This may have been due to the more porous nature of the RCA. Similarly, the effects of using SF were also observed in the NCA 100 SF, which had lower porosity than the NCA 100 C sample. Therefore, it can be stated that the use of RCA in making a PC will cause increases in the ITZ Materials 2019, 12, x FOR PEER REVIEW 14 of 20 porosity. However, the addition of SF may compensate for this, and achieve a denser cement matrix. Unitary accumulated porosity (%/µm) NCA 100 C 0.035 RCA 100 SF RCA 50 C (a) RCA 50 C (b) 0.030 RCA 50 C (c) RCA 50 SF (a) RCA 50 SF (b) RCA 50 SF (c) 0.025 0.020 0.015 0.010 0.005 0.000 10 20 30 40 50 Thickness of the ITZ (µm) Figure11. Figure Theporosity 11.The porosityof ofthe thesamples samplesanalyzed analyzedin inthe theITZ ITZwith withseveral severaltheoretical theoretical thicknesses. thicknesses. Figure 12 Figure 12shows showsthethe percentage of unitary percentage accumulated of unitary anhydrous accumulated materialmaterial anhydrous at different at theoretical different ITZ thicknesses in the PC samples studied. The trend shows a direct, rising, connection theoretical ITZ thicknesses in the PC samples studied. The trend shows a direct, rising, connection between the increase between of this parameter the increase and the and of this parameter increase in the theoretical the increase ITZ thickness. in the theoretical It should ITZ thickness. be noted It should be that the samples with SF have lower percentages of anhydrous material. The SF can noted that the samples with SF have lower percentages of anhydrous material. The SF can react react with the with cement the hydration cement products hydration and form products new ones, and form new which show low ones, which show percentages of non-reacting low percentages material. of non-reacting The RCA The material. 50 C RCA sample50(b), has the most C sample anhydrous (b), has the mostmaterial: at 10material: anhydrous µm an important at 10 μm concentration an important of concentration of 0.0062%/µ m can be seen, reaching 0.0106%/µ m when the theoretical thickness is 50 μm. Additionally, comparing NCA 100 C and NCA 100 SF, the former is shown to contain a far greater percentage of anhydrous material at all theoretical ITZ thicknesses than the latter. Therefore, to sum up, it can be said that the use of SF contributes to the formation of more hydration products in the cement paste; this material can fill pores, thereby leading to lower percentages of anhydrous Materials 2019, 12, 2201 14 of 19 0.0062%/µm can be seen, reaching 0.0106%/µm when the theoretical thickness is 50 µm. Additionally, comparing NCA 100 C and NCA 100 SF, the former is shown to contain a far greater percentage of anhydrous material at all theoretical ITZ thicknesses than the latter. Therefore, to sum up, it can be said that the use of SF contributes to the formation of more hydration products in the cement paste; Materials 2019, 12, x FOR PEER REVIEW 15 of 20 this material can fill pores, thereby leading to lower percentages of anhydrous material. NCA 100 C Unitary accumulated anhydrous 0.010 NCA 100 SF RCA 50 C (a) RCA 50 C (b) RCA 50 C (c) RCA 50 SF (a) 0.008 RCA 50 SF (b) material (%/µm) RCA 50 SF (c) 0.006 0.004 0.002 0.000 10 20 30 40 50 Thickness of the ITZ (µm) Figure12. Figure 12. Anhydrous Anhydrousmaterial materialof ofthe thesamples samplesanalyzed analyzedin inthe theIZT IZTwith withvarious varioustheoretical theoreticalthicknesses. thicknesses. The ThePHPH(the (theresult resultofofsubtracting subtractingthethe porosity and porosity andthethe anhydrous anhydrous material fromfrom material the paste) that that the paste) can be detected can in thein be detected ITZ studied the usuallyusually ITZ studied has a low hasvalue, a lowasvalue, porosity dominates as porosity in an ITZ.in dominates Figure 13 shows an ITZ. Figure the percentage 13 shows of unitary pH the percentage at different of unitary pH at theoretical different ITZ thicknesses theoretical (%/µm). The ITZ thicknesses RCA (%/µ m).50The C samples RCA 50 (a C and b) have samples the highest (a and b) havepercentage the highestofpercentage PH, whichof corroborates PH, which previous paragraphs. corroborates previousThis sample paragraphs. showed low percentages This sample showed low of porosity and percentages anhydrous of porosity andmaterial. anhydrous Thematerial. NCA 100 TheC has NCA a high 100 Camount of has a high PH and shows a significant increase at theoretical ITZ thicknesses of between 40 and amount of PH and shows a significant increase at theoretical ITZ thicknesses of between 40 and 50 50 µm. μm. The samples with SF (NCA 100 SF, RCA 50 SF (a and b)) have the lowest amounts of pH and correlate The the results samples of the with mechanical SF (NCA 100 SF,properties RCA 50 SF (they areb)) (a and thehave leastthe resistant). Their mechanical lowest amounts of pH and properties were correlate the low compared results with the samples of the mechanical made properties fromare (they Portland. the least resistant). Their mechanical properties were low compared with the samples made from Portland. Materials 2019, 12, 2201 15 of 19 Materials 2019, 12, x FOR PEER REVIEW 16 of 20 0.0075 Unitary accumulated PH (%/µm) NCA 100 C NCA 100 SF 0.0060 RCA 50 C (a) RCA 50 C (b) RCA 50 C (c) RCA 50 SF (a) 0.0045 RCA 50 SF (b) RCA 50 SF (c) 0.0030 0.0015 0.0000 10 20 30 40 50 Thickness of ITZ (µm) Figure13. Figure Hydrationproducts 13.Hydration productsof ofthe thesamples samplesanalyzed analyzed in in the the IZT IZTwith with various various theoretical theoretical thicknesses. thicknesses. 4. Conclusions 4. Conclusions The physical properties evaluated show that using RA in the PC leads to a reduction in mechanical The physical properties evaluated show that using RA in the PC leads to a reduction in properties when compared to the reference PC (32%, 30% and 6% for compression, flexure and mechanical properties when compared to the reference PC (32%, 30% and 6% for compression, permeability, respectively), as these old aggregates are nearing the end of their life cycle and have less flexure and permeability, respectively), as these old aggregates are nearing the end of their life cycle mechanical strength than natural aggregates. It is also shown that the use of SF in these concretes does and have less mechanical strength than natural aggregates. It is also shown that the use of SF in these not lead to an improvement in compressive strength, flexural strength or permeability, as they do not concretes does not lead to an improvement in compressive strength, flexural strength or permeability, react with the CH crystals (which dissolve due to the high porosity of this concrete). as they do not react with the CH crystals (which dissolve due to the high porosity of this concrete). By means of image analysis with the NI Vision Assistant software, it was possible to prepare By means of image analysis with the NI Vision Assistant software, it was possible to prepare a a script to make a mask of the aggregate and the cement paste. This was then used to analyze the script to make a mask of the aggregate and the cement paste. This was then used to analyze the percentage of porosity, the anhydrous material and the hydration products of the ITZ of several PC percentage of porosity, the anhydrous material and the hydration products of the ITZ of several PC samples. This analysis confirmed that the PC with RA increased the porosity by 32% at 50 µm ITZ samples. This analysis confirmed that the PC with RA increased the porosity by 32% at 50 µ m ITZ thickness between the new aggregate and the new paste. This is due to the RA being less dense, thickness between the new aggregate and the new paste. This is due to the RA being less dense, more more porous and higher water-absorbent. It also shows that the use of SF caused an 89% decrease porous and higher water-absorbent. It also shows that the use of SF caused an 89% decrease in in porosity, mainly in the reference concrete (at 50 µm ITZ thickness in the new aggregate-new paste porosity, mainly in the reference concrete (at 50 µ m ITZ thickness in the new aggregate-new paste area), acting as a filling material for microporosity. The above information has shown that porosity area), acting as a filling material for microporosity. The above information has shown that porosity increases when there is a thicker ITZ and that the most porous area of the PC is to be found between the increases when there is a thicker ITZ and that the most porous area of the PC is to be found between new aggregate and the new paste. This method is an option for analyzing the microporosity of many the new aggregate and the new paste. This method is an option for analyzing the microporosity of materials. The evidence of the image analysis was corroborated by mercury intrusion porosimetry. many materials. The evidence of the image analysis was corroborated by mercury intrusion The use of RCA in making permeable concrete leads to higher porosity in the cement paste and the ITZ; porosimetry. The use of RCA in making permeable concrete leads to higher porosity in the cement but the use of SF in the concrete also helps reduce the porosity, as it fills the gaps in the microstructure. paste and the ITZ; but the use of SF in the concrete also helps reduce the porosity, as it fills the gaps in 5. the microstructure. Future Research To find 5. Future a correlation of XRD with SEM to locate specific compounds and their relationship with Research mechanical behavior. To find a correlation of XRD with SEM to locate specific compounds and their relationship with mechanical behavior. Materials 2019, 12, 2201 16 of 19 To obtain a numerical quantification of the amount of old mortar adhered to the recycled aggregates and its connection with other mechanical and durability properties. Regarding the technique of image analysis, there are parameters that will be necessary to calibrate in the definition of these masks; among the most complex and therefore the most demanding future study due to its great diversity of application alternatives are morphological filtering and noise filtering. The use of scripts for image analysis can be used to measure porosities and failure sites (cracking) for a wide range of materials (ceramic, metallic and polymeric). Images can be obtained in different layers and at different depths of the sample and develop scripts so that the material can be considered statistically homogeneous (the results are representative of the sample volume). Author Contributions: Conceptualization, J.M.G.-S. and S.P.A.-R.; methodology, J.M.G.-S., R.C.-H. and S.P.A.-R.; validation, S.P.A.-R. and R.C.-H.; formal analysis, M.J.C.-C. and C.A.R.-C.; investigation, M.J.C.-C. and R.C.-H.; resources, J.M.G.-S. and R.C.-H.; data curation, C.A.R.-C. and M.J.C.-C.; writing—original draft preparation, M.J.C.-C. and J.M.G.-S.; writing—review and editing, J.M.G.-S. and M.J.C.-C.; supervision, S.P.A.-R. and C.A.R.-C. Funding: This research received no external funding Acknowledgments: The authors would like to thank CONACYT for its master and doctoral scholarship program, the Barcelona School of Building Construction-UPC, the Department of Architecture Technology-EPSEB-UPC and the Faculty of Engineering Mochis-UAS. Conflicts of Interest: The authors declare no conflict of interest. References 1. Zaetang, Y.; Sata, V.; Wongsa, A.; Chindaprasirt, P. 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