The Effect of Superabsorbent Polymers on the Microstructure and Self-Healing Properties of Cementitious-Based Composite Materials

Featured Application: Superabsorbent polymers of novel structure have been used in cementitious-based composite materials improving their self-healing behavior by an index of 60%. Abstract: Cementitious structures have prevailed worldwide and are expected to exhibit further growth in the future. Nevertheless, cement cracking is an issue that needs to be addressed in order to enhance structure durability and sustainability especially when exposed to aggressive environ-ments. The purpose of this work was to examine the impact of the Superabsorbent Polymers (SAPs) incorporation into cementitious composite materials (mortars) with respect to their structure (hybrid structure consisting of organic core—inorganic shell) and evaluate the microstructure and self-healing properties of the obtained mortars. The applied SAPs were tailored to maintain their functionality in the cementitious environment. Control and mortar/SAPs specimens with two different SAPs concentrations (1 and 2% bwoc) were molded and their mechanical properties were determined according to EN 196-1, while their microstructure and self-healing behavior were evaluated via microCT. Compressive strength, a key property for mortars, which often degrades with SAPs incorporation, in this work, practically remained intact for all specimens. This is coherent with the porosity reduction and the narrower range of pore size distribution for the mortar/SAPs specimens as determined via microCT. Moreover, the self-healing behavior of mortar-SAPs specimens was enhanced up to 60% compared to control specimens. Conclusively, the overall SAPs functionality in cementitious-based materials was optimized.


Introduction
Cementitious materials have been widely used over the years in the construction sector due to the cement abundance, its low cost and excellent durability [1,2]. In 2017 the global cement production came up to 4.1 billion tones. China and India which currently represent two of the most rapidly growing countries worldwide produced 57% and 7% of the global cement production in that year, while 6% of the same production is attributed to Europe [3]. It is estimated that the cement production will double in the decades to come [2,4].
Nevertheless, cement is prone to cracking as a result of both inherent material properties such as low tensile strength and application related factors such as tension inducement during the infrastructure service life thus compromising its integrity and durability [1,[5][6][7]. Aggressive environment conditions (i.e., wide ambient temperature fluctuation, rich presence of ions, pH levels etc.) combined with external loading favor crack propagation often leading to the formation of an interconnected crack network that allows corrosive factors to tion/desorption behavior, morphology (particles shape, size and size distribution) and dosage in the cementitious matrix, as well as water to cement (w/c) ratio and the incorporation procedure adopted [9,15,17,31,35]. In most cases SAPs absorption/desorption has been examined in extracted or synthetic solutions. Nevertheless, there have also been some studies on the in-situ evaluation of this behavior when SAPs are incorporated in a porous cementitious matrix. The results from these studies revealed that SAPs desorption when in contact with a porous, cementitious material, is effected by the bonding between SAPs particles and the cementitious matrix and is governed by diffusion between SAPs particles and capillary sorption in the matrix [36][37][38]. Consequently, it is imperative to investigate the correlation between SAPs used and the response of the cementitious system with respect to its mechanical properties and its microstructure.
The vast majority of commercial SAPs are copolymeric networks based on acrylic acid or acrylamide that may or may not have been partially neutralized [10,11,14,15,17,18,[20][21][22][23]31,35]. Because of the diversity of the parameters that have to be satisfied in the construction applications to enhance their internal curing action, SAP particles should preferentially have a homogeneous spherical shape, size in the submicron area so that the voids left behind after their deswelling are smaller and don't affect the structure mechanical properties and chemical affinity to the cement matrix so that they are more easily dispersed homogeneously in the cement paste [23].
The aim of this work was to examine the impact of the incorporation of tailored SAPs with respect to their structure (hybrid structure consisting of organic core-inorganic shell) on mortars mechanical strength in terms of flexural and compressive strength, microstructure and self-healing behavior. The synthesis and characterization of the incorporated SAPs have been thoroughly discussed in previous authors' work. More specifically, the SAP particles used were spherical in the submicron range based on poly (methacrylic acid) crosslinked with ethylene glycol dimethacrylate which were synthesized via radical polymerization and later encapsulated with CaO-SiO 2 inorganic shell via the sol-gel method [25,39]. The incorporation of SAPs in mortars was conducted in two different dosages, 1% and 2% by weight of cement (bwoc). The results obtained in this work revealed that the flexural strength improved by 3%, while the compressive strength remained practically intact for the mortar/SAP composite materials compared to the control specimens. Moreover, the total and closed porosity of the mortar/SAPs specimens were reduced by about 0.5% and 2.5% for mortar-SAPs-1 and mortar-SAPs-2, respectively, while self-healing behavior was enhanced for both SAPs concentrations (in the case of mortar-SAPs-1 by 60% and in the case of mortar-SAPs-2 by 10% compared with mortar-reference specimens).
The added value of this work resides in the optimization of the SAPs functionality in cementitious-based materials and the improvement of the cementitious materials selfhealing properties due to tailored SAPs structure, which are easy to fabricate via the combination of the sol-gel process, radical polymerization and the coprecipitation method. Finally, a new approach for the quantitative evaluation of mortars self-healing behavior was proposed.

Materials
Sand grade in accordance to CEN, EN 196-1 standard, cement CEM I 52.5 N and in-house synthesized SAPs were used to manufacture conventional mortar specimens. As mentioned earlier, the SAPs used were synthesized according to previous authors' work [39]. In Figure 1 the novel SAPs structure which consists of a hybrid organic core of poly(methacrylic acid) crosslinked with ethylene glycol dimethacrylate encapsulated with a composite inorganic shell of silicon-calcium oxide, P(MAA-co-EGDMA)@CaO-SiO 2 (Figure 1a,b) is shown. Their size ranges from 190 to 320 nm ( Figure 1c) while their maximum water absorbance ratio in cement slurry filtrate is determined 1100% their initial dry weight. Before their incorporation in mortars, SAPs are ground to fine powder form, in order to dismantle agglomerates and enhance their performance. poly(methacrylic acid) crosslinked with ethylene glycol dimethacrylate encapsulated with a composite inorganic shell of silicon-calcium oxide, P(MAA-co-EGDMA)@CaO-SiO2 (Figure 1a,b) is shown. Their size ranges from 190 to 320 nm ( Figure 1c) while their maximum water absorbance ratio in cement slurry filtrate is determined 1100% their initial dry weight. Before their incorporation in mortars, SAPs are ground to fine powder form, in order to dismantle agglomerates and enhance their performance.

Manufacture of Mortar/SAPs Specimens
Two specimen series of mortar/SAPs composites were manufactured, prismatic and cylindrical. Prismatic test specimens were fabricated according to the EN 196-1 [40] using a Teflon mold, while cylindrical specimens were using a medical disposable syringe [41][42][43]. The specimens were cast from a batch of mortar paste with a water/cement ratio (w/c) of 0.50. In accordance to literature review and previous authors' work on cement/SAPs composites [7,10,11,15,16,20,21,23,31,35,39], SAPs were incorporated in mortar in two different dosages, 1 and 2% by weight of cement (bwoc). The SAPs incorporation in the mortar paste was accomplished in succession to the sand fraction and the obtained mortar paste was mixed until the visually monitored dispersion was acceptable. In all cases mixing time needed was more than 30 s. A mortar mixer (MATEST-E094) was used for the dry mechanical mixing of the mixture components and a jolting apparatus (MATEST-E130) to compact the specimens. The molded samples were stored in the moist air room for 24 h and after demolding they were submerged horizontally in a suitable container at 20 °C for 28 days. The composition of the specimens is shown in Table 1. The flexural strength of mortar/SAPs composites was evaluated using prism specimens with dimensions 40 mm × 40 mm × 160 mm according to EN 196-1 using a universal testing machine of capacity 300 KN (Instron 300DX-B1-C4-G6C) [40]. For each SAPs dosage as well as for control samples, triplicates were manufactured, demolded and tested after 28 days curing in water. The prism halves from the flexural strength tests were used for the compressive strength tests, which were also performed according to EN 196-1.

Manufacture of Mortar/SAPs Specimens
Two specimen series of mortar/SAPs composites were manufactured, prismatic and cylindrical. Prismatic test specimens were fabricated according to the EN 196-1 [40] using a Teflon mold, while cylindrical specimens were using a medical disposable syringe [41][42][43]. The specimens were cast from a batch of mortar paste with a water/cement ratio (w/c) of 0.50. In accordance to literature review and previous authors' work on cement/SAPs composites [7,10,11,15,16,20,21,23,31,35,39], SAPs were incorporated in mortar in two different dosages, 1 and 2% by weight of cement (bwoc). The SAPs incorporation in the mortar paste was accomplished in succession to the sand fraction and the obtained mortar paste was mixed until the visually monitored dispersion was acceptable. In all cases mixing time needed was more than 30 s. A mortar mixer (MATEST-E094) was used for the dry mechanical mixing of the mixture components and a jolting apparatus (MATEST-E130) to compact the specimens. The molded samples were stored in the moist air room for 24 h and after demolding they were submerged horizontally in a suitable container at 20 • C for 28 days. The composition of the specimens is shown in Table 1. The flexural strength of mortar/SAPs composites was evaluated using prism specimens with dimensions 40 mm × 40 mm × 160 mm according to EN 196-1 using a universal testing machine of capacity 300 KN (Instron 300DX-B1-C4-G6C) [40]. For each SAPs dosage as well as for control samples, triplicates were manufactured, demolded and tested after 28 days curing in water. The prism halves from the flexural strength tests were used for the compressive strength tests, which were also performed according to EN 196-1.

Microstructure of Mortar/SAPs Specimens
X-ray micro computed tomography (microCT) was utilized to evaluate the mortar/SAPs composites microstructure. This technique is based on the correlation between X-rays absorption, material density and atomic number. High-density materials absorb X-rays more profoundly and produce light grey projection images. On the other hand, low-density materials are visualized as darker projection images. During the scanning process, angular projections of the specimen were acquired and saved. After the angular projections acquisition was completed, their reconstruction followed. The reconstruction process was executed by the "NRecon" visualization program via the implementation of Feldkamp algorithm. As a result, a 3D reconstructed model of the scanned specimen was produced. Moreover, quantitative parameters were determined using densiometry and morphometry evaluation, the latter based on image segmentation (black and white) which was done via a global threshold method, the Otsu method [44].
In this work, the specimens were scanned using SkyScan 1272 X-ray micro-tomograph at the age of 28 days. The specimens' geometry was chosen to be cylindrical; their diameter was 10 mm and their height ranged between 20 and 30 mm. In order to enhance contrast in microCT images and improve grey scale histogram segmentation iodine was utilized as a contrast agent. Therefore, all specimens were treated with a 3% iodine solution in ethanol prior to their scans. More specifically, at the age of 28 days, the cylindrical specimens were submerged in the iodine solution for 48 h and then, they were dried in an oven at 80 • C for 24 h. If the specimens were not immediately scanned, they were stored in a desiccator [45]. The acquisition settings of the scans are presented in Table 2. In addition, for each scan the flat field correction was applied. The reconstruction and the porosity analysis were performed using NRecon (version 1.6.6.0) and CTAnalyzer (version 1.13) softwares, respectively. The selected volumes of interest were 441, 105 and 350 mm 3 for mortar-reference, mortar-SAPs-1% and mortar-SAPs-2%, respectively. Moreover, the challenging issues of material phase segmentation and their quantitative analysis were addressed via 2D and 3D reconstructed images data, and more specifically via the grey scale intensity thresholds. In particular, even though microCT does not support chemical analysis, differences in brightness are recorded in the form of Grey Scale Histograms (GSH) due to density and atomic mass variations for different materials. As a result, phase segmentation depends on the thresholding method followed in each case [46]. In this work, GSH were obtained during the scan data processing via CTAnalyzer Software. The Grey Scale Values ranged from "0" to "255". The lower grey scale values Moreover, the challenging issues of material phase segmentation and their quantitative analysis were addressed via 2D and 3D reconstructed images data, and more specifically via the grey scale intensity thresholds. In particular, even though microCT does not support chemical analysis, differences in brightness are recorded in the form of Grey Scale Histograms (GSH) due to density and atomic mass variations for different materials. As a result, phase segmentation depends on the thresholding method followed in each case [46]. In this work, GSH were obtained during the scan data processing via CTAnalyzer Software. The Grey Scale Values ranged from "0" to "255". The lower grey scale values correspond to black color in reconstructed images and were attributed to the lack of material or pores. On the other hand, the higher values correspond to white and were attributed to unhydrated cement phases and/or SAPs aggregates, while the intermediary grey scale values are depicted as grey and were attributed to different hydrated cement phases [44,[46][47][48][49][50]. In this work, taking into consideration that all hydrated cement phases, mainly Calcium Silicate Hydrates(C-S-H) are expected to show peaks at similar grey scale values, a deconvolution procedure was engaged to identify and quantify the different material phases (pores, hydrated cement phases, unhydrated cement phases and SAPs) assuming that the grey scale values distribution for each phase is Gaussian. Then, a fitting model distribution, comprising a set of four (n = 4) Gauss distributions was numerically fitted to the GSH using an algorithm implemented in Software "Magic Plot" (student version 2.5.1).

Self-Healing Evaluation of Mortar/SAPs Specimens
Cementitious structures can be damaged in a variety of ways, the most common being cracking. MicroCT has been utilized as a laboratory-scale method to evaluate their damage extent and interpret healing mechanisms. In the past more conventional methods, such as SEM (Scanning Electron Microscopy), have been used to evaluate crack morphology and mitigation but they can provide insight only on the specimen surface, whereas microCT can provide useful information on the bulk of the specimen and consequently evaluate internal cracks and internal self-healing [46].
In this work, after 28 days of curing the cylindrical specimens that were previously examined via microCT were precracked under compressive load using a hydraulic press. During their compression, the load was applied in a controlled and smooth manner but the specimens were completely split in two halves in all cases. Therefore, prior to this procedure they were wrapped tightly in a polypropylene based film in order to avoid the complete separation of the two halves but instead to form a crack as shown in Figure 3. Afterwards, the specimen circumference and base were coated with an epoxy resin (a mixture of Sinmast J 158 (component A) and Sinmast S2 liquid primer (component B) by Sintecno in a ratio A:B 77:23) in order to secure the two halves together. The top surfaces of the cylinders were untreated, so that the formed crack could interact with healing agents (water). The epoxy resin was cured at ambient temperature for 1 day and then at 60 • C for 2 h.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 20 procedure they were wrapped tightly in a polypropylene based film in order to avoid the complete separation of the two halves but instead to form a crack as shown in Figure 3. Afterwards, the specimen circumference and base were coated with an epoxy resin (a mixture of Sinmast J 158 (component A) and Sinmast S2 liquid primer (component B) by Sintecno in a ratio A:B 77:23) in order to secure the two halves together. The top surfaces of the cylinders were untreated, so that the formed crack could interact with healing agents (water). The epoxy resin was cured at ambient temperature for 1 day and then at 60 °C for 2 h. In order to estimate their self-healing behavior, the cracked specimens were submerged in water at ambient temperature and they were evaluated at different time slots (0 and 8 days). The self-healing efficiency of mortar/SAPs composites was quantified and visualized via microCT analysis. Prior to microCT evaluation the specimens were dried at 50 °C for three days and were stored in a desiccator. The cracked mortar specimens were scanned using the SkyScan 1272 desktop microCT at 25.0 μm pixel resolution with 0.5 mm In order to estimate their self-healing behavior, the cracked specimens were submerged in water at ambient temperature and they were evaluated at different time slots (0 and 8 days). The self-healing efficiency of mortar/SAPs composites was quantified and visualized via microCT analysis. Prior to microCT evaluation the specimens were dried at 50 • C for three days and were stored in a desiccator. The cracked mortar specimens were scanned using the SkyScan 1272 desktop microCT at 25.0 µm pixel resolution with 0.5 mm aluminum filter. The scanned images were reconstructed via NRecon software, while threedimensional evaluation was conducted by CTAnalyzer software following a methodology proposed by Nicole Y.C Yu et al., appropriately adjusted for cracked mortar specimens [51]. The main outcomes discussed in this work were the evolution of connectivity density and the percent object volume versus healing time. These parameters were adopted because they allow the quantitative evaluation of crack healing in 3D (not only in 2D), with respect to crack closure in terms of changes in morphology and density and are embedded in the CTAnalyzer software.

Mechanical Properties of Mortar/SAPs Specimens
During the flexural strength test an abrupt, brittle failure was observed in mortars mainly due to deformation localization by the coalescence of narrow microcracks that ultimately lead to macrocracks that expand to the entire specimen [52]. The flexural strength evaluation of the mortar-based composites with SAPs in dosages 1 and 2% bwoc as well as control mortar specimens at the age of 28 days is demonstrated in Figure 4a  On the other hand, the compressive strength of the mortar based composite materials with the SAPs incorporation for the same mortar age (28 days) and w/c ratio (0.5) is depicted in Figure 5a,b. More specifically, Figure 5a depicts the load applied during the test as a function of the head displacement and Figure 5b shows the calculated compressive strength of the mortar based composite materials with the SAPs incorporation for the same mortar age (28 days) and w/c ratio (0.5). As shown in Figure 5b the compressive strength of the mortar-based composite materials practically remains intact for both SAPs dosages (1% and 2% bwoc) in comparison with the control materials. On the other hand, the compressive strength of the mortar based composite materials with the SAPs incorporation for the same mortar age (28 days) and w/c ratio (0.5) is depicted in Figure 5a,b. More specifically, Figure 5a depicts the load applied during the test as a function of the head displacement and Figure 5b shows the calculated compressive strength of the mortar based composite materials with the SAPs incorporation for the same mortar age (28 days) and w/c ratio (0.5). As shown in Figure 5b the compressive strength of the mortar-based composite materials practically remains intact for both SAPs dosages (1% and 2% bwoc) in comparison with the control materials. picted in Figure 5a,b. More specifically, Figure 5a depicts the load applied during the test as a function of the head displacement and Figure 5b shows the calculated compressive strength of the mortar based composite materials with the SAPs incorporation for the same mortar age (28 days) and w/c ratio (0.5). As shown in Figure 5b the compressive strength of the mortar-based composite materials practically remains intact for both SAPs dosages (1% and 2% bwoc) in comparison with the control materials.

Microstructure Analysis
In this work, the effect of SAPs incorporation on the mortar microstructure in means of porosity and phase segmentation was examined via microCT. In cementitious materials, porosity affects greatly key properties such as mechanical and transport properties [46]. Moreover, the dispersion quality of SAPs in the cementitious matrix can be evaluated through porosity determination. The porosity of cementitious materials is usually divided into gel pores (ranging from a few nanometers to 0.2 μm), capillary pores (ranging from 0.2 to 10 μm), and air voids (above 10 μm) [50,55,56]. In this work, the pore analysis from micro-CT applies only to the pores with diameters larger than 9 μm. The specimen size dictates the minimum distance that can be reached from the X-ray source during scanning and consequently the accuracy of the method. Therefore, the pore analysis from micro-CT scans includes partially capillary pores and the air voids. Air voids are mainly of interest  In this work, the effect of SAPs incorporation on the mortar microstructure in means of porosity and phase segmentation was examined via microCT. In cementitious materials, porosity affects greatly key properties such as mechanical and transport properties [46]. Moreover, the dispersion quality of SAPs in the cementitious matrix can be evaluated through porosity determination. The porosity of cementitious materials is usually divided into gel pores (ranging from a few nanometers to 0.2 µm), capillary pores (ranging from 0.2 to 10 µm), and air voids (above 10 µm) [50,55,56]. In this work, the pore analysis from micro-CT applies only to the pores with diameters larger than 9 µm. The specimen size dictates the minimum distance that can be reached from the X-ray source during scanning and consequently the accuracy of the method. Therefore, the pore analysis from micro-CT scans includes partially capillary pores and the air voids. Air voids are mainly of interest in this work, as in literature, it is reported that the incorporation of SAPs in cementitious matrixes creates air voids around them as a result of the water absorption/desorption by them combined with their poor dispersion in cement and therefore the formation of large SAPs agglomerates [9,10,15,21,31,35,38]. The statistical pore analysis of mortar specimens and the corresponding pore size distribution are exhibited in Table 3 and Figure 7, respectively. The statistical pore analysis parameters calculated are delineated as follows.

Image Segmentation and Phase Identification
There are a number of different approaches on the matter of image segmentation and phase identification via microCT Analysis. One that stands out as it is widely used is the global thresholding method in GSH. The GSH with the Gaussian deconvolution procedure performed for the mortars examined in this work, are given in Figure 8. • "Volume of closed porosity" (mm 3 ) is the total volume of all closed pores within the Volume of Interest (VOI), when a closed pore in 3D is a connected assemblage of space (black) voxels that is fully surrounded on all sides in 3D by solid (white) voxels. • "Closed porosity" (%) is the volume of closed pores (as defined above) as a percent of the total of solid plus closed pore volume, within the VOI. An open pore is defined as any space located within a solid object or between solid objects, which has any connection in 3D to the space outside the object or objects.
• "Open porosity" (%) is the volume of open pores (as defined above) as a percent of the total VOI volume • "Volume of total porosity" (mm 3 ) is the total volume of all open and closed pores within the VOI • "Total porosity" (%) is the volume of all open plus closed pores (as defined above) as a percent of the total VOI volume.

Image Segmentation and Phase Identification
There are a number of different approaches on the matter of image segmentation and phase identification via microCT Analysis. One that stands out as it is widely used is the global thresholding method in GSH. The GSH with the Gaussian deconvolution procedure performed for the mortars examined in this work, are given in Figure 8. The deconvolution of the initial GSH as described earlier, produced four distinct curves in each case, curves 1 to 4 and the corresponding peaks (Figure 8). Curve 1-Peak 1 were attributed to pores, curve 2 and curve 3 were assigned to hydrated cement phases and curve 4 was ascribed to unhydrated cement phases and SAPs. We observed that for all specimens the main peak in the GSH was deconvoluted in two separate peaks which The deconvolution of the initial GSH as described earlier, produced four distinct curves in each case, curves 1 to 4 and the corresponding peaks ( Figure 8). Curve 1-Peak 1 were attributed to pores, curve 2 and curve 3 were assigned to hydrated cement phases and curve 4 was ascribed to unhydrated cement phases and SAPs. We observed that for all specimens the main peak in the GSH was deconvoluted in two separate peaks which were attributed to different hydrated products probably owed to the main hydrated cement products which are Calcium Silicate Hydrates, C-S-H and Calcium Hydroxide, Ca(OH) 2 .
Then, using the GSH data, the quantitative determination of the different phases identified in the mortar specimens was performed and the corresponding results were tabulated in Table 4. Taking into consideration the data in Table 4, it is observed that in the case of mortar-SAPs-1 the unhydrated cement products together with the incorporated SAPs (1% bwoc) represented only the 5% of the total material when the corresponding values for both mortar-reference and mortar-SAPs-2 were 36%. This indicates that the progress of the hydration reactions for this material was remarkably enhanced compared to mortar-reference and mortar-SAPs-2, as a result of SAPs incorporation in the cementitious matrix. This is attributed to more effective SAPs dispersion in the cementitious matrix and therefore enhancement of their functionality to extend hydration reactions during cement curing, thus promoting this as the optimal SAPs concentration in mortars in respect to microstructure evaluation.

Self-Healing Evaluation
In Figure 9 the crack surface and the larger voids in respect to cracks and large holes in the bulk of the specimens immediately after they were cracked (0 days) and after 8 days of healing treatment, are shown for the mortar specimens examined in this work. Then, using the GSH data, the quantitative determination of the different phases identified in the mortar specimens was performed and the corresponding results were tabulated in Table 4. Taking into consideration the data in Table 4, it is observed that in the case of mortar-SAPs-1 the unhydrated cement products together with the incorporated SAPs (1% bwoc) represented only the 5% of the total material when the corresponding values for both mortar-reference and mortar-SAPs-2 were 36%. This indicates that the progress of the hydration reactions for this material was remarkably enhanced compared to mortar-reference and mortar-SAPs-2, as a result of SAPs incorporation in the cementitious matrix. This is attributed to more effective SAPs dispersion in the cementitious matrix and therefore enhancement of their functionality to extend hydration reactions during cement curing, thus promoting this as the optimal SAPs concentration in mortars in respect to microstructure evaluation.

Self-Healing Evaluation
In Figure 9 the crack surface and the larger voids in respect to cracks and large holes in the bulk of the specimens immediately after they were cracked (0 days) and after 8 days of healing treatment, are shown for the mortar specimens examined in this work. Figure 9. Initial microCT images depicting voids (in respect to both large pores and cracks) in red color for mortar-reference (a1), mortar-SAPs-1 (b1) and mortar-SAPs-2 (c1) and corresponding images after 8 days of healing treatment for mortar-reference (a2), mortar-SAPs-1 (b2) and mortar-SAPs-2 (c2).
MicroCT imaging allows both to visualize the self-healing process, but also to quantitatively analyze it in terms of changes in morphology and density using methods and functions embedded in CTAnalyzer software properly adjusted for mortar specimens. The methodology followed in this work is described as follows.
The region surrounding a crack is rich in products with a vast variety of thicknesses ranging from thick intact mortar to fresh self-healing products that can be thinner or thicker structures. Binarizing or segmenting the structures in this region can therefore be compromised. An effective solution, which has the effect of artificially diminishing the attenuation of thin structures, is the method of adaptive thresholding in CTAnalyzer custom processing procedure. Then, the (Region of Interest) ROI shrink-wrap and stretch over holes functions were performed. As a result, the (Volume of Interest) VOI was Figure 9. Initial microCT images depicting voids (in respect to both large pores and cracks) in red color for mortarreference (a 1 ), mortar-SAPs-1 (b 1 ) and mortar-SAPs-2 (c 1 ) and corresponding images after 8 days of healing treatment for mortar-reference (a 2 ), mortar-SAPs-1 (b 2 ) and mortar-SAPs-2 (c 2 ).
MicroCT imaging allows both to visualize the self-healing process, but also to quantitatively analyze it in terms of changes in morphology and density using methods and functions embedded in CTAnalyzer software properly adjusted for mortar specimens. The methodology followed in this work is described as follows.
The region surrounding a crack is rich in products with a vast variety of thicknesses ranging from thick intact mortar to fresh self-healing products that can be thinner or thicker structures. Binarizing or segmenting the structures in this region can therefore be compromised. An effective solution, which has the effect of artificially diminishing the attenuation of thin structures, is the method of adaptive thresholding in CTAnalyzer custom processing procedure. Then, the (Region of Interest) ROI shrink-wrap and stretch over holes functions were performed. As a result, the (Volume of Interest) VOI was wrapped around the boundary of complex and porous objects such as thin self-healing products and the wrapped VOI was prevented from penetrating into the porous spaces of the object but instead only the complex outer margins were marked. Figures 10 and 11 depict the resulting banalization-segmentation images after applying the adaptive thresholding method and shrink-wrap function for mortar-reference, mortar-SAPs-1 and mortar-SAPs-2 specimens at 0 and 8 days of healing treatment, accordingly.
Then, a full 3D analysis was run. The morphometric parameters calculated to quantitatively evaluate self-healing of the cracked mortar specimens were (a) percent object volume (%) and (b) connectivity density (mm −3 ) [51,57,58]. The first parameter shows the specimen volume variations, while the second one is sensitive to structure complexity changes versus healing time. The corresponding results are shown in Figure 12.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 14 of 20 specimen volume variations, while the second one is sensitive to structure complexity changes versus healing time. The corresponding results are shown in Figure 12.

Discussion
The overall effect of SAPs incorporation in cementitious materials highly depends on the SAPs properties. In this study, in house synthesized SAPs were used with tailored properties [39]. Firstly, different SAP particle sizes introduce different effects. For the same SAP concentration and w/c ratio, the larger the SAP particle size, the lower the determined strength was in means of flexural and compressive strength. This is directly correlated with the formation of larger SAP voids left behind in the cement structure when SAPs release the amount of water they absorbed during mixing [21]. The size of the SAPs used in this work laid in the submicron scale as shown in Figure 1c, whereas the particle sizes of SAPs tested by other researchers were several micrometers [8][9][10]15,18,21,23].
Additionally, SAPs chemical constitution triggers different behaviors in the cementitious matrix. As mentioned earlier, most commercially available SAPs are copolymeric networks based on acrylic acid or acrylamide that may or may not have been partially neutralized [10,11,14,15,17,18,[20][21][22][23]31,35] which present a low affinity with cement and as a result form large aggregates when incorporated in it. In this work, SAPs that have been tested had a hybrid organic core-inorganic shell structure. The introduction of an inorganic coating on the organic SAP core has been proposed by Kanellopoulou I. et al. (2019), via the encapsulation of the polymeric core with an inorganic CaO-SiO2 shell via solution-gelation technique [31]. Other research groups have investigated the behavior of coated SAPs via the Wurster process but the compensation of the strength reduction achieved was only partial [9].
On the contrary, as indicated by the total porosity reduction and the fact that no mechanical strength degradation was determined with respect to flexural and compressive strength for the mortar/SAPs composite materials, the tailored SAPs in this work presented enhanced chemical affinity with the cementitious matrix. More specifically, due to the enhancement of the chemical affinity between SAPs and cement, large SAP agglomerates were not formed and as a result the size of the SAP voids (porosity) when water was released from their particles were smaller. Additionally, the limited water absorption capacity of the coated SAPs compared to the corresponding capacity of the uncoated particles also favored the size limitation of the SAP voids. The total porosity reduction in both cases of SAPs incorporation in mortars as shown in Table 3 indicates that:


SAPs were homogeneously dispersed in mortars due to enhanced chemical affinity with the cementitious matrix and  SAPs promoted hydration reactions in the cement matrix, thus forming denser and consequently more durable structures, since stress inducing points (voids) in the cement matrix were smaller. This is coherent with the mechanical properties behavior discussed later.

Discussion
The overall effect of SAPs incorporation in cementitious materials highly depends on the SAPs properties. In this study, in house synthesized SAPs were used with tailored properties [39]. Firstly, different SAP particle sizes introduce different effects. For the same SAP concentration and w/c ratio, the larger the SAP particle size, the lower the determined strength was in means of flexural and compressive strength. This is directly correlated with the formation of larger SAP voids left behind in the cement structure when SAPs release the amount of water they absorbed during mixing [21]. The size of the SAPs used in this work laid in the submicron scale as shown in Figure 1c, whereas the particle sizes of SAPs tested by other researchers were several micrometers [8][9][10]15,18,21,23].
Additionally, SAPs chemical constitution triggers different behaviors in the cementitious matrix. As mentioned earlier, most commercially available SAPs are copolymeric networks based on acrylic acid or acrylamide that may or may not have been partially neutralized [10,11,14,15,17,18,[20][21][22][23]31,35] which present a low affinity with cement and as a result form large aggregates when incorporated in it. In this work, SAPs that have been tested had a hybrid organic core-inorganic shell structure. The introduction of an inorganic coating on the organic SAP core has been proposed by Kanellopoulou I. et al. (2019), via the encapsulation of the polymeric core with an inorganic CaO-SiO 2 shell via solution-gelation technique [31]. Other research groups have investigated the behavior of coated SAPs via the Wurster process but the compensation of the strength reduction achieved was only partial [9].
On the contrary, as indicated by the total porosity reduction and the fact that no mechanical strength degradation was determined with respect to flexural and compressive strength for the mortar/SAPs composite materials, the tailored SAPs in this work presented enhanced chemical affinity with the cementitious matrix. More specifically, due to the enhancement of the chemical affinity between SAPs and cement, large SAP agglomerates were not formed and as a result the size of the SAP voids (porosity) when water was released from their particles were smaller. Additionally, the limited water absorption capacity of the coated SAPs compared to the corresponding capacity of the uncoated particles also favored the size limitation of the SAP voids. The total porosity reduction in both cases of SAPs incorporation in mortars as shown in Table 3 indicates that: • SAPs were homogeneously dispersed in mortars due to enhanced chemical affinity with the cementitious matrix and • SAPs promoted hydration reactions in the cement matrix, thus forming denser and consequently more durable structures, since stress inducing points (voids) in the cement matrix were smaller. This is coherent with the mechanical properties behavior discussed later.
The slight flexural strength increment (~3%) of the mortar-SAPs specimens in comparison to the control specimens was evaluated taking into account as corroborating evidence the aforementioned porosity parameters and was attributed to the water absorbed by SAPs during the mixing process, which became available later during cement curing, thus promoting and accelerating hydration reactions. The space surrounding SAP particles was enriched with hydrated cement phases. As a result, mitigation of autogenous shrinkage and early age cracking were observed, while the densification of the cement microstructure led to the enhancement of the mortar/SAPs composites flexural strength [8,52,54,59].
Moreover, compressive strength is considered a key concrete property since it is directly correlated with concrete quality. Nevertheless, it must be clarified that the determined value of each measurement set highly depends on a variety of parameters namely the w/c, additives incorporation and curing conditions, i.e., humidity levels and temperature profiles. Curing conditions can affect drastically the compressive strength of a cementitious material. More specifically, compressive strength degradation has been correlated with low moisture levels during the first day of curing or high temperatures in the initial curing state which is responsible for lower quality hydration products [52]. Furthermore, w/c ratios used in mortars formulation have also been directly correlated with the voids formed in the concrete matrix and consequently with their mechanical strength. More specifically, increased w/c ratios have been known to lead to increased voids in the cementitious matrix and thus to the degradation of mechanical properties [1]. The w/c ratio used in this work (0.5) is high compared to the corresponding ratios examined by other research groups.  [54]. Nonetheless, since the effective w/c ratio is not the same as the total w/c ratio, when SAPs are incorporated in mortars, excess of water must be added to counterbalance the amount of water absorbed by SAPs. Considering the dosage of SAPs used in this work (1 and 2% bwoc) the high w/c ratio of 0.50 was chosen. The w/c ratio chosen combined with the curing conditions of the mortar specimens and notably, low moisture during the first day after casting as well as low temperature during the 28 days specimen curing the value of the compressive strength of the control specimens was calculated below 52.5 MPa.
In literature, it is often reported that cementitious materials containing SAPs show poorer compressive performance compared to the corresponding materials without SAPs during all curing periods and for SAPs dosages even lower than those examined in this work, while this behavior became more pronounced for increased SAPs dosages. [1,7,9,10,15,17]. On the contrary, in this work compressive strength was not influenced by the SAPs incorporation in mortars.
The relationship between the determined flexural and compressive strength for the mortar specimens as depicted in Figure 6 was also indicative of the fact that SAPs incorporation in the mortars affected flexural and compressive strength in a similar manner and that their incorporation did not negatively influence the mortar microstructure and hence the mortar strength was not degraded [53,54].
According to the results in Table 3, the percentage of total porosity was reduced by 0.5% and 2.5% in the cases of mortar-SAPs-1 and mortar-SAPs-2, respectively. Moreover, the percentage of open porosity was also reduced in the case of mortar-SAPs-2 compared to mortar-reference by 0.5%. Open porosity represents the pores which are in direct contact with the specimen environment thus allowing humidity and/or other harmful factors (e.g., corrosive factors) to penetrate the bulk of the specimen and cause the material deterioration. As a result, the open porosity reduction in cementitious materials promotes their mechanical integrity and their sustainability.
Additionally, as shown in Figure 7, the size of the pores in the mortar-reference specimens was greater than that of the mortar-SAPs-1 and mortar-SAPs-2 specimens, while the smallest pores were recorded in the case of mortar-SAPs-2. The pore sizes in the mortarreference specimens showed a wide distribution ranging from 9 to almost 500 µm. On the contrary, the pore sizes in mortar specimens in which SAPs had been incorporated showed a narrower distribution ranging from 9 to 333 µm and from 9 to 171 µm for mortar-SAPs-1 and mortar-SAPs-2, respectively. The more uniform the pore profile the more homogeneous the materials, which is consistent with improved mechanical properties.
Furthermore, the Gaussian deconvolution procedure of the GSH of all the mortar specimens examined in this work, showed that the minimum unhydrated cement phases (5%) were found in the case of the mortar-SAPs-1 specimens, whereas the corresponding phases in mortar-reference (36%) and mortar-SAPs-2 (35%) were almost identical (see Table 4). Firstly, this indicates that SAPs were more homogeneously dispersed in the cementitious matrix for the concentration of 1% bwoc thus allowing them to function as water reservoirs and cement hydration promoting agents and therefore led to denser and more durable cementitious structures. Secondly, it is safe to assume that even greater SAPs concentrations, if needed to render different functionalities to the mortars, will not compromise the mortar behavior.
In mortars, apart from one single crack several other microcracks are formed and distributed in the bulk of the specimens, when they are subjected to compressive loads, which sometimes may not be easy to detect using conventional methods. On the contrary, by running a full 3D analysis via microCT, apart from crack healing, the 3D volume of the specimen was also be thoroughly evaluated. As mentioned earlier, the quantitative analysis of the self-healing progress in pre-cracked mortar specimens took place via microCT Analysis. More specifically, a morphological and an architectural parameter were calculated for all mortar specimens before and after eight days of healing treatment. These parameters were: • Morphometric parameter, "Percent Object Volume" (%) which shows the percent of the specimen volume variation during the healing treatment. The value of this parameter increases with healing time, since healing products that are formed cover progressively the crack volume in part, thus representing the healing progress with time [51,57,58]. • Architecture parameter, "Connectivity Density" (mm −3 ) comprising a sensitive indicator of the change in texture and complexity associated with the healing progression. More specifically Connectivity Density is an indicator of complex, very highly porous structures. The value of this parameter decreases with healing time, since crack healing provides denser structures within the crack volume [58,60].
In order to estimate the relative self-healing enhancement of mortar/SAPs specimens versus the control specimens, the relative self-healing enhancement index h i (%) was calculated [61], based on the results of Connectivity Density since this architecture parameter is very sensitive to texture and complexity changes. where: h i : relative self-healing enhancement index for specimen, i CD i,n : value of Connectivity Density for specimen, i after n days of self-healing treatment CD i,0 : value of Connectivity Density for specimen, i before self-healing treatment (at 0 days) More specifically, when no self-healing is observed h i value is "0", as the final and the initial value of connectivity density are the same. On the other hand, when selfhealing is promoted, h i values become higher and approach the value "100", since the ratio CD i,n /CD i,0 approaches "0" as CD i,n decreases. The corresponding values of h i for the mortar/SAPs and control specimens are shown in Table 5 and it is revealed that the self-healing index was 84% and 31% for mortar-SAPs-1 and mortar-SAPs-2, respectively, while it was 20% for mortar-reference specimen. Comparatively, self-healing was enhanced by about 60% and 10% for mortar-SAPs-1 and mortar-SAPs-2, respectively compared to the mortar-reference specimen. These results reveal that healing efficiency was optimized for the specimen mortar-SAPs-1. This is attributed to the more effective dispersion of SAPs in the cementitious matrix for the SAPs concentration 1% bwoc and therefore the enhancement of their functionality as self-healing agents. This conclusion is in agreement with that drawn by the phase identification in mortars via image segmentation obtained by microCT scans. Table 5. Indexes h i for the mortar/SAPs specimens and control specimens.

Specimen h i (%)
Mortar-reference 20 Mortar-SAPs-1 84 Mortar-SAPs- 2 31 At this point, it must be taken into consideration that the specimen mortar-SAPs-1 was more severely damaged during the artificial inducement of the crack by the application of compressive load, leading to a very high initial value for connectivity density.
Our future goal is to expand this preliminary study on the evaluation of the selfhealing behavior of mortars containing the proposed SAPs for longer treatment durations and attain a more profound insight on the healing mechanisms.

Conclusions
The presented work, took into consideration the overall behavior of the mortar-SAPs composites manufactured and examined. It also comprises the determination of their mechanical properties, their microstructure evaluation, as well as the evaluation of their self-healing behavior yielding a series of conclusions. Even though it is often reported in literature that the incorporation of SAPs in mortars causes degradation in mechanical properties and specifically in compressive strength [1,7,9,10,15,17], within the manuscript it is shown that, the incorporation of tailored SAPs with respect to their structure (hybrid organic core-inorganic shell structure, spherical shape in the submicron scale) did not negatively influence neither the flexural nor the compressive strength of the mortars. This is directly correlated with the microstructure and porosity evaluation of the mortars, which took place via microCT analysis. In particular, the total porosity was reduced by about 0.5% and 2.5% for mortar-SAPs-1 and mortar-SAPs-2, respectively, while the range of the pore size distribution became narrower for both SAPs concentrations compared to the control specimens. As a result, these SAPs enhanced cement hydration reactions when incorporated in the mortars without introducing more stress inducing sites (macropores left behind in the matrix because of the deswelling of SAPs particles during cement curing) and consequently not compromising the mortars strength. Moreover, the Gaussian deconvolution procedure of the GSH of all the mortar specimens examined in this work, showed that the minimum unhydrated cement phases (5%) after 28 days of aging, were found in the case of mortar-SAPs-1, which also revealed the more pronounced self-healing behavior.
Conclusively, the overall SAPs functionality in cementitious-based materials was optimized while, the SAP concentration of 1% bwoc was promoted as the premium one in reference to mortar composite strength, microstructure and self-healing enhancement.