## 2 Materials and Methods
### Methods
#### 2.2.3 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 \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Cement (g)** & **Sand (g)** & **Water (g)** & **SAPs (g)** \\ \hline Mortar-reference & 450 & 1350 & 225 & 0.0 \\ Mortar-SAPs-1 & 450 & 1350 & 225 & 4.5 \\ Mortar-SAPs-2 & 450 & 1350 & 225 & 9.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Composition of the mortar specimens. Figure 1: Polymerization reaction to form the organic core P(MAA-co-EGDMA (**a**), schematic representation of organic core—inorganic shell (**b**), SEM image of the synthesized SAPs/P(MAA-co-EGDMA)@CaO-SiO\({}_{2}\) (**c**), revealing the homogenous spherical morphology of SAPs particles in the nanoscale. 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 \({}^{\circ}\)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. Figure 2 shows representative 2D and 3D reconstructed images of the scanned specimens ((a), (b) mortar-reference; (c), (d) mortar-SAPs-1%; (e), (f) mortar-SAPs-2%). Moreover, the challenging issues of material phase segmentation and their quantitative analysis were addressed via 2D and 3D reconstructed images data, and more specifically \begin{table}
\begin{tabular}{c c c} \hline \hline
**Acquisition Settings** & & **Value** \\ \hline Source Voltage & kV & 60 \\ Source Current & uA & 120 \\ Image Pixel Size & um & 9 \\ Filter & & Al 0.5 + Cu 0.038 \\ Rotation Step & deg & 0.2 \\ Rotation & deg & 180 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Acquisition settings of the X-ray micro-tomography scans. Figure 2: 2D and 3D reconstructed images of the scanned specimens of: mortar-reference (**a,b**); mortar-SAPs-1% (**c,d**); mortar-SAPs-2% (**e,f**). 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].