## 3 Results and Discussion
### Microscopic Analysis  
To investigate the changes in the microstructure and chemical composition of cement hydrates due to the treatment with the 30% strontium nitrate solution, SEM and EDS analyses were conducted. These analyses aimed to examine the altered microscopic morphology of cement hydrates treated with the strontium nitrate solution over a sufficient period. The specimen S7, along with the control specimen H7, was analyzed for comparison. Specimens fractured in the flexural strength test were used for this study. A JEOL JSM-6480 SEM (JEOL, Tokyo, Japan) equipped with a 25 kV EDS was utilized in this research. Figure 11a,b, respectively, show the SEM images (magnification: \(\times\)500) of the H7 and S7 specimens. The structure of the specimens additionally cured in the 30% strontium nitrate solution can be visually observed to be denser compared to that of the cement paste specimens cured in deionized water for the same period. This densification is presumed to be due to the strontium nitrate solution penetrating the surface of the specimens and reacting with the already formed cement hydrates, transforming or generating different forms of hydrates. These new hydrates are believed to fill the voids, resulting in a denser structure. Figure 12 shows the results of EDS analyses conducted on six different areas of the S7 specimen. Areas that appeared to differ from the typical components of cement hydrates were selected for EDS analysis, and the atomic percentages of five elements (O, Si, Ca, Al, Sr) were analyzed. As shown in the analysis results, the Sr element was detected in all areas. Areas 1, 3, and 5, with almost no aluminum element, are presumed to be primarily altered C-S-H hydrates, while areas 2, 4, and 6 are believed to be hydrates formed from reactions between the strontium and the aluminum-containing cement hydrates, such as C-A-S-H (calcium alumino-silicate hydrate), AFm, and AFt. Additionally, the typical needle-like appearance of the AFm and CH hydrates, commonly found in normally cured hydrated cement composites, was hardly observed. Figure 13 presents the results of EDS element mapping analysis for the S7 specimen. Figure 13a shows the analyzed specimen, with the left end being the surface where the strontium nitrate solution penetrated, and penetration progressing towards the right, analyzed up to a depth of approximately 5 mm. Figure 13b,c, respectively, show the distributions of calcium and strontium elements. It was observed that the concentration of strontium increases closer to the surface of the specimen, while the concentration of calcium decreases. Strontium elements were detected throughout the analyzed depth (i.e., up to 5 mm beneath the surface). Figure 11: SEM analysis. XRD analysis was conducted to investigate the crystalline phase changes in a hydrated cement matrix upon interaction with strontium nitrate solution. For this study, a Rigaku D/MAX XRD II was utilized, performing 2-theta scanning from 5\({}^{\circ}\) to 80\({}^{\circ}\) with a step size of 0.02\({}^{\circ}\), in accordance with ASTM C295 [36]. Three samples were prepared for the XRD analysis to identify phase changes over time: a hydrated cement paste cured for 28 days in deionized water, S3, and S7. Each sample was ground into powder prior to the analysis. The results are shown in Figure 14. A decrease in the relative peak intensities of Sr(NO\({}_{3}\))\({}_{2}\) at specific 20 degrees over time was observed, indicating its consumption within the hydrated cement matrix. This was particularly evident in the comparison of samples S3 and S7, supporting the hypothesis of Sr(NO\({}_{3}\))\({}_{2}\)'s active involvement in the chemical reactions within the matrix. Subsequently, the amount of portlandite (Ca(OH)\({}_{2}\)) was found to have dramatically diminished after the treatment, as observed in S3 and S7. This suggests the consumption of Ca(OH)\({}_{2}\) in the formation of new hydrates, possibly involving strontium.