*4.2. Microstructural Analysis*

Scanning electronic microscopy (SEM) was used to investigate changes in microstructure in samples subjected to different silica sand proportions. As shown in Figure 5a–e, micro-cracks found by SEM on the cement paste region might explain the apparent surface cracks in samples such as CM0.2 (Figure 2a), CM0.4 (Figure 2b), and CM0.6 (Figure 2c). The dehydration of cement paste was generated by heat affected by laser irradiation. Micro-cracks were generated in the cement hydrate region as can be seen Figure 5a,b. More cracks were formed in cement paste region of samples with low silica content, such as CM0.2 and CM0.4. Furthermore, Figure 5d,e show that micro-cracks formed more densely in the silica sand particles and less densely in the cement paste region, indicating that the occurrence of the cracks on the top surface (Figure 2e) decreased as the increasing of silica sand proportion in the cement mortar. Due to the higher melting temperature of silica sand, the generation of a glassy layer with the major component being silicon in the silica sand restricted the heat transmission of laser beam.

**Figure 5.** SEM images of heat-affected zone: (**a**) CM0.2; (**b**) CM0.4; (**c**) CM0.6; (**d**) CM0.8; (**e**) CM1.5.

Figure 6 shows the SEM images in the processed zone of cement mortar samples. The size of the pores grew with increasing of silica sand proportions in the cement mortar. Furthermore, samples with a higher silica sand content had a denser appearance of microcracks near the pores. Due to that, it can be assumed that the porosity of the glassy layer in samples CM0.8 and CM1.5 is higher than in other samples. It also implies that the glassy layer's strength is decreased and easily removed with an increasing of silica sand proportion.

**Figure 6.** SEM images of processed zone: (**a**) CM0.2; (**b**) CM0.4; (**c**) CM0.6; (**d**) CM0.8 and (**e**) CM1.5.
