## 5 Results and Discussion
### Microstructural Investigation  
SEM images presented in Figure 12 show the morphology of the cement matrix on the crack plane of the pre-cracked samples that were loaded at 7-days. Figure 12b shows the morphology of the cement matrix for 100%OPC that is similar to the dry soil pattern, which indicates that the sodium silicate solution was dispersed and covered the flank surface area of the crack. The formation of ettringtite filling the pores within the cement matrix of 100%OPC cement paste was also noted in Figure 12a. A similar cement matrix morphology nature of 100%OPC was observed at the crack surface of 95% + 5%SF sample, as can be seen in Figure 12d. This confirmed the leakage of sodium silicate on the cement matrix surface. The dispersion of sodium silicate started to crystallize instantly due to the moisture availability within the cement matrix [18,56]. This crystallization is caused by the reaction of silica with the oxygen available in water, and the formation of silica oxide crystals. The deposition of silica oxide crystals as the result of sodium silicate reaction on the cementitious matrix can be seen in Figure 12d [18]. Moreover, SEM images had also shown the availability of common chemical products such as calcite (CaCO\({}_{3}\)), portlandite (Ca(OH)\({}_{2}\)), ettringtite, and CSH gel. Furthermore, Figure 12c showed the availability of un-hydrated silica fume particles on the crack plane of 95%OPC + 5%SF sample at the age of 7 days. The availability of such un-hydrated silica fume particles is the main reason for the autogenous healing ability at a later age. SEM images of the pre-cracked samples that have undergone the healing period of 28-days are presented in Figure 13. The microstructure observed the availability of portlandite (Ca(OH)\({}_{2}\)), CSH gel, and calcite as the hydration product of the cementitious composite for both samples, 100%OPC and 95%OPC + 5%SF. Denser layers of CSH were observed in 100%OPC samples especially for the sample containing sodium silicate capsules, as shown in Figure 13a,b. The finding suggests that the sodium silicate enhanced the chemical reactivity of the cementitious compound that promoting the healing process and this resulted in the increment of strength recovery. The EDX elemental analysis of 100%OPC without and with sodium silicate capsules reveals the presence of silica (Si) of 8.72% and 11.44%, respectively. The increment of silicon atomic ratio led to a reduction of Ca/Si ratio from 1.54 to 1.35, which is an indication of a high amount of healing products that contribute to strength recovery. The lower the Ca/Si ratio results in the higher strength of cementitious composite ([PERSON] et al., 2017). Figure 11: Strength recovery performance of cementitious combinations with various capsules’ surface modification. Nevertheless, a denser cementitious microstructure was observed on 95%OPC + 5%SF, as shown in Figure 13c,d. This indicates the existence of the additional formation of calcium silicate hydrate (CSH) as the result of pozzolanic reactivity and sodium silicate solution reacted with the available un-hydrated calcium particles and Ca(OH)\({}_{2}\) on the crack planes. As understood, this chemical reactivity produces CSH gel and Ca(OH)\({}_{2}\) that healed the cracks. Similarly, the increment in Si atomic ratio due to the presence of silica fume and sodium silicate solution had reflected on the Ca/Si ratio of cementitious composites for the samples without and with capsules. A further reduction of Ca/Si ratio was observed from 1.47 to 1.18 for 95%OPC + 5%SF samples without and with capsules, respectively. This confirmed the high hydration reactivity obtained due to the combination of silica fume and healing agent, sodium silicate solution. The availability of calcium (Ca) element in the composition of healing product is also an indication of the chemical reaction between the present Ca ions due to the presence cementitious compounds and the presence of sodium silicate, which these formed additional amounts of CSH, as observed in Figure 13c,d.