## 5 Results and Discussion
### Capsule's Bonding Strength  
One of the most significant aspects related to capsule compatibility is the bonding strength of the capsule within the cement matrix [42; 50]. This characteristic is paramount due to the importance of the breakage of the embedded capsules for the release of the healing agent solution within the concrete to heal the cracks. This breakage of the capsule could only occur without the slipping of the capsule from the cement matrix. In relation to this, the study examined the effect of surface modification of the capsule with mortar matrix on the bonding characteristics. As can be seen in Figure 10, the modifications of the capsule surface helped to enhance the bonding strength regardless of mortar mixes. The results found that the bonding strength for 100%OPC mortar with rough capsules increased to 5.7 MPa, with an increment of 78.1% compared to smooth capsules. The bonding strength of modified PS capsule increased about 60-125% than those of non-surface treated PS capsule observed by [PERSON] et al. [19]. The result also indicated that combining the surface treatment with blended cement matrix improved the bonding strength of PS capsule, which the bonding strength increased about twice than bonding strength of 100%OPC mortar, regardless of surface modification type. Figure 10 also shows that, regardless of surface modification, the polystyrene (PS) capsules had higher bonding strength compared to both glass and ceramic capsules obtained by previous studies [19]. This could be associated with the brittle nature of the capsule shell material, where both glass and ceramic can rupture more easily compared to PS. As expected, the weak lines capsules had the higher bonding strength regardless of mortar mixes. It was noted that the bond strength of weak lines capsule with 100%OPC mortar increased about 39% compared to those of the 100%OPC mortar with rough capsules. This increment in bonding strength is related to the increase in the interface area between the capsule surface and mortar matrix due to the surface modification effect. Furthermore, the formation of engraved lines on the capsule walls enhanced the interlocking effect between the capsule's interface area and the mortar matrix, which made the capsule intact within the mortar matrix. This finding agreed with work done by [PERSON] et al. [51], [PERSON] et al. [52] and [PERSON] et al. [53], which discovered that the deposition of polymerization resulted in a rough texture microcapsule that improved the capsule's bonding within the cementitious matrix.  
Nevertheless, it is also noted that the PS capsule with smooth surface texture, which had no surface treatment, had lower bonding strength compared to those found by [PERSON] et al. [19] except to the mortar blended with silica fume. Furthermore, the findings also discovered that the existence of silica fume gave the highest bond strength regardless of capsule surface modification. It is noted that the presence of pozzolan increased the formation of CSH gel [54; 55], which densities the cement microstructure and consequently improved the interfacial zone between capsule's shell and mortar-cement matrix. The adhesion between capsule and cement matrix needs to be strong to allow the breakage of the capsule during concrete cracking [42]. According to [PERSON] et al. [34], the improvement of cement matrix microstructure and capsule's shell influenced the stability of the capsule within  
Figure 9: Capsules after mixing with high mixing speed level (**a**) rough capsules and (**b**) weak line capsules.  
the cement matrix and enhanced the potential self-healing efficiency and the mechanical strength of the cementitious matrix.