*3.4. Optical Observation of Cracks*

The prisms produced for this research were pre-cracked after 28 days of curing under the two regimes. Representative images of optical observation under the curing regimes are given in Figure 7. The images were captured when the cracks were formed and correspond to 0 days. The images captured at 14 and 28 days after cracking correspond to 28 + 14 and 28 + 28 days total age. Table 6 shows representative measurements of the crack width evolution during the curing.

The images of neat cement depict the autogenous self-healing that the system could perform under these specific conditions. The width decrease is not significant (Table 6) in both curing conditions. At 14 days, there are open parts in the cracks that have not healed, while at 28 days, the situation is improved. However, unhealed areas still remain.

The water immersion promoted the formation of crystals in NC-cement pastes and allowed the healing of up to 14 days. The mean crack width was 0.156 mm at 0 days and is healed, as depicted in Figure 7. The newly formed material also overflowed on the surface of very small cracks. The 28 days captions showed a progression of this crystallization. A greater quantity of the material was observed on the surface of the cracks and in a broader area around them. The system presented healing to a smaller degree when it was subjected to wetting-drying cycles. The crack width decreased significantly up to 14 days, from 0.106 mm to 0.031 mm, and it was eliminated at 28 days.

The combination of nanoparticles showed efflorescing phenomena after 14 days of cracking, but the cracks were not healed at that age. Healing was recorded up to 28 days for samples cured in the water, and a satisfactory crack closure was recorded for samples kept in the wetting-drying regime.


**Table 5.** EDS analysis of the micrographs shown in Figure 6.

**Figure 7.** *Cont.*

**Figure 7.** *Cont.*

**Figure 7.** Optical microscopy observation of the cement pastes at 28 + 0 days of the cracking, at 28 + 14 days and 28 + 28 days, cured in water immersion (w) and wetting-drying cycles (c) (Scale equals to 1 mm).


**Table 6.** Crack width evolution (mean values of 14 measurements).
