*3.1. Compressive Strength and Open Porosity*

The compressive strength values and the variation of these values compared to the reference are given in Figure 2. Each nano-modified system was compared to the reference cement paste. The equation used for the calculation of the variation (V) is V (%) = (N − R) × 100/R, where N is the value of the nano-modified paste, and R is the value of the reference cement paste.

**Figure 2.** (**a**) Compressive strength values of the pastes at 28 and 90 days; (**b**) Compressive strength variation of nano-modified cement pastes, in respect to the reference.

The curing conditions slightly affected compressive strength values. At three months, the strength was higher in relation to the values recorded at 28 days in all cases. Also, neat cement pastes have lower strength when subjected to cycles in relation to water immersion. In other reports, the curing regimes and w/c influenced the results concerning the physicomechanical properties of the cement pastes. Abusnina et al. suggested that both compressive strength and porosity were modified when cement pastes with varying proportions of oil-contamination were evaluated in the age of curing [24].

Nevertheless, the cement pastes cured in water immersion had slightly greater compressive strength than the pastes cured under wetting-drying cycles, mainly for the reference and the cement pastes with NS presence. On the other hand, the presence of the nanoparticles also affected this property. The addition of NC slightly improved compressive strength at 28 days recording, by 3.58% and 3.45%, at 28 and 90 days, respectively. On the contrary, the combination of NS and NC decreased the compressive strength significantly in both curing conditions. This reduction was approximately 30% at 28 days and more than 30% at 90 days. The increased water demand in these systems could have influenced the strength. Another possible cause could be the high amount of the incorporated nanoparticles (1.5%wt NC + 1.5%wt NS). This condition did not favor the distribution of them, causing agglomerations [25,26].

The open porosity evolution and the variation of open porosity at 28 and 90 days are given in Figure 3. The open porosity values are typical of cement pastes and in agreement with previous works [17,21]. Porosity decreases with time. The curing conditions seem to influence this property. NC-cement paste presented higher open porosity when cured in water than wetting-drying cycles, as the nano-calcium oxide hydrates and forms calcium hydroxide. When the samples remain in the water, the calcium hydroxide content remains hydrated.

**Figure 3.** (**a**) Open porosity evolution of the pastes at 28 and 90 days; (**b**) Open porosity variation of nano-modified cement pastes, in respect to the reference.

On the other hand, in wetting-drying cycles, NC hydrates and then dries in the air and forms calcium carbonate. As a result, the porosity of NLw was found to increase at 28 days. Additionally, all the systems cured in cycles have smaller porosity values and smaller variations than the reference (Figure 3b). The addition of NC favored open porosity closure at 90 days, as the variation recorded was −16.36% when cured in water and −48.58% when cured in cycles. In contrast, the combination of nanoparticles did not benefit the reduction of porosity, both at 28 and 90 days, but still, the open porosity values of NSLw and NSLc are low.

The open porosity results are in agreement with compressive strength results. The physicomechanical properties of NC-cement pastes were benefited, compared to neat cement paste. In previous work, when NC was added in cement pastes with a w/b ratio of 0.32, the open porosity results had demonstrated an extreme reduction of porosity by −50% [21]. The combined nanoparticles addition leads to a compressive strength decrease and a slight open porosity increase, especially when cured in water. With time, the porosity is similar, regardless of the curing regime. In neat samples, water assists cement hydration and the porosity is quickly reduced.
