## 3 Results and Discussion
### Dimensional Stability, Porosity and Phases Formed
The volume of big pores (Figure 4b), which can be the most dangerous for compromising the mechanical properties or enhancing chloride diffusion, do not show significant changes in the different materials. It is worth mentioning that HN-4% have a more refined porosity than those manufactured with 8%, which could seem surprising at first sight. Moreover, it must also be stressed that HC-6% have a volume of large pores similar to or higher than these determined in more porous materials such as OPC, and higher than HC-3% (Figure 4b). This last fact can be coherent with cracks generated during the beginning of the curing (Figure 2), which can also progress during the drying step. The analysis of hysteresis loops during the Hg intrusion measurement (due to intrusion and extrusion of Hg in the pores) informs that the five materials under study present predominantly inkbottle pore shapes, together with ultimate mercury entrapment after the extrusion of mercury (see Supplementary Materials, Figure S1). The use of other techniques such as computed tomography could allow a better understanding of porosity in construction materials [50]. On the other hand, it is clear that the effect of nitrates in the porosity is different from that observed for carbonates, at least for the highest activator concentration (Figure 4). It has been reported that the addition of nitrates in traditional cements increases the initial curing heats [38], which in turn favors the hydration of the slag present in the hybrid binder [51]. The nitrate additions also boost the formation of large amounts of needle-type hydrates, such as nitrate-AFt compounds, in traditional binders [38], favoring their early strength development. The quickly formed hydrates partially fill the pores and explain the pore reduction monitored even after short curing periods [52]. However, it has also been demonstrated that the addition of calcium nitrates can interfere in the usual hydration process of the traditional cements, decreasing the formation of C-S-H gel and portlandite [38], especially when the nitrates are added in relevant amounts. This addition can have either a predominately positive or negative effect, essentially depending on its concentration. Hence, bearing in mind the obtained results (Figure 4), the addition of nitrates in 8% concentrations to the precursor under study seems to be high enough to negatively affect the expected beneficial effect of the CEM I in the mix (Table 3). The reaction of nitrates with the clinker can negatively affect the formation of gels relevant for the decrease in the porosity in the long term, as has been confirmed (Figures 3 and 4a). A SEM study was carried out to evaluate the possible occurrence of microcracks in the dried mortars. Figure 5 summarizes the most significant observations. The presence of cracks is frequent in alternative mortars, such as AAMs [9] or hybrids [53]. These cracks have been associated with high stresses related to the very refined porosity generated by the heavy activation employed to achieve the high mechanical properties [9]. Cracks or microcracks can act as preferential pathways for chloride diffusion and may significantly affect the corrosion behavior of the embedded metal reinforce [23]. The mortars formulated in the present work, even after the drying step, generally show homogenous non-cracked microstructures. The detailed study carried out confirms that HN (Figure 5d,e) and OPC (Figure 5a) mortars can be considered microcrack-free after the curing and drying processes. On the other hand, a reduced number of microcracks have been identified in some observed sections of HC mortars (Figure 5b,c), with their presence being clearly more relevant in HC-6%. These cracks in HC mortars can have different origins. The first reason can rely on the contribution of the expansion process taking place at the beginning of the curing for HC (Figure 2). The expansive reactions shown, related to alkali-carbonate reaction [46] can generate small microstructural defects.