## 3 Results and Discussion
### Dimensional Stability, Porosity and Phases Formed
The porosity volume in all the mortars under study was determined by the Archimedes method, and the obtained results are plotted in Figure 3. The hybrid mortars show higher porosity values after the drying process than just after the curing. This can be understood bearing in mind that, after curing, some water remains in the pores of the mortars blocking the He penetration. Hence, the He-pycnometer measurements are unable to distinguish between dense mortar and pores with remaining water. An intensive, prolonged drying at 40% RH completely eliminates water from the pores, erasing this interference and increasing the measured porosity values. The porosity of slag-based AAMs activated with NaOH or water-glass after curing and measured in our laboratory following the same procedure [9] ranges between 5 and 2%, which is similar to the values measured for HN mortars and lower than the values determined for the HC binders (Figure 3). The drying effect in the porosity is clearly evident for the HN mortars (Figure 3), but seems masked for HC, especially for the HC-6%. The expansion process taking place at the beginning of the curing (Figure 2) can have generated microcracks that affect the porosity obtained for HC and become evident after drying or that can even grow from the stresses caused by water evaporation. On the other hand, during the drying step of OPC, two competitive processes affect the porosity: hydration and drying. The curing kinetics of OPC, which lacks activators in its formulation, is slower than that of hybrid mortars. The OPC mortar curing process lasts more than the 28 days that the samples are exposed to 99% RH [47], and continues at 40% RH, as long as water remains in the pores [44]. In Figure 3, it can be confirmed that OPC decreases its porosity during the drying process by hydration. For OPC, the measured porosity values are lower after the drying step than just after curing, as the effect of curing dominates over the effect of a complete water removing from the pores in the porosity results for this traditional cement. Additional information about the pore structure of the dried mortars has been obtained with Hg intrusion porosimetry (Figure 4). The global porosity values obtained with this technique (Figure 4a) show a trend similar to that of the porosity values obtained with the Archimedes method after the same exposure condition (Figure 3). The results obtained confirm that the use of Na2 CO3 is more effective than the use of NaNO3 (for identical Na amounts) to reduce the porosity volume of hybrid mortar (Figures 3 and 4a), but it must be stressed that both activators allow mortars denser than OPC to be obtained. In spite of the expansive reactions taking place at short curing times (Figure 2), the HC mortars show low porosity. As occurs with sulfates, carbonates affect the curing process  
Figure 4: Mercury intrusion porosimetry results obtained for the cured and dried mortars. (**a**) Total porosity and percentage of pores with diameters in different ranges; (**b**) % of mortar volume occupied by pores with different diameters. Figure 3: Porosity values determined by the Archimedes method. Striped bars plot the porosity values of the mortars just after curing, while full color bars plot the porosity of the mortars after the drying step. of the binders [48], favoring the fast development of denser and more cohesive gels than those generated by OPC hydration. Increasing the activator concentration seems to have a positive effect on the porosity reduction in HC after the drying process, which is coherent with the results obtained for mortar activated sulfates at similar Na concentrations [42]. There are no dramatic differences in pore size distribution between OPC and the hybrid mortars (Figure 4). Most of the pore volume in the five mortars studied is formed by pores of small diameter. This fraction of pores that can significantly contribute to the physisorption of chloride on pore walls [49] is smallest for the HC mortars (Figure 4b).