## 3 Results and Discussion  
### Dimensional Stability, Porosity and Phases Formed  
The results of the dimensional variation in the hybrid mortars (Figure 2) indicate that all materials experience global shrinkages during the curing and drying processes. For the five mortars under study, the shrinkages suffered during the drying were 40% RH higher than those previously experienced during the curing at high RH. While curing at 99% RH, the pores of the mortars retain water, which favors the hydration and formation of cementitious products [25]. However, when mortars are subsequently exposed to a drier environment (40% RH), water evaporates from the pores, generating internal stresses that cause shrinkage [44]. The drying shrinkages are meaningful for approximately the first 20 days at 40% RH, which suggests that the greatest loss of water in the systems occurs during this period, before equilibrium with the environment is reached. Figure 1: Schematic of the chloride diffusion test and chloride ion determination used for hybrid mortars: (**a**) NaCl immersion test; (**b**) depth profiling of the specimen for sample extraction; (**c**) chloride determination by means of a selective electrode. Figure 2: Dimensional variation in hybrid mortars during their curing and drying periods. Although OPC shows slightly better dimensional stability during curing than the hybrid materials, the mortar manufactured from commercial cement undergoes more significant shrinkage than the ecological alternatives studied during the drying process. Moreover, it must be stressed that the studied hybrid mortars show global shrinkages during the curing and drying periods that are very similar to those suffered by OPC. This is a clear advantage of the hybrid mortars as opposed to other traditional cement alternatives manufactured from slag such as AAMs [45]. During curing, hybrid mortars activated with Na\({}_{2}\)CO\({}_{3}\) undergo an initial expansion followed by stabilization. This expansive reaction can be related to the well-known expansive reaction of the carbonates in alkali media [46], and this must be borne in mind as it can affect the final properties of the material. The early expansion observed in HC mortars (Figure 2) is attributed to the interaction between Na\({}_{2}\)CO\({}_{3}\) and dissolved calcium species, which promotes the precipitation of calcium carbonate and other intermediate carbonate phases under high pH conditions. These reactions can lead to localized supersaturation and transient CO\({}_{2}\) gas release, generating internal stresses during the initial curing stage. Although the systems do not contain carbonate aggregates (thus excluding a classical alkalial-carbonate reaction, ACR), the expansive behavior shares some characteristics with ACR, in the sense that it involves carbonate phase instability in alkaline environments. Although the formation of ettrin spite could also generate expansions in cementitious systems, the absence of an external sulfate source, the low SO\({}_{3}\) content in the slag precursor, and the clear correlation between the use of Na\({}_{2}\)CO\({}_{3}\) and the observed initial expansion suggest that carbonation reactions are the most probable mechanism responsible for the initial expansion observed in HC mortars under the conditions of this study. Curing shrinkage values of hybrid mortars can be compared with those of AAMs based on slag. NaOH and waterglass as activators of AAMs provoke important shrinkages stresses in the materials, that mortars do not withstand [9]. After the curing period at high RH, length increases of 0.01-0.02% (depending on the activator and its amount) have been monitored and related to the extensive microcracking caused by the huge stresses these AAMs suffer during curing. The global dimensional changes taking place after curing (Figure 2) correspond to shrinkage in hybrid mortars; so, if microcracking appears, it must take place in a more reduced amount than in slag-based AAMs, and it does not determine the global length change.