*2.2. Methods*

In all cases, various tests were performed in both non-alkaline (A) and alkaline activated mortars (PO, SC, WGS, and WGN) of different ages (28, 90, 180, and 365 days) after manufacture. The behavior of the mortars against water penetration was examined by the conduction of capillary absorption (UNI EN15801:2010), Karsten tube penetration (EN 16302:2013), and porosity (RILEM CPC11.3) tests. Capillary absorption tests were performed at the ages of 180 days and 365 days, while water absorption through Karsten tubes was performed at 90 days and 365 days. Moreover, the open porosity tests were

conducted at all ages previously mentioned, with the use of heptane instead of water [34], due to the sensitive nature of the material against direct water contact. In this study, however, the results of the 180 days and 365 days are reported.

The capillary coefficient was calculated, while immediately after the completion of the capillary test, the drying test was carried out (EN 16322:2013). The drying index (ID) was then calculated, while the results are presented as a function of time expressed in hours. The Mi-t chart represents the first drying phase of the samples, while the drying index is calculated through the integral under this curve (EN 16322:2013). Thus, the equation for the calculation of the drying index is the following (Equation (1)):

$$ID = \int\_{0}^{t\_f} \frac{M\_i dt}{M\_{\text{max}} \* t\_f} \text{ where } : M\_i = \frac{m\_i - m\_f}{A} \tag{1}$$

The symbol *mi* (kg) stands for the mass of the sample at a given time *ti* (h). Thus, mf stands for the final mass of the sample recorded at the final time *tf* (h). Consequently, the residual amount of water of the sample at a given time *ti* per unit area in kg/m2 is symbolized as *Mi*. Furthermore, *Mmax* is the maximum mass difference of the sample that occurs at the beginning of the test at time *t*<sup>0</sup> (kg/m2) [35,36]. The drying index can then be calculated by Equation (1) using the simplified Equation (2) according to the European standard 16322:2013 [36]:

$$ID = \frac{\sum\_{i=1}^{i=n} \left[ (t\_i - t\_{i-1}) \* \left( \frac{M\_{i-1} + M\_i}{2} \right) \right]}{M\_{\max} \* t\_f} \tag{2}$$

Furthermore, through the Karsten tube test, the average water penetration values are calculated. The conduction of this experiment was at the ages of 90 days and 365 days. These ages were decided to have representative values both at an early age and in the long term. At the age of 28 days, the experiment was not conducted due to the vulnerable structure of the samples.

In the ages of 180 days and 365 days, the compressive and flexural strength of the mortars was examined (EN1015-11). The results recorded at the early age of 28 days and 90 days are presented in a previously published study [29]. Concerning the volume stability of the mortars, the linear shrinkage (DIN 18947:2013-08) and volume change were measured. For the latter experiment, the specimens were cured in a chamber with specific temperature and humidity conditions (23 ± 2 ◦C, 50% ± 5% relative humidity). The change in the dimensions and weight of the specimens was recorded daily until stabilization.

Durability tests were also carried out when the mortars reached the age of 90 days. This certain age was decided in order to allow the mortars of aerial nature (such as clay mortars) to gain strength and mass stability. Additionally, the freeze–thaw and wet–dry cycles applied were designed, taking into consideration the vulnerable nature of clay mortars and the realistic scenarios of exposing the mortars to deteriorating agents. These tests included freeze–thaw and wet–dry cycles, where the final mass loss, compressive strength values, porosity, and surface alteration through stereoscopic observation were recorded. The stereoscopic observation was conducted through a LEICA WILD M10 (Thessaloniki Greece) microscope for all mortars at the ages of 90 days and 365 days. Any surface modifications, including cracking and color alterations, as well as the roughness of the mortars, using qualitative, comparatively images under the microscope were recorded.

Furthermore, to define the modification of the inner structure, a microscopic examination by SEM (JEOL840A JSM, Thessaloniki, Greece) equipped with an EDS device was performed. Thus, the molar ratios of SiO2/Al2O3, CaO/SiO2, and M2O/Al2O3 (*M* = Na or K) at an early age and after the completion of one year were estimated indicatively.
