*2.1. Materials and Mortars Design*

In total, five different compositions were cast, including the reference one (recorded as A) with no activators, used for comparison reasons. The manufactured specimens were rectangular with dimensions (40 mm × 40 mm × 160 mm) that were used in every test conducted in this study besides open porosity. The specimens for the open porosity test were approximately 40 mm × 40 mm × 50 mm). The soil used was extracted from the island of Crete and has been characterized using XRD analysis, particle size distribution, chemical analysis, and Atterberg limits. The XRD analysis indicated that the soil particles consisted of quartz, calcium aluminum hydroxide, calcite, cancrinite, and a small percentage of muscovite. Moreover, the high content of calcium oxide of 25% was reported by the results of the chemical analysis using atomic absorption technique [29]. The specific gravity of the dry material was 1.96 g/cm<sup>3</sup> (ASTM-C188-95) [30], and the color characterization by Munsell charts is 5Y 7/1 light grey.

Moreover, the particle size distribution (Malvern 2000, Mastersizer, Thessaloniki, Greece) depicted that the particles of 2–500 μm size prevailed in the binder's mass (Figure 1). The above analysis indicated the high volumetric percentage of silt contained in the soil (2–63 μm) of approximately 56%, with a 31% being fine sand (63–250 μm) and 13% clay particles (<2 μm). Atterberg limits were estimated (ASTM-D4318-00), while the Plasticity Index of the soil was calculated at 23.2. The liquid limit was calculated at 47.2% and the plastic limit at 24%. Thus, using the plasticity chart by Casagrande (ASTM-D2487-17) [31], the soil was classified as lean clay (CL).

**Figure 1.** The particle size of Cretan clay used.

The details of the mortars' manufacture, as well as the activators used, can be found in a previous publication [29]. To clarify the acronyms PO stands for potassium metasilicate, which was used as such, SC for sodium carbonate solution, in a ratio of 70:30 water: sodium carbonate, WGS for sodium metasilicate, which was used as such, and WGN for sodium hydroxide solution mixed with water–glass at a 1:1 ratio. The water–glass used was of sodium silicate nature, with a molar ratio SiO2/AlK2O of 3.45, a pH concentration at 20 ◦C of 11.1, and a viscosity of 68 m·Pas at 20 ◦C. The activating solutions used in this work are of low viscosity (liquid form), and they are commercial products. These activators are primarily used for activation in slag, fly ash, bentonite, and pure kaolinitic clay systems [3,32,33]. However, they have not been thoroughly tested in such an impure clay system. The mortars' composition can be seen in Table 1, while the pH values, the details of the mortar mixtures, and curing conditions can be found in a previously published research paper [29]. The workability achieved was recorded by the flow table, as mentioned in EN 1015-3.



<sup>1</sup> L/S = liquid/solid.
