**2. Materials and Samples**

Two types of mixtures were tested: one containing 100% fly ash (FA), designated as M0-K, and the second one containing 50% fly ash and ground granulated blast furnace (GGBSF) slag, designated as M50-K. In addition to the two raw materials, sand with a grain size of 0/2 mm was used in the two mixtures. The oxide composition of the raw materials is presented in Table 1.


**Table 1.** Oxide compositions for the Raw materials used for the research.

Potassium Silicate (K-Sil) was used as an Alkali source, which is a commercially available liquid, and a specific amount of water was added into the mixture. The compositions of the tested mixtures are presented in Table 2.


**Table 2.** Composition of the two mixtures in kg per m3.

A procedure involving the preparation of the binder was applied to the manufacture of geopolymer mortars. First, an alkaline-activating solution was prepared. The amount of liquid silicate was measured, then water was added into this liquid to obtain the assumed water/binder ratio of 0.3. Subsequently, FA was mixed with an Alkaline-activating solution for 10 min. Then, GGBFS was added, and mixing continued for 5 min. Next, the rotary mixer was switched off for 1 min while the binder mixing continued to remove the solids stuck onto the container. Finally, at a low mixing speed, sand was gradually added to the mixture for 3 min. The total mixing time was approximately 20 min. The manner of preparation was selected based on reports in the literature and optimized following the team's previous experience in the propagation of geopolymer blends.

The geopolymer mortars were cast into prismatic 40 × 40 × 160 mm specimens, as shown in Figure 1a. The samples were compacted on a shaking table and covered with plastic lids. Twenty-four hours after casting, the specimens were removed from the molds and stored at room ambient temperature with a relative humidity of about 75%. Plastic was used to protect them from water evaporation and to limit moisture exchange with the environment.

**Figure 1.** (**a**) Samples after being heated and cooled down, (**b**) Electric Furnace NABETHERM.
