2.1.4. Activator Solution

As an activator, an acid solution of o-phosphate (H3PO4) with 85 wt.% was diluted in distilled water to obtain a corresponding activation solution for an H3PO4/Al2O3 ratio of 1:1. According to previous studies [12,13], this ratio exhibits the best performance in terms of the compressive strength of obtained geopolymers.

Using the materials presented above, five types of geopolymers were synthetized. Table 2 shows the experimental mixes used in this study. The samples were designed to observe the influence of 50% by the mass addition of each raw material onto another. Accordingly, geopolymers with 100% of the solid component of coal ash (CA–geo) and metakaolin (MK–geo) were obtained, while because of the low geopolymerisation potential of mine tailings, a geopolymer with 100% mine tailing in the solid component could not be synthesized (the sample could not be cured at room temperature). In order to evaluate the addition of each component, three types of blended geopolymers have been designed, one from a mixture of metakaolin and coal ash (CAMK), one from a mixture of mine tailings and coal ash (MTCA) and another one with mine tailings and metakaolin (MTMK). Accordingly, these samples were obtained from a mixture of two of the raw materials at a 1 to 1 weight ratio.


**Table 2.** Mix design and parameters.

All the mixtures have been activated with a phosphorous solution at a solid/liquid ratio of 0.9 (to assure workability). Moreover, to assure better homogeneity, the geopolymers with multiple aluminosilicate sources were mixed in a dry state for 2 min, and for 5 min after the liquid addition, according to the procedure presented in [18,19]. Accordingly, the mixtures were poured into 20 × <sup>20</sup> × 20 mm<sup>3</sup> moulds, covered with a thin layer of plastic (to avoid moisture loss) and cured at room temperature (22 ± 2 ◦C) for 28 days before testing. The schematic representation of samples obtained is presented in Figure 2.

**Figure 2.** Process flow diagram of the obtained samples.
