2.1.1. Coal Ash (CA)

In this study, local coal ash from CET II Holboca, Iasi, Romania was collected and processed. To ensure experimental repeatability, the collected powder was firstly dried in a chamber with a static atmosphere, i.e., without ventilation, to avoid fine particle removal, and was secondly sifted to remove the particles with a diameter above 100 μm. The drying method is common and is presented in the literature [20], while the sifting method has been presented in a previous study [4]. The coal ash used in the study had a particle size distribution of 3.4 μm (d50) and a specific surface area of 1.6 m2/g as determined by a Coulter LS 200 laser scattering particle size distribution analyser (Beckman Coulter Inc., Pasadena, CA, USA). In addition, the bulk density of coal ash was 2.16 ± 0.01 g/cm3 evaluated with a Le Chatelier densimeter (Recherches & Realisations Remy, Montauban, France).

According to XRF analysis, the coal ash used in this study belongs to class F fly ashes. From a microstructural point of view (Figure 1), the aluminosilicate waste collected is a mixture of fly ash and bottom particles, as both spherical and irregular porous particles can be seen.

**Figure 1.** Coal ash morphology showing bottom and fly ash particles.

Coal ash is a mineral residue resulting from coal combustion in thermal power plants, it has small particles in the range of 0.01 to 300 μm. The chemical composition of coal is critical since it influences the ultimate properties of geopolymers. Silicon dioxide, aluminium oxide, iron oxide, and calcium oxide are the chemical compounds with the highest concentrations in their composition. Its chemical composition, on the other hand, changes depending on the type of coal and the furnace operation.

#### 2.1.2. Metakaolin (MK)

The metakaolin used in this research was produced by the calcination of commercially available kaolin clay [12] at low temperatures (heated up to 700 ◦C at a rate of 10 ◦C/min and maintained for 30 min.). As a result, the starting material was converted into an

aluminosilicate source with strong pozzolanic activity. According to the XRF analysis, the metakaolin contains a high concentration of silicon and aluminium oxides (Table 1). The metakaolin used in the study had a particle size distribution of 9.2 μm (d50), a specific surface area of 16.8 m2/g and a bulk density of 0.22 ± 0.01 g/cm3.

**Table 1.** Oxide composition of raw materials, coal ash (CA), metakaolin (MK) and mine tailings (MT).


Oth.\*—oxides in a concentration lower than 0.1% (traces of S, Cl, Cr, Zr, Ni, Sr, Zn and Cu). L.O.I.\*\*—Loss on ignition.
