*2.1. Materials and Samples Preparation*

Fly ash was obtained from the combustion of lignite coal from the Belchatow Power Plant (Belchatow, Poland). These ashes are characterized by relatively high calcium content. Other precursors used in the production of geopolymers were construction sand (Swietochlowice, Poland) and diatomite dust, obtained from an open-pit diatomite mine in Jawornik Ruski (Zohatin, Poland). The studies were conducted on non-calcined and calcined diatomite dust at 650 ◦C.

Table 1 shows the results of the particle size distribution for raw diatomite and fly ash dust. Investigations were carried out using a particle size analyzer (AntonPaar GmbH, Graz, Austria).


**Table 1.** Particle size distribution.

The average particle size for diatomite dust is smaller than for fly ash from Belchatow and is approximately 12 μm. For the fly ash from Belchatow, it is approximately 21 μm. Figure 1 shows the particles of fly ash from Belchatow from the lignite combustion process and the particles of diatomite dust.

The alkaline activator was a 10 and 14 mol sodium hydroxide solution (PCC Rokita SA, Brzeg Dolny, Poland), and the water glass was sodium R-145 (STANLAB, Gliwice, Poland) with a molar modulus of 2.5 and a density of about 1.45 g/cm3; the Na/Al ratio was 1:2. To prepare the mass, the precursors were mixed with the activator for about 10 min and poured into molds. The molds were placed on a vibrating table to eliminate air bubbles. After preparing the masses, the samples were tightly covered with foil and then placed in a laboratory dryer (SLW 750 STD, Pol-Eko-Aparatura, Wodzislaw Slaski, Poland) for 24 h at 75 ◦C. After 24 h, the samples were unmolded and cured under laboratory conditions (temperature about 20 ◦C, relative humidity about 50%) for 28 days. Strength tests were carried out in the next stage. Table 2 shows the names of the samples to better systematize the mixing ratios.

**Figure 1.** Particle morphology: (**a**) Belchatow fly ash (1000× magnification), (**b**) diatomite dust (1000× magnification).

**Table 2.** Composition of geopolymer samples based on fly ash from Belchatow, sand, and diatomite dust (non-calcined (DN) and calcined (DK)).


The sand used in the produced geopolymers functioned as a filler. Furthermore, the amount of alkaline solution added increased with the percentage of diatomite dust in the mixture. Diatomite is commonly used as a sorbent [13]. A higher amount of alkaline solution was necessary to achieve the proper workability and consistency of the geopolymer mixture.

#### *2.2. Research Methods*

2.2.1. Phase Composition of Precursors

The PANalytical Aeris instrument (Malvern PANalytical, Lelyweg 1, Almelo, The Netherlands) was used to investigate the mineralogical composition of the geopolymer samples prepared. Quantitative analysis was carried out using the Rietveld method, which was implemented in HighScore Plus software (version: 4.8, Malvern PANalytical B.V., Almelo, The Netherlands). The International Centre for Diffraction Data (ICDD) PDF-4+ database was used during the analysis. Measurements were recorded in the range of 10–100◦, with a step size of 0.003◦ (2θ) and a time per step of 340 s, using Cu Kα radiation.
