*2.1. Materials*

Geopolymer samples were produced based on mine tailings for coal production from a PGG Sp. z o.o. Branch KWK 'ROW' Ruch 'MARCEL' coal mine in the Silesia region (Radlin, Poland) and quartz sand as a fine aggregate. This mine was founded in 1949 and is still operating today. Currently, the mine employs 3,168 people, and its net daily output is 11,000 tons.

The main tailings were delivered in the form of rocks with dimensions ca. 20 cm. Firstly, they were fragmented into smaller particles by using a crushing machine. In the second step, they were ground using an ultra-centrifugal mill. The size reduction takes place by impact and shearing effects between the rotor and the fixed ring sieve. As a result of this operation, the fine powder (fraction < 40 μm) was obtained. In the next step, the obtained material was calcinated using a laboratory furnace. The calcinations were performed for 24 h at a temperature of 700 ◦C. The materials were heated and cooled together in a furnace. The main aim of this process was to increase the material reactivity by obtaining the appropriate microstructure. Additionally, the process made it possible to remove volatile substances, oxidize a portion of the mass of the material, and decrease the carbon content.

#### *2.2. Samples Preparation*

The calcined waste was mixed, in a ratio of 1:1, with quartz sand. The dry components were mixed in a slow-speed mixer (Geolab, Warsaw, Poland) cement mortar mixer for about 5 min. The prepared mix was activated with an activator solution consisting of a 10 M NaOH solution (technical sodium hydroxide flakes) and an aqueous sodium silicate solution—density 1.45 g/cm<sup>3</sup> (water glass solution, R-145)—in which the ratio of the sodium base solution to water glass solution was 1: 2.5. Tap water was used to prepare the solution. After the addition of liquid components, the process of mixing was continued for about 15 min, up to obtaining a homogeneous paste. The prepared geopolymer masses were poured into a set of molds with dimensions 50 mm × 50 mm × 200 mm (prismatic forms). Then, in order to remove air bubbles, the molds with the materials were placed on a vibrating table for 15 min. Subsequently, to prevent too-quick water evaporation, each mold was wrapped in a polyethylene film. For the acceleration of the curing process, the samples were placed at 75 ◦C for 24 h. After this time, they were cooled to room temperature, unmolded, and stored in laboratory conditions for 28 days at ambient temperature (ca. 23 ◦C). After this time, the mechanical properties were investigated—Figure 1.

**Figure 1.** Scheme for sample preparation.
