**1. Introduction**

Fly ash is a waste material that settles in electrostatic precipitators as a residual after the combustion of coal dust in the furnaces of a power plant or thermal power plant. Fly ashes vary in chemical and phase composition. The best-known fly ashes are those with a high silica or calcium content. In this paper, the latter will be presented. Fly ash from the Bełchatów mine is ash with a high calcium oxide content. This material is a by-product of lignite coal combustion. This ash can come from conventional furnaces or be generated by dry flue gas desulfurization (from conventional or fluidized bed furnaces). The concentration of reactive calcium oxide required as a pozzolan and hydraulic component in the whole mixture is more than 10% for lignite fly ash [1–3]. According to EN 197-1 [4], only the fly ash from boiler furnaces can be used. However, EN 206 [5] states that lignite fly ash cannot be used as a Type II active additive.

In Poland, lignite fly ash is a by-product of combustion mainly in the boilers of the Bełchatów Power Plant and Combined Heat and Power Plant, as well as P ˛atnów, Adamowa and Konin (the PAK region). Table 1 shows the chemical composition of the lignite fly ash from the Bełchatów Power Plant and, for comparison, Table 2 shows the coal fly ash from the Skawina Power Plant. Tables 3 and 4 show the phase composition of the fly ash from the Bełchatów Power Plant and, for comparison, the phase composition of fly ash from hard coal. The data presented in Tables 1–4 highlight the great diversity in domestic

**Citation:** B ˛ak, A.; Bazan, P.; Pławecka, K.; Łach, M. Geopolymers Based on Fly Ash from the Bełchatów Power Plant. *Mater. Proc.* **2023**, *13*, 17. https://doi.org/10.3390/ materproc2023013017

Academic Editors: Katarzyna Mróz, Tomasz Tracz, Tomasz Zdeb and Izabela Hager

Published: 14 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

fly ashes. The fly ash from the Bełchatów Power Plant has the characteristic of calcium aluminosilicates and is characterized by a high content of silicon, aluminum and calcium oxide and a small amount of other oxides [6,7].

**Table 1.** Oxide composition of the Belchatów lignite fly ash.


**Table 2.** Oxide composition of the Skawina charcoal fly ash.


\*\*\* **F.A. Skawina**—Fly ash from Skawina.

**Table 3.** Phase composition of the Belchatów lignite fly ash.


**Table 4.** Phase composition of the Skawina charcoal fly ash.


Fly ash from Bełchatów lignite is characterized by a more complex mineral composition compared to fly ash from Skawina coal. As shown in Tables 3 and 4, this observation applies to both the glass phase and minerals. The main minerals in lignite fly ash are quartz, gelenite, anorthite, anhydrite and calcium oxide. Typical cement clinker phases, i.e., C12A7, C3A and C4A3S, have also been identified [ ˆ 6]. These phases determine the hydraulic properties of fly ash. The pozzolanic and hydraulic properties of lignite fly ash are also related to the concentration of the amorphous phase. In the case of lignite fly ash, aluminosilicate glass is identified as the amorphous phase [8]. Fly ash from lignite Bełchatów is

characterized by a relatively high variability in chemical and phase composition. This is also true for other characteristics that affect their functional properties [9].

The purpose of this paper is to study lignite fly ash from the Bełchatów Power Plant and Combined Heat and Power Plant as a replacement for hard coal fly ash from the Skawina Power Plant and Combined Heat and Power Plant. In this paper, the base material was tested and then specimens were made for bending and compression tests, so the mechanical strength of the finished structural material was shown. For the alkaline activation of the raw material, the same activator with different molar concentrations of 10 and 14 M NaOH was used.
