**1. Introduction**

The production of Portland cement contributes about 7% of the world's CO2 emissions, instigating the search for alternatives to this type of material [1]. Alkali-activated binders or geopolymers are examples of such alternatives. These materials are formed by the polycondensation of aluminosilicates (containing silicates with alkali) and are characterized by an amorphous or semi-crystalline structure [2,3]. In the technology of cement and concrete, in addition to the already used and standardized mineral additives, attempts are being made to use undeveloped industrial waste [4,5]. Lach et al. investigated the possibility of immobilizing waste from municipal waste incinerators in geopolymers. The results of the study showed a high level of immobilization of compounds and elements such as sulfates, chlorides, fluorides, zinc, and barium [6]. Fly ash and slag have an advantage over other materials because, as finished waste from energy processes, they do not require additional reactivation during geopolymerization [7]. Moreover, their properties depend on their composition, the technology of production, or the raw material used in the combustion process. In addition, their use as a by-product of the energy industry is in line with the policy of a closed-loop economy [8]. Another group of potential mineral additives is power industry co-products, commonly known as calcium fly ash [9]. Calcium fly ash has a more complex composition than silica fly ash. The vitreous phase of calcium fly ash is characterized by a rich content of silica and aluminum. In their structure, there may also be glasses of the C-F-S structure (CaO-Fe2O3-SiO2) [10]. Among the main phase components of calcareous fly ash are gehlenite, anorthite, anhydrite, tricalcium aluminate, quartz, calcium sulfate-aluminate, and unbound calcium oxide (CaOx) [11].

Over recent times, diatomite material has become increasingly popular due to its unique properties and various applications. For example, the diatomite material is used in

**Citation:** Pławecka, K.; B ˛ak, A.; Hebdowska-Krupa, M.; Łach, M. The Use of Calcined Diatomite as an Additive to Geopolymeric Materials. *Mater. Proc.* **2023**, *13*, 28. https://doi.org/10.3390/ materproc2023013028

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

Published: 15 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/).

filtration processes and the production of insulating or sound-absorbing materials [12,13]. However, the most well-known example of the material's applications is its ability to sorb various types of petroleum substances. Diatomite can also support the synthesis of zeolite structures [12]. Thus, it is possible to obtain zeolites from waste raw materials that have significant SiO2 content in their composition [14,15]. Researchers have also focused on the use of diatomite as an additive to cement binders or concrete [16]. Investigations have shown that the addition of diatomite lowers the consistency of fresh concrete mixes. The initial mechanical properties of mortars modified with diatomite additives were low, but after 28 days of seasoning, the strength increased [17].

This paper presents a comparison of the properties of geopolymers based on lime fly ash from an incinerator in Belchatow and sand, modified with different contents of diatomite dust—10%,15%, and 30% by weight. Alkali activation was carried out using two different activators: aqueous sodium glass solution and 10 and 14 mol sodium hydroxide. Diatomite dust was introduced into geopolymer mixtures in calcined and non-calcined forms. Investigations into the produced geopolymer materials showed great potential in using diatomite as one of the precursors in geopolymers. These investigations are important for providing new knowledge not only on the possibility of using local resources to produce construction materials by geopolymerization but also on the possibility of using industrial waste. Activities such as these are part of the environmentally friendly policy of a closed-loop economy.
