**1. Introduction**

Geopolymer composites are a "green" alternative to traditional cementitious materials. It is estimated that the production of geopolymers creates four to eight times less carbon dioxide than cement production [1]. The process requires twice less energy compared to the manufacture of Portland cement [1]. It seems that geopolymers have low emissions of CO2, SO2, and NOX. In spite of the mechanical resemblances, the chemical and morphological differences in this material create challenges when comparing it to ordinary Portland cement (OPC), particularly in light of the current standards.

The term "geopolymers", concerning aluminosilicate binders, was introduced in the late 1970s, and developed by the French scientist and engineering prof. Joseph Davidovits in 1978 [2]. Geopolymers are inorganic [3] and belong to the family of alkali-activated materials, which, unlike cementitious materials, require alkalis to harden. They are also amorphous aluminosilicate materials with three-dimensional frameworks of SiO4 and AlO4 tetrahedra that can be produced via alkali activation even at low temperatures (20 and 120 ◦C, according to Davidovits [1]) and at low pressure. Geopolymers are synthesized via geopolymerization. The mineral raw materials dissolve in the alkaline environment at room temperature or higher. As a result, an amorphous phase and three-dimensional aluminosilicate structure are formed. The mechanisms of geopolymerization are not fully understood at present, but most hold that the geopolymer binding process can be presented in three main stages [4–8]:


**Citation:** Issa, T.M.M.; Sitarz, M.; Mróz, K.; Rózycki, M. ˙ Geopolymers—Base Materials and Properties of Green Structural Materials. *Mater. Proc.* **2023**, *13*, 43. https://doi.org/10.3390/ materproc2023013043

Academic Editors: Tomasz Tracz, Tomasz Zdeb and Izabela Hager

Published: 9 March 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/).

• Gelation/Polycondensation: Polymerization and precipitation of the system, i.e., the formation of a three-dimensional network of silico-aluminate that forms a geopolymer.

The production of a hard geopolymer involves mixing aluminosilicate powder that meets specific requirements for particle size distribution and specific surface area with an alkali solution (such as NaOH, KOH, or waterglass), resulting in the formation of a gel-like substance that solidifies quickly [9–11]. This alkali-activated material boasts exceptional physical, chemical, and mechanical properties, including low density, micro- and nanoporosity, high mechanical strength, thermal stability, fire resistance, and chemical resistance. Geopolymer materials allow non-polluting production and do not emit toxic gases or fumes. They can be reinforced, for example with carbon fibers, which provide excellent adhesive properties to the reinforcement.
