Effects of potassium-silicate, sands and carbonates concentrations on metakaolin-based geopolymers for high-temperature applications

Abstract

Geopolymers can be optimized to have interesting physical characteristics, which are influenced by the raw materials and additives utilized in their formulations. For this reason, various formulations of two metakaolin-based binders with filler additions of different natures, i.e., sands and carbonates, were tested. The aluminosilicate source used is a commercial metakaolin (Si/Al = 1.44). The feasibility of the consolidated samples and their thermal behaviors were evaluated. The viscosity tests of the fresh geopolymer pastes indicated that their setting times are controlled by their potassium concentrations. The evaluation of the thermal stabilities of consolidated samples was carried out before and after heat treatment at 1000 °C by means of compressive tests. Geopolymers with low potassium concentrations showed mechanical strengths that were 16% greater (37–43 MPa) than geopolymers with high potassium concentrations, both before and after heat treatment. The exposure to different conditions did not influence the final mechanical properties of the geopolymers. XRD analysis of the geopolymers was performed and the crystallization of kalsilite was found in samples high potassium concentrations. Different mechanisms during geopolymerization, specifically the kinetic and thermodynamic mechanisms of geopolymers with high and low potassium concentrations, respectively, can explain the diverse mechanical responses of the geopolymers at room temperature, while the presence of kalsilite can explain the differences of geopolymers at high temperatures. Hence, it is possible to develop fire-resistant geopolymers by controlling the concentrations of potassium and fillers.