Physical and chemical processes in alkali-activated aluminosilicates at high temperatures and their effect on functional properties: A critical review

Abstract

Alkali-activated aluminosilicates (AAA) are considered a potential environmentally friendly alternative to traditional concrete. Previous studies also showed their better high-temperature performance as compared with cement-based composites. In this paper, a comprehensive assessment of AAA's behavior at high temperatures is presented. A detailed analysis of physical and chemical processes taking place in AAA in a high-temperature range is performed at first. The findings gathered make then it possible to summarize how exposure to high temperatures changes AAA's microstructure and phase composition and how it affects macroscopic properties. The great potential of AAA for a broad range of applications requiring high-temperature resistance is confirmed. The advantages of AAA stem mainly from the fact that their structure is stable even without water molecules (or OH⁻ groups) and further from the ability to self-heal during geopolymerization, which is enabled both by temperature-induced geopolymerization and, at higher temperatures (typically above 800 °C), by sintering. Mortars with high-temperature resistant aggregate and suitable fibers can be considered as the optimal form of AAA, as pastes exhibit high shrinkage during high-temperature exposure. The critical analysis of extensive datasets reveals some gaps in the current state of knowledge, such as the lack of a consolidated understanding of phase transformations and microstructure-property relationships at high temperatures. Additionally, certain aspects remain unclear, among them the suitability of using low-melting or natural fibers that would be able to create paths for water release during heating from the dense matrix, the optimal Si/Al ratio, or the optimal activator solution.