A Review of the Thermal and Mechanical Characteristics of Alkali-Activated Composites at Elevated Temperatures

Alkali-activated materials (AAMs) are promoted as a sustainable alternative to ordinary Portland cement (OPC). They not only have excellent resistance to high temperatures and chemical corrosion, but they can also help to reduce greenhouse gas emissions and reduce energy consumption. Despite their superior resistance to high temperatures compared to conventional cement-based concretes, studies have indicated that AAMs still face challenges related to performance degradation under elevated temperatures. This paper systematically reviews and summarizes the thermal properties (i.e., thermal expansion, thermal stability, and thermal conductivity), mechanical performance, and deterioration mechanisms of various alkali-activated composite systems. The findings reveal significant variability in resistance to high temperatures among different AAM systems, originating from the diversity of precursor materials used. Generally, the strength deterioration of various AAMs below 400 °C is minimal or even slightly increased, while between 600 °C and 800 °C, the strength degradation is significantly accelerated. Upon reaching 800 °C, the rate of the strength deterioration of AAMs tends to stabilize, with some alkali-activated composites even exhibiting signs of strength recovery. After exposure to high temperatures of 800 °C, the retentions of the compressive strength and flexural strength of alkali-activated composites are in the ranges of about 20–60% and 20–40%, respectively. The degradation mechanisms at elevated temperatures primarily include crystalline-phase transformation, microstructural changes, and thermal incompatibility arising from the differing coefficients of thermal expansion between the matrix and the aggregates. Finally, this paper discusses effective strategies to enhance the resistance of AAMs to high temperatures and highlights both the opportunities and challenges for future research in this field.

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