Refractory Metakaolin Geopolymers: Achieving High Mechanical Strength and Near-Zero Shrinkage After Exposure to 800°C

Abstract

Metakaolin-based geopolymers are candidate binders for refractory and fire-resistant components because they combine ceramic-like thermal stability with low-temperature processing. Their use is still restricted by a trade-off: Formulations that develop high strength shrink substantially near 800°C, while formulations that remain dimensionally stable are mechanically weak. This study addresses that trade-off by combining refractory particle reinforcement (alumina, bauxite, zircon, and ground refractory waste) with the systematic control of alkali concentration (10 and 20 wt.% K₂O), soluble silicate content (molar ratio SiO₂/K₂O, Ms of 0.50 and 1.25), and heating rate (10°C/min and 20°C/min). Compressive, flexural, and shear strength were assessed before and after exposure to 800°C and correlated with dilatometry, thermal analysis, and microstructure. The system with 20 wt.% K₂O and Ms of 1.25 reached 111 MPa in compression and 16 MPa in flexure after heating, but shrank by up to 2.55%, whereas 10 wt.% K₂O combined with the refractory waste exhibited a linear shrinkage up to 0.06%. The reversible $\propto$ and $\beta$ quartz transition by the refractory waste at 573°C was identified as the mechanism that compensated matrix shrinkage, and the same particle reduced the shrinkage of the high-strength system to 1.53% while retaining 97 MPa in compressive strength. These relationships define a quantitative design window for geopolymers-based components in which dimensional tolerance at 800°C is a design constraint, such as molds and tooling for casting and injection process.