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.% K 2 O), soluble silicate content (molar ratio SiO 2 /K 2 O, 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.% K 2 O 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.% K 2 O combined with the refractory waste exhibited a linear shrinkage up to 0.06%. The reversible and 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.