Pure model geopolymers present a promising potential for high-temperature binders for structural or coating applications. However, their well-known chemical stability, which can reach 1400 ​°C in the case of K-activated geopolymer, is accompanied by deleterious dimensional behavior, with significant shrinkage on heating. The work in the present paper focuses on the impact of tailored calcined argillite added to geomaterial formulations for fireproofing applications. Calcined argillite acts both as a filler and a reagent, with a supplementary delayed reaction activated by humidity and heat. K-activation of a mix of one-third metakaolin with two-thirds calcined argillite enables the shrinkage rate to be limited, and is accompanied by improved self-healing during viscous creep (allowing to preserve mechanical resistance at high temperature). This composite formulation preserves chemical stability, improves physical behavior, and maintains interesting mesoporosity for hydric properties. These different aspects are beneficial for a high temperature application.