Metakaolin-based geopolymers as consolidation products for the conservation of historic mortars

Recent advances in conservation science have highlighted geopolymers as sustainable alternatives to traditional binders for the repair of historic mortars; however, their use as consolidants has so far received limited attention. This study investigates the development of low-viscosity, metakaolin-based geopolymer formulations specifically designed for consolidation of deteriorated mortars. Sodium- and potassium-activated systems were synthesized using alkaline silicate and hydroxide solutions and systematically evaluated in terms of gel formation, chemical stability, workability, injectability, and mechanical performance. A comprehensive experimental program, including integrity tests, pH and electrical conductivity monitoring, setting time, shrinkage, mineralogical and microstructural analyses, and injection trials on simulated degraded mortars, was carried out. The results show that stable geopolymeric gels can be obtained at high liquid-to-solid ratios, combining adequate fluidity for injection with setting times compatible with on-site application requirements. Potassium-based formulations consistently outperformed sodium-based ones, exhibiting superior injectability, higher compressive strength, and enhanced chemical stability in aqueous environments. Microstructural observations revealed good adhesion and homogeneous distribution of the consolidant within both siliceous and calcareous aggregates, with calcium incorporation confined to interfacial zones and no evidence of deleterious secondary phase formation. Overall, the findings confirm the strong potential of metakaolin-based geopolymers, compatible and environmentally sustainable consolidants, for the preservation of historic mortars.