Evaluation of Self-Healing in Metakaolin-Based Geopolymers: Influence of Lime Content and Curing Environments

Abstract

This study investigates the self-healing performance of metakaolin-based geopolymers incorporating hydrated lime and polyvinyl alcohol fibers (PVA) fibers. Because metakaolin provides a low-calcium matrix, healing mechanisms can be evaluated in isolation, and the gradual addition of hydrated lime enables a controlled assessment of how calcium availability influences healing and strength recovery. In parallel, this study systematically examines how distinct curing environments—air, immersion, and wet–dry cycles—govern the kinetics and stability of healing products, providing and integrated view that remains limited in current literature. Healing efficiency was quantified using two numerical indices—Index of Strength Recovery (ISR) and Index of Crack Sealing (ICS) over a 60-day period, with ISR reaching 87% for the mixture with 20% lime and ICS values between 27% and 37% at 60 days under water exposures. The results reveal that curing conditions and lime content influence healing behavior. Notably, the lime-free mixture demonstrated substantial self-healing, particularly under cyclic exposure, achieving an ICS of 34% and an ISR of 78%, indicating the potential of autogenous healing driven by unreacted aluminosilicates and fiber interaction. Microstructural analyses confirmed the formation of sodium (calcium) aluminosilicate hydrate [N(C)-A-S-H] and carbonate phases and revealed the role of PVA fibers as nucleation sites. These insights advance the understanding of healing mechanisms in metakaolin-based geopolymer binders and support the development of durable, lime-free binders for civil infrastructure applications.