Linking early-stage dissolution kinetics to long-term network reorganization in metakaolin-based geopolymers

Metakaolin (MK)-based geopolymers represent promising low-carbon alternative binders to ordinary Portland cement (OPC); however, their real-time dissolution kinetics and phase-resolved geopolymerization mechanisms remain experimentally unresolved. While the recent Coarse-Grained Monte Carlo (CGMC) model has demonstrated predictive capability for the full geopolymerization sequence, from asymmetric Al and Si dissolution through Ostwald ripening-driven gel nucleation, network formation, and solidification, direct experimental validation at the nano- and microstructural level remains lacking. This study addresses this gap by providing multiscale, real-time experimental validation of the CGMC framework, quantifying dissolution rate, gel growth kinetics, and nano-to-microstructural evolution in hydroxide silicate (H/S) activated MK geopolymers using multiscale advanced characterization techniques. Solution chemistry with ICP-MS revealed rapid early-stage dissolution of Al and Si species, with rates of 0.605 and 4.697 μmol L −1 s −1 , respectively, followed by a progressive decline as the system approached supersaturation, marking the transition from dissolution-controlled kinetics to polycondensation. The higher Si concentration and decreasing Si/Al ratio suggested greater silicate mobility and preferential incorporation of Al into the developing N–A–S–H gel network. AFM-derived surface topography demonstrated continuous nanoscale surface smoothing and densification, with roughness parameters decreasing with age, consistent with Ostwald ripening–controlled gel growth and microstructural refinement. Concurrent improvements in thermal stability, stabilization of the Si/Al ratio, and a reduction in the Na/Al ratio reflected ongoing charge balancing and structural reorganization, ultimately resulting in the formation of a dense, chemically stable, and highly polymerized N–A–S–H gel network. These findings provide quantitative evidence linking dissolution kinetics, gel formation, and microstructural evolution, confirming the progressive development of a dense, chemically stable, and charge-balanced geopolymer network.