Exploring the aluminium paradox in calcium-aluminosilicate-hydrate decalcification.

Decalcification, a prevalent form of mineral dissolution, drives the long-term degradation of cementitious materials in civil and energy infrastructures and controls the weathering of silicates in Earth's crust. The impact of aluminium (Al) incorporation on the decalcification of complex minerals like calcium-silicate-hydrates, the predominant binding phase in concrete, remains debated. This study combines metadynamics simulations, density functional theory calculations, and experimental characterizations to unravel the atomistic mechanisms of Al governing calcium-aluminosilicate-hydrate (C-A-S-H) decalcification. We reveal a two-fold effect of Al incorporation stemming from aluminosilicate chain variations. Increasing Al at a constant Ca/(Si+Al) ratio distorts aluminosilicate chains, weakening Ca-O bonds and accelerating decalcification. Conversely, increasing Al at a constant Ca/Si ratio leads to longer aluminosilicate chains, enhancing Ca surface restraints and hindering decalcification. Additionally, the preferential dissolution of Na ions effectively suppresses the dissolution of Ca ions. Our calculated activation free energies quantitatively predict experimental C-(A)-S-H decalcification kinetics across various temperatures. This work provides a framework for tailoring the composition of complex minerals for controllable degradation and offers fundamental knowledge relevant to broader geological and environmental processes.