Applicability of Ammonium Chloride Attack for Simulating Long-Term Leaching in CAC-Based Materials

To assess the applicability of ammonium chloride ( NH 4 Cl ) solution as an accelerant for simulating the long-term leaching of calcium aluminate cement (CAC)–based materials, this study systematically investigates the chemophysical degradation pathways induced by NH 4 Cl attack. Although natural leaching in water is a slow, diffusion-controlled process, exposure to a NH 4 Cl solution was found to trigger a rapid, reaction-controlled degradation, a fundamental mechanistic shift confirmed by kinetic modeling. Advanced characterization revealed further significant divergences. The NH 4 Cl attack induces the formation of atypical secondary products (Friedel’s salt) and alters the evolution of primary hydrates, leading to a complex formation–recrystallization pathway for gibbsite rather than simple dissolution. Furthermore, 29 Si nuclear magnetic resonance ( Si 29 NMR) analysis showed that the calcium alumino-silicate hydrate (C─(A)─S─H) gel formed in the NH 4 Cl environment has a distinct structure, with a longer mean chain length and greater aluminum incorporation, resulting from a complex interplay of decalcification-driven formation and acid attack–driven degradation. These findings demonstrate that although NH 4 Cl is a highly effective accelerator, the induced chemical pathways and resulting material structures differ profoundly from those in natural leaching. This work provides a critical mechanistic framework for understanding the accelerated testing of CAC, highlighting both its utility in rapid assessment of chemical resistance and the crucial considerations needed when interpreting its results for long-term durability prediction.