Ammonium chloride for accelerated leaching of calcium aluminate cement paste: Effects of concentration and temperature

This study investigates the feasibility and temperature-dependent applicability of NH 4 Cl solution as an aggressive agent for the accelerated leaching of Calcium Aluminate Cement (CAC) paste. To achieve this, some hardened CAC paste specimens were immersed in NH 4 Cl solutions of varying concentrations (0.1–2.0 M) at two distinct temperatures: 20 ℃ and 60 ℃, which respectively represents the pre- and post- conversion states. The degradation of the specimens was comprehensively monitored through mass loss, phase composition (XRD-Rietveld and DTG), microstructural evolution (MIP) and solution chemistry (pH and Ca 2+ concentration). Results show that, there includes two distinct, temperature-governed degradation pathways of hardened CAC paste in NH 4 Cl solutions. At 20 ℃, NH 4 Cl induces a continuous and profound degradation, the rate and severity of which are directly proportional to the solution concentration. This is characterized by a holistic leaching mechanism, where the dissolution of the primary CAC hydrates leads to the concurrent physical detachment of all solid phases. At 60 ℃, the degradation was rapid but self-limiting, and it was governed by a selective leaching mechanism, where the initial concentration-dependent dissolution of the converted C 3 AH 6 is followed by the formation of a stable gibbsite-rich residual framework that arrests further degradation. Higher NH 4 Cl concentrations not only accelerate this initial dissolution but also promote the formation of this framework by maintaining a lower pH. This study validates NH 4 Cl solution as an effective accelerated leaching agent for CAC. However, it critically demonstrates that the mechanism being accelerated is fundamentally dependent on the temperature. At 20 ℃, it accelerates a comprehensive matrix collapse, while at 60 ℃, it accelerates the selective leaching of the converted paste. These findings are crucial for developing reliable, temperature-aware accelerated testing protocols for CAC-based materials.