Design of alkali-activated phosphorus slag binders for Cr(VI) Immobilization: Thermodynamic modelling and experimental insights

Abstract

Alkali-activated materials offer a low-carbon and cost-effective approach for heavy metal immobilization. In this study, alkali-activated phosphorus slag (AA-PS) mixtures were designed to immobilize Cr(VI). This research integrates thermodynamic modelling and experiments to investigate the reaction processes, immobilization mechanisms, and leaching behaviors. The results show that the hydration products of NaOH-activated PS include C-(N)-A-S-H gel, hydrotalcite, and strätlingite. Among them, hexagonal plate-like hydrotalcite is the key mineral phase to immobilize Cr(VI), which is first directly observed in a real AA-PS hydrated binder. A 6 % NaOH content achieves satisfactory compressive strength and high Cr(VI) immobilization efficacy exceeding 99.99 % even at 3 days. Under acidic conditions, the hydration products provide buffering capacity, and hydrotalcite can efficiently immobilize Cr(VI) when the inner pH is above 10.5. Meanwhile, CaCrO₄ can contribute to Cr(VI) immobilization when the pH is above 7. Only when the internal pH drops below 7, significant Cr leaching begins. These findings provide scientific basis for the value-added utilization of phosphorus slag and the immobilization of Cr(VI).