Effect and Mechanism Analysis of Phosphate on the Enhancement of Copper Tailings Activity

Copper tailings are large-volume and highly polluting solid wastes. They exhibit low pozzolanic reactivity and require activation to be used as supplementary cementitious materials (SCMs). While alkali metal salts are commonly used for chemical activation, the role of anion species, particularly phosphate (PO43−), remains underexplored. This study compares the activation of copper tailings using two inorganic salts containing phosphate anions—sodium tripolyphosphate (STPP) and sodium hexametaphosphate (SHMP)—and investigates the mechanism of phosphate activation. The results reveal that 0.075 wt.% STPP and SHMP increase the 28-day activity index of copper tailings by 74.48% and 66.34%, respectively. The cumulative heat release for the copper tailings–cement paste is 202.54 J/g with STPP and 186.32 J/g with SHMP. STPP proves to be a more effective activator. PO43– anions enhance the pozzolanic reactivity of copper tailings by reducing particle size. The capture of Ca2+ ions by PO43– anions delays the hydration induction period, increases peak heat release, and promotes additional C-S-H gel formation, thereby improving hydration and compressive strength. These findings support the sustainable use of copper tailings as cost-effective and eco-friendly SCMs in cement.