pH-controlled Cd2+ immobilization in cement paste using cadmium-containing wastewater as mixing water

This study evaluated the feasibility of using cadmium-containing wastewater (CCW) as mixing water for cement paste and elucidated the coupled relationship among hydration evolution, pore structure evolution, permeability variation, and Cd 2+ transport under different pH conditions. The results demonstrated that the initial pH of CCW significantly influenced cement hydration and pore connectivity, thereby controlling Cd 2+ transport and leaching behavior. Specifically, H + ions inhibited cement hydration and etched the hydration products, resulting in a porous structure while OH - ions accelerated the hydration and increased the amount of hydration products, refining the pore structure. It is worth noting that as the pH increases, the dominant Cd species gradually shifted from the highly mobile free Cd 2+ (which prevails under strongly acidic conditions, though minor hydroxy complexes like Cd(OH) + may also coexist) towards less soluble hydroxide complexes and ultimately to stable precipitates such as Ca 0.67 Cd 0.33 CO 3 , which further enhanced pore tortuosity and inhibited Cd 2+ migration. Furthermore, the Katz-Thompson model revealed the pH-regulated evolution of permeability-controlling pore structures, showing a strong linear correlation with Cd leaching (R 2 =0.979). Permeability-controlling pores shifted from bimodal distributions (1–10 μm and 0.003–0.01 μm) under acidic conditions to a concentrated micropore range (0.003–0.01 μm) under alkaline conditions. These findings established a predictive permeability-controlled framework that quantitatively links micro structure to leaching behavior, providing theoretical support for the safe utilization of CCW in construction materials.