Interfacial-engineered Pb2+ sequestration in calcium sulfoaluminate hybrid binders: coupled precipitation–ettringite uptake for transport regulation and strength gain

Lead-bearing residues from mining and smelting pose persistent environmental risks due to the long-term mobility of Pb2+. Herein, we uncover a dual role of Pb2+ in calcium sulfoaluminate (CSA) binders governed by coupled interfacial retention and association with ettringite-related hydrates. CSA pastes incorporating 0–2.0 wt% Pb2+ equivalent were characterized by compressive strength, setting, N2 adsorption, isothermal calorimetry, chemical shrinkage, XRD/TG, SEM–EDS, XPS, leaching assessment, and thermodynamic simulations. Results indicate that Pb2+ initially undergoes hydrolysis and forms hydroxide-rich interfacial deposits that suppress ion transport and delay ettringite-dominated heat release, thereby retarding hydration and weakening early-age strength. With ongoing hydration, Pb2+ retention is reinforced by uptake associated with ettringite-related hydrates, accompanied by refined pore and reduced connectivity of coarse pores (50–100 nm). At 0.2% Pb2+, the fraction of pores >50 nm decreased by 26.5% and compressive strength rose by 10.3% at 120 d relative to the control. Across all tested dosages, Pb2+ was effectively immobilized in the CSA matrix, with fixation rates above 99.9% and leachate concentrations well below regulatory limits. These findings show that trace Pb can act as a functional hydration modulator in CSA binders while maintaining high immobilization efficiency, providing a mechanistic basis for designing CSA-based matrices for Pb-bearing waste. Pb2+ rapidly undergoes hydrolysis and forms hydroxide-rich interfacial deposits that reduce ionic flux and extend the CSA induction period. Pb retention is reinforced by uptake in ettringite-related hydrates, modifying their local chemistry and morphology and refining pore connectivity. At 0.2 wt% Pb2+ equivalent, early-age retardation converts into a 120 d strength gain (+10.27%) by suppressing coarse mesopores (50–100 nm). Pb is immobilized (>99.9%) through coupled interfacial retention and hydrate-associated uptake, and leachate Pb remains well below the hazardous-waste leaching threshold.