Modification of lead-influenced basic magnesium sulfate cement: Performance enhancement and mechanism

Addressing the performance degradation of Basic Magnesium Sulfate Cement (BMSC) caused by Pb(II) is important for its application in solid waste treatment. While previous studies demonstrated the adverse effects of Pb(II) on BMSC performance, the underlying mechanism and effective modification strategies remain underexplored. This study systematically compares two modification approaches for Pb(II)-affected BMSC: increasing the citric acid (CA) to Pb molar ratio (from 0.11 to 1) and incorporating 517-type basic magnesium sulfate whiskers (0.5–5 wt.%). The setting time, compressive strength, hydration phases, leaching toxicity, and microstructure were systematically evaluated. Increasing CA content (n CA :n Pb = 1) improved the 28-day compressive strength of Pb-influenced BMSC (5 wt.% Pb) from 20.2 MPa to 40.4 MPa but delayed initial setting to 11.9 h with negligible 1-day strength. In contrast, modification with 517 whiskers achieved superior performance: 1 wt.% whiskers increased compressive strength by 74.78% at 28 days, while 5 wt.% whiskers further raised it to 50.4 MPa. The Pb leaching concentration was reduced from 1.2287 mg/L to 0.3361 mg/L. This enhancement mechanism is attributed to 517 whiskers not only serves as nucleation sites that promote processes associated with 5·1·7 phase formation but also functions as an immobilizing agent that rapidly removes Pb(II) during the early hydration stage. This prevents Pb(II) from primarily reacting with SO₄²⁻, CA⁻, and OH⁻, thereby enhancing the development of 5·1·7 phase crystals.