Lead retardation on cement hydration: Inhibition and re-acceleration of clinker dissolution

Lead-bearing solid wastes strongly retard the hydration of Portland cement. In this work, the effect of Pb(NO3)2 and PbCl2 on the hydration of Portland cement and tricalcium silicate (C3S) was investigated. The results show that both Pb(NO3)2 and PbCl2 prominently delay the end of the dormant period, the setting time of cement paste, and decrease compressive strength especially at lead dosages of 1%. Conductivity, pH, and elemental concentration in C3S suspension underlined a strong effect of lead on C3S reaction kinetics. XRD and SEM-EDS measurements indicated the formation of lead silicates and chloride (hydr)oxides. Alite dissolution is probably retarded by the formation of surface complexes between Pb(II) with silanol groups on clinker surface. The increase of pH and precipitation of less soluble Pb-hydrates lowers Pb concentrations and the inhibition induced by Pb(II).