Long-term behaviour of Portland cement blended with Pb: Persistent retardation effects from early hydration to 2 years

The integration of industrial solid wastes into binder formulations offers a promising strategy to support the decarbonization of cement industry. However, the presence of heavy metals, particularly lead (Pb), poses significant challenges. Pb has long been reported to retard the early hydration of Portland cement, prolonging the induction period and hindering the dissolution of clinker phases. While several mechanisms have been proposed, the underlying causes of this negative effect remain debated, and its long-term consequences are yet to be fully understood. This study investigates the hydration behaviour of Pb-blended Portland cement systems over a period of up to two years, with the aim of elucidating the persistence and mechanisms of Pb-induced retardation. Results show that at early ages (6 h), Pb is enriched as Pb/Ca hydroxides, which consume Ca2+ and OH−, inhibit the formation of Ca cation pairs on clinker surfaces, and inhibit silicates dissolution and C-S-H nucleation. Over time, the gradual dissolution of Pb/Ca hydroxides facilitates a progressive redistribution of Pb, transitioning from initial localized enrichment near cement particles to a more homogeneous dispersion within the cementitious matrix, as confirmed by WDS and EDS analyses. pH-dependent leaching tests and geochemical modelling further demonstrate that Pb retention is governed by C-S-H and portlandite, but decreases under highly alkaline conditions due to soluble Pb hydroxyl complex formation. Overall, the findings provide mechanistic insights into the dynamic evolution of Pb-Portland cement interactions and establish a scientific basis for assessing the long-term performance of Pb in cementitious systems.