Mechanisms and modelling of antimonate leaching in hydrated cement paste suspensions

Antimonate (Sb(OH)6−) leaching in hydrated cement pastes (HCP) was investigated by adsorption isotherms with cement minerals, EXAFS, geochemical modelling and by synthesising the antimonate AFm end-member, Ca4Al2[Sb(OH)6]2(OH)12·3H2O. Antimonate forms inner-sphere complexes with portlandite and ettringite surfaces. Much stronger interaction of antimonate with monosulphate was most likely AFm solid solution formation, whereas the mechanism of strong sorption by C–S–H was unclear. Modelling Sb leaching from HCP containing ca. 1000 mg kg− 1 Sb and EXAFS analysis suggested the variable structure of calcium antimonate may explain the Sb leaching increase at decreasing pH or extended carbonation times. In HCP with lower Sb concentrations (ca. 300 mg kg− 1) leaching is lower probably because it is controlled by a combination of AFm solid solution and/or C–S–H adsorption that can ensure long-term stabilisation, whereas calcium antimonate precipitation appears sensitive to weathering reactions such as carbonation.