Uptake and retention of molybdenum in cementitious systems

Abstract

The uptake of molybdenum, present in aqueous cementitious environments in the form of the molybdate ion (MoO42-), by various cement hydration phases such as calcium-silicate-hydrates (C–S–H), monosulphate (AFm), ettringite (AFt), as well as hardened cement paste made from Ordinary Portland cement (OPC), was studied in batch-type sorption experiments under anoxic conditions. Uptake kinetics were generally fast, leading to sorption equilibrium in less than 30 days. In particular, a strong uptake of molybdate by AFm phases was observed as well as a distinct contribution of C–S–H phases to the molybdate retention in cementitious systems, the latter depending on the Ca/Si-ratio of the C–S–H and the alkali content in solution. In systems containing hydrogarnet, the neo-formation of a molybdate-bearing AFm phase was identified as an additional process contributing to molybdate retention. The findings have implications for selecting grouts for the immobilisation of radioactive waste streams containing Mo-93 and also enhance the data available on molybdate sorption and retention in cementitious systems.