Sr 2+ uptake and leachability study on cured aluminum-substituted tobermorite–OPC admixtures

Abstract

Synthetic calcium silicate hydrate (C-S-H), which is similar to 1.1 nm tobermorite mineral, has demonstrated capacity to selectively pick up Sr 2+ cations from mixed cationic solutions in the presence of 1000 times concentration of Na +, K +, Ca 2+ and Ba 2+ ions. The selective uptake of Sr 2+ from mixed cationic solution has been quantified as distribution coefficient ( K d) and decontamination factor ( D f). The mass balance data have been generated from the solution analysis for Ca 2+⇆Sr 2+ system after equilibration. The data show that ratio of the amount of Sr 2+ incorporated in the matrix to the amount of Ca 2+ released from the tobermorite is approximately equal to 1:1. The position of Al and Si in the aluminum-substituted tobermorite has been characterized by 27Al and 29Si magic angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. Because 1.1 nm tobermorite is a powder, experiments were carried out on monolithic samples solidified with various amounts of ordinary portland cement (OPC). Leach rate studies were performed to study the retention power of Sr 2+ in these monoliths. The leach rate of Sr 2+ decreases drastically due to the addition of 10–40% of tobermorite to the OPC. The Sr 2+ retention capacity is expressed in terms of meq/100 g of exchanger. The leach rate studies show that Sr 2+ retention by various tobermorite containing OPC blocks increases with increasing weight percentage of this 1.1-nm Al-substituted tobermorite, which may be used as an additive to make the waste form.