Solidification/stabilisation behaviours of Zn2+ in magnesium potassium phosphate cement: Experiments and density functional theory study.

The solidification/stabilisation behaviours of Zn2+ in magnesium potassium phosphate cement (MKPC) have not been thoroughly investigated. Herein, a series of experiments and a detailed density functional theory (DFT) study were conducted to investigate the solidification/stabilisation behaviours of Zn2+ in MKPC. The results showed that the compressive strength of MKPC reduced with the addition of Zn2+ because the formation of MgKPO4·6H2O (the main hydration product in MKPC) was delayed with the addition of Zn2+, as discovered by the crystal characteristics, and because Zn2+ exhibited a lower binding energy in MgKPO4·6H2O compared to Mg2+, as revealed by DFT results. Additonally, Zn2+ had little influence on the structure of MgKPO4·6H2O, and Zn2+ existed in MKPC as the formation of Zn2(OH)PO4, which was decomposed in the range of around 190-350 °C. Moreover, there were a lot of well-crystallised tabular hydration products before the addition of Zn2+, but the matrix was comprised of irregular prism crystals after adding Zn2+. Furthermore, the leaching toxicity of Zn2+ of MKPC was much smaller than the requirements of Chinese and European standards.

Atomistic insights into structure evolution and mechanical property of calcium silicate hydrates influenced by nuclear waste caesium.

The fundamental mechanisms underlying the influence of nuclear wastes on concrete properties remain poorly understood, especially at the molecular level. Herein, caesium ions (Cs+) are introduced into calcium silicate hydrates (CSH) to investigate its effect using molecular dynamics simulation. Structurally, a swelling phenomenon is observed, attributed to the CSH interlayer expansion as Cs+ occupies larger space than Ca2+. The diffusion of interlayer water, Ca2+ and Cs+, following an order of water > Cs+ > Ca2+, is accelerated with increasing Cs+ content, owing to three mechanisms: expanded interlayer space, weakened interfacial interaction, and loss of chemical bond stability. Mechanically, the Young's modulus and strength of CSH are degraded by Cs+ due to two mechanisms: (1) the load transfer ability of interlayer water and Ca2+ is weakened; (2) the load transfer provided by Cs+ is very weak. Additionally, a "hydrolytic weakening" mechanism is proposed to explain the mechanical degradation with increasing water content. This study also provides guidance for studying the influence of other wastes (like heavy metal ions) in concrete.