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

Abstract

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.

Keywords: Accelerated diffusion; Mechanical degradation; Radioactive ions (Cs(+)); Swelling phenomenon.