A thermodynamic analysis of cementitious materials for low-level radioactive wastes

Radioactive wastes resulting from scientific research, industries and healthcare activities require a proper treatment and storage procedures. As for Low Level Wastes (LLW), the aqueous solutions containing radioisotopes are often used as mixing water for the casting of cementitious products. Sulphates already present in the solution can react with hydrated constituents of the cement (i.e. C-S-H gel, portlandite, hydrated calcium aluminates), giving rise to the so called internal sulphate attack. After several years such a process can determine cracking with a correlated loss of stiffness and strength, compromising the integrity of the cement layer. To predict the behaviour of temporary disposal facilities along hundred years, mechanical models must be coupled with reliable simulations of the chemical reaction. As an alternative to kinetic formulations based on diffusion-reaction equations with an incremental formulation, computationally expensive and depending on numerous parameters difficult to estimate, in this communication a thermodynamic approach based on the minimization of Gibbs free energy of reaction is proposed, apt to provide in one step the final concentrations of the chemical species at the equilibrium.