Multiscale thermo-elastic modeling of hydrating binders for waste confinement

Alkali-activated materials based on ground granulated blast furnace slag (GGBFS) are investigated as low-carbon hydraulic binders for immobilizing long-lived, low-activity nuclear waste (LL-LA) containing graphite. This study presents a multiscale approach to characterize the early-age behavior of sodium hydroxide-activated GGBFS mortars and ordinary Portland cement (OPC) mortars incorporating graphite. At the mesoscale, a Representative Volume Element (RVE) models the matrix and graphite inclusions, enabling the calculation of thermo-elastic deformations, hydration-induced chemical shrinkage, and the evolution of elastic modulus with hydration degree. At the macroscale, a 3D finite element simulation evaluates thermal gradients and early-age deformations, highlighting greater endogenous shrinkage and lower initial stiffness in alkali-activated mortars compared with OPC. The presence of non-hydrating, highly conductive graphite significantly alters heat dissipation, thereby reducing early-age stresses. This methodology provides a robust framework for optimizing the performance of confinement matrices for nuclear waste applications.