Reactive Transport Modelling of the Aggregate Degradation During ASR

In the context of massive concrete structure ageing, such as nuclear power plant containments or hydroelectric dams, the study of cementitious material durability is of great interest. In particular, the Alkali-Silica Reaction (ASR) can drastically reduce the durability of such structures. A reactive transport model (species transport and chemical reaction) of ASR able to simulate the progressive aggregate dissolution, the silica gel precipitation and its localization depending on the aggregate characteristics (chemical composition, diffusion properties, size, and morphology) is proposed. The objective is to consider mainly physical measurable parameters and thermodynamic constants. The ion transport is given by Fick's second law of diffusion and the geochemical system models the aqueous complexation and solid reactions. The thermodynamic equilibrium of chemical processes is assumed, except for the dissolution of the reactive silica, that is modeled by a kinetic reaction. Two main reaction products of ASR are considered, a low Ca/Si ratio C-S-H and an expansive alkali-silica gel. The application focuses on the cement paste and aggregate interaction. The results fit well with the experimental observations where the ASR gel forms inside the aggregate particle and the C-S-H precipitates at the interface between paste and the particle. The effect of aggregate composition and particle size on the overall ASR kinetic and gel precipitated localization is investigated.