Simulated microstructure and transport properties of ultra-high performance cement-based materials

Ultra-high performance cement-based materials expected to be used in nuclear waste containers were submitted to a leaching test in order to evaluate their long-term durability. Reactive powder concretes (RPC) were attacked by de-ionized water. Previous studies revealed a superficial degradation after leaching with a sound zone underneath an altered porous zone in which anhydrous silicates C₃S and C₂S were dissolved. To predict the long-term durability of RPC, the hydration rate of cement minerals, pozzolanic reactivity of silica fume, pore structure, and mechanisms of chemical reactions were needed. So first, the microstructure of RPC matrix was simulated using the NIST microstructural model. Then the transfer of Ca ions through percolating water was estimated using DIFFU-Ca, a model based on the local chemical equilibrium. This double modeling validates the damage process related to an instantaneous dissolution of anhydrous cement silicates at the degradation front which results in a higher connected pore space, and is in good agreement with experimental results. The long-term behavior is expressed as the depth of the altered zone, the leaching kinetics and the evolution of Ca concentration in the material.