Numerical modelling of degradation of cement-based materials under leaching and external sulfate attack

A finite element model is developed in this paper to simulate the degradation of cement-based materials when subjected to aggressive agents, including deionized water and sodium sulfate solutions. The proposed model includes three main modules, i.e. the ionic diffusion module, the chemical reaction module, and the damage quantification module. A modified Poisson-Nernst-Planck model is developed to model the diffusion of multiple ions in the ionic diffusion module. The cracking suction effect is accounted for the suction action caused by newly-formed unsaturated cracks with potential to accelerate the entire diffusion process. In the chemical reaction module, chemical interactions between ions and cement hydration products are solved based on a local equilibrium assumption. Different approaches in dealing with the simulation of the decalcification process of ill-crystallized Calcium Silicate hydrate (C-S-H) are discussed. The entire reactive diffusion mechanism is achieved by using the operator splitting approach to couple the ionic diffusion and the chemical modules. In the damage quantification module, the diffusivities of material are evaluated at the end of each time step based on the change of porosity and propagation of cracking. Two mechanisms of precipitation of hydration products, i.e. through-solution and topochemical reactions, are discussed in the light of their distinct contributions to the cracking propagation. The proposed model is applied to model the experiments reported in the literature and the computed results are also compared with those obtained from other available models. It is found that the results obtained from the proposed model agree very well with those from experiments and generate more accurate predictions than other models.