Water dynamics in cement paste : insights from lattice Boltzmann modelling

Transport properties are often used as indicators of durability because water underpins most of the degradation mechanisms of concrete structures. The objective of this thesis is to understand the link between the microstructure and the permeability of cement paste, the binder phase of concrete. To achieve this goal, the lattice Boltzmann method is used to simulate the flow through three-dimensional model cement paste microstructures generated with the hydration modelling platform μic. First, it is confirmed that standard lattice Boltzmann (LB) models fail to predict the permeability and the water isotherms of cement paste. This is because the calcium silicate hydrate (C-S-H) has a complex and uncertain structure and is usually, out of computational necessity, treated as an impermeable solid. The LB model is consequently extended with an effective media approach to incorporate the flow through the nano-porous C-S-H. Accordingly, to calculate the permeability of cement paste, the C-S-H is assigned an intrinsic permeability. It is found that when the capillary porosity is completely saturated with a fluid (either water or gas), the calculated intrinsic permeability is in good agreement with measurements of gas permeability on dried samples (10−17−10−16 m2). However, as the water saturation is reduced, the calculated apparent water permeability decreases and spans the full range of experimentally measured values (10−16 −10−22 m2). Thus, the degree of saturation of the capillary porosity is likely the major cause for variation in the measurements of the permeability of cement paste. Further, it is found that the role of the weakly-permeable C-S-H, omitted in earlier modelling studies, has a non-linear effect on the permeability of cement paste and is critical at a low capillary porosity and / or low capillary saturation. Finally, to calculate the water isotherms of cement paste, the LB model is extended from isothermal to non-ideal fluids as described by an equation of state. Using a novel method, the C-S-H is assigned an intrinsic permeability and effective wetting properties. It is found that, in agreement with experiments, the calculated water isotherms show two main steps, the first corresponding to the capillary pores and the second to the smaller gel pores.