Numerical simulations of permeability of plain and blended cement pastes

Abstract

The durability of a concrete structure is principally dependent on its permeability, which governs the rate of transport of fluid through the concrete pore network. This paper presents a numerical model based on the lattice Boltzmann method (LBM) to calculate the saturated permeability of cement pastes, including blended cement pastes, wherein cement is replaced partially by an inert filler. The LBM uses a discretized form of the Boltzmann equation to simulate the pressure-gradient induced flow of a fluid through a porous microstructure. The accuracy of the algorithms implemented in the LBM are verified against analytical solutions of permeability of simple geometries. A microstructural model is used to generate three dimensional (3D) microstructures of cement pastes. The effects of liquid-to-solid ratio (l/s), degree of hydration of cement (DOH), filler content, and physical properties of the cement and filler (e.g., PSD: particle size distribution, affinity towards nucleation of hydrates on their surfaces) are evaluated. The simulations are verified against experimental measurements and numerically derived values of permeability published in literature. Results obtained from the simulations show that in both plain and blended pastes, the permeability increases and decreases monotonically, though in a nonlinear manner, over several orders of magnitude, in relation to the effective capillary porosity and the DOH, respectively. At equivalent DOHs, pastes consisting of finer anhydrous (i.e., cement or filler) particles have lower permeability compared to those prepared with coarser particles. This is attributed to the improved packing of finer particles at the time of mixing, which increases the likelihood of 3D percolation of solid phases and, ultimately, results in a higher connected volume of solid phases and a poorly connected capillary pore space within the microstructure. It is shown that in blended pastes, those prepared using a filler with a greater affinity towards nucleation and growth of hydrates on their surfaces have a higher volume of percolated solid phases, and, hence, lower permeability.