Towards realistic microscale simulation of reactive transport in cement pore space using advanced LBM
Concrete is the most ever used synthetic material by humans. For infrastructure or for super structures such a material produces 5% to 10% of world carbon emissions (https://cee.mit.edu/onbalance/2011/april), this actual percentage comes from the process of manufacturing cement which is the binder constituent of the concrete. The limestone breaks down into Carbon dioxide and lime. This heavy carbon footprint emerges from the huge production of cement, for the strategic aim to decrease this percentage, one must think of increasing the durability of concrete. Before pursuing this objective, the study of the nature of this material and the influence of transport phenomena in its pores in determining its age is a necessity. Concrete is a porous material partially saturated with water in the pore space and may contain aggressive ions (e.g. Chloride, CO2, Sulphates) making it vulnerable to chemical attacks from such ions resulting in decreasing resistance to degradation. These ions transferred into the solid matrix both through pores and microcracks (< 0.1 mm in width) (Abyaneh et al. 2016) acting as channels extending the reach of such ions into the concrete and thereby accelerating deterioration. Microcracks evolve due to structural and thermal loading, cycles of wetting/thawing etc. and are ubiquitous in concrete and unlike larger cracks they cannot be controlled by the presence of steel reinforcement. Transport due to microcracks prevails over that of capillary pores. However, their local and integral effect on transport is not well understood until today.
Previous models were limited to sound concrete or large cracks (> 0.1 mm) and thus these models are only of limited use for realistic cement pastes which inevitably contain microcracks. Furthermore, only simplified Lattice Boltzmann models have been used to investigate transport in resolved cement paste pore spaces to model combined multi-phase and multi-component advective-diffusive transport.
In this work, we propose a state-of-the-art approach based on advanced LBM variants using the VirtualFluids software package, which is a research code developed in our group that runs in parallel on CPUs and GPUs, to simulate the details of transport phenomena in hardened cement paste (HCP) such as permeation, diffusion taking into account adsorption desorption isotherms, multiphase flow, dynamic contact angle between solid and fluid phases and its effect on migrants transport. Our model is based on the Cumulant LBM, proposing an advanced collision scheme overcoming usual cons of previous LB models.
The microstructure used in the simulation is a bending-cracked sample obtained using μ-CT tomography. The reconstructed three-dimensional geometry is a greyscale voxel-based image, each greyscale value represents the attenuation of the intensity of the electromagnetic waves penetrating the sample i.e. different values means different materials, a pre-processing scheme is done for the segmentation of void and solid phases by analysing the greyscale informative histogram, which describes the relative voxel count against greyscale value.