A reactive transport simulator for cement pastes

Abstract

In order to reduce the carbon footprint of cement and concrete, supplementary cementitious materials such as fly-ash or blast-furnace slag, are added to the formulation. However, these products modify the mechanical and chemical properties of cementitious products. To guarantee the long-term durability of these new formulations, we need to understand the long-term degradation mechanisms and how they relate to the evolution of the microstructure. Inside a porous medium, the different phenomena are coupled by the microstructure and its evolution. For example, in coupled simulation of drying and carbonation, the entry of CO2 and release of water will occur through the carbonated layer which has distinct properties from the non-carbonated core.

To solve this problem, we propose a new reactive transport implementation which can integrate a custom-made microstructure model adapted to the problem at hand. This model is used to compute the macroscopic parameters used in the governing equations. In our framework, we can integrate the advancement of the chemical reactions to follow the evolution of the microstructure. This feature has many strong numerical challenges and it leads to a robust and flexible reactive transport framework. We demonstrate its capabilities

through two set of simulations: the leaching of cement paste and the coupled carbonation and drying of cement paste.

A unified description of leaching is proposed for low and high pH, and low and high concentration of CO2 in the leaching solution. The different rates of degradation observed in the experiments are reproduced in our simulations and an explanation is proposed for each mechanism.

The main new feature of our carbonation model is the introduction of a two layer microstructure model to the differences between the non-carbonated and carbonated layer. Using a rigorous sensitivity analysis, we show that the water transport properties define the overall kinetics of carbonation. To obtain quantitative predictions, the capillary pressure and the relative transport properties must be computed accurately in the carbonated layer. Our flexible simulator can be used to perform successive iterations between the model and the experiments to improve the model and identify the experiments necessary to validate the model.