Numerical Modelling of Cement Grout Degradation in a Single Rock Fracture

The construction of infrastructures such as dams requires a stable ground for the foundations to lay on, which also have to be protected from erosion deriving from groundwater exposure. One way to achieve this is to grout the rock fractures present in the bedrock using cement. However, the continuous flow of groundwater will, over time, cause the deterioration of the grout by dissolving and mobilising the mineral phases resulting from the hydration process. This study aims to determine the variations in porosity and hydraulic conductivity induced by the dissolution of portlandite present in the grout under the influence of groundwater flow. Two different cases have been simulated in COMSOL Multiphysics®, one being a two-dimensional fracture with infinite extent, while the other is a three-dimensional geometry. Each model has two main components, one for the flow simulation and one for the transport. The COMSOL Multiphysics® modules used in this study are Darcy’s Law and Transport of Diluted Species in Porous Media. The software PHREEQC was used to determine the chemical species concentration for both the initial and boundary conditions. The results show that, at the end of 20 years, the mineral concentration decreases from the initial molarity of 1035.98 mol/m^3 to values lower than 20 mol/m^3, with the 2D model reporting the strongest variation. The pH values dropped from 12.951 to 8.65 or less. This leads to an increase in porosity by 3.45%, while hydraulic conductivity increases by 9.64%.