Modelling decalcification of cement paste in ammonium nitrate solution

Leaching process of cement-based materials is relevant for the long-term durability of cement based barrier materials used in radioactive waste disposal systems. Ca leaching changes the properties by reducing the pH, increasing the porosity which leads to the variation of transport properties. This process is extremely slow under environmental conditions. To accelerate the leaching kinetics, the use of ammonium nitrate (NH4NO3) solution becomes popular. The use of ammonium nitrate (NH4NO3) solution to accelerate the leaching kinetics has become popular because of the extremely slow nature of the degradation process under environmental condition. In this study, a one-dimensional diffusion-based transport model was proposed to simulate the leaching process of saturated hardened cement paste in contact with a NH4NO3 solution. The model helps to better understand the transient state of leaching which is difficult to be observed by experimental work. This model is based on macroscopic mass balances for Ca in aqueous and solid phases which are linked together by applying solid-liquid Ca equilibrium. The model only considers the leaching of portlandite and calcium silicate hydrates (C-S-H) which are the main hydrated phases in typical CEM I Ordinary Portland cement paste. The hydration during leaching is not taken into account in the model because of the limited time duration of the experiment. Besides the prediction of the leached depth, portlandite and C-S-H contents, and the amount of leached Ca, the model also enables to simulate the variation of water permeability over the domain at different immersion periods in NH4NO3 solution. The model is verified by accelerated leaching experiments in 6 mol/l NH4NO3 solution on CEM I cement paste samples. Preliminary verification with experimental results shows a good agreement.