Modelling calcium leaching kinetics of cement asphalt paste

Calcium leaching is an important durability problem for cement asphalt (CA) mortar used in ballastless slab track which is under the coupled action of train-induced dynamic load and rainwater erosion. In this paper, a model based on the calcium mass conservation and the thermodynamic equilibrium of calcium was developed to describe the calcium leaching of CA paste in both deionized water and ammonium nitrate solution. In this model, the spatial–temporal distribution of solid calcium content in skeleton, porosity and diffusivity, which are directly or indirectly relevant to the calcium concentration in pore solution, were considered. Finite difference method was utilized to solve this model, and a three layer Crank–Nicolson differential scheme was applied to improve the stability. The model was experimentally verified using the Ca/Si molar ratio in solid and the calcium leaching rate characterized by SEM–EDX and potentiometric titration respectively. It was found that asphalt content does not change the spatial distribution of solid calcium in CA paste. Asphalt affects the leaching rate by reducing solid calcium concentration and changing the activation energy of micro-pore wall. Accelerating factors by ammonium nitrate solution were experimentally determined and predicted by the model.