Numerical simulation of sulfate attack in cement based materials: Considering dynamic boundary calcium concentration

The coupling of sulphate attack and calcium leaching has been considered in various existing models. Whereas, the boundary condition in terms of calcium phases is commonly assumed as constant. In the present study, the runoff of calcium on the boundary is treated as a dynamic behaviour and accordingly, a flux-based boundary condition is proposed. Given this, the diffusion-reaction-damage model is established further to simulate the combined sulphate attack and calcium leaching in cement-based systems. By comparing the numerical results with experimental observations, the rationality of the proposed model has been fairly verified. Moreover, setting the boundary calcium concentration as zero or csatu has been found to either underestimate or overestimate the sulphate-induced degradation in cement-based systems, particularly within the region close to the exposed surface. The results obtained with the proposed flux-based condition are between them and are much more reasonable. Besides, under the coupling of sulphate attack and calcium leaching, the consumption of calcium ions in the formation of gypsum and ettringite is found much greater than the amount of runoff due to calcium leaching. This implies the dominant role of expansive cracking in such durability concerns. Through monitoring the cracking period for a certain clear cover, the result generated from the present study is between those modelled with the aforementioned two constant boundary calcium concentrations. This clearly confirms that fixing the boundary calcium concentration as a constant value will misestimate the service life of cement-based systems when subjected to the combined sulphate attack and calcium leaching.