Sulfate ingress in Portland cement

The interaction of mortar with sulfate solutions leads to a reaction front within the porous material and to expansion. Thermodynamic modelling coupled with transport codes was used to predict sulfate ingress. Alternatively, “pure” thermodynamic models – without consideration of transport – were used as a fast alternative to coupled models: they are more flexible and allow easy parameter variations but the results relate neither to distance nor to time. Both transport and pure thermodynamic modelling gave comparable results and were able to reproduce the changes observed in experiments. The calculated total volume of the solids did not exceed the initial volume of the paste indicating that not the overall volume restriction leads to the observed expansion but rather the formation of ettringite within the matrix and the development of crystallisation pressure in small pores. The calculations indicate that periodic changing of the Na2SO4 solution results in more intense degradation.