Microstructural origins of cement paste degradation by external sulfate attack

• A new microstructure model simulates reaction-induced expansion in sulfate attack.
• Constrained AFt grows at the expense of a series of less stable Al-phases.
• Localized deformation and the onset of damage are predicted.
• Spatial distribution of monosulfate influences the severity of damage.
• The model predicts mitigation of damage by adding Mg or carbonates.

A microstructure model has been applied to simulate near-surface degradation of portland cement paste in contact with a sodium sulfate solution. This new model uses thermodynamic equilibrium calculations to guide both compositional and microstructure changes. It predicts localized deformation and the onset of damage by coupling the confined growth of new solids with linear thermoelastic finite element calculations of stress and strain fields. Constrained ettringite growth happens primarily at the expense of calcium monosulfoaluminate, carboaluminate and aluminum-rich hydrotalcite, if any, respectively. Expansion and damage can be mitigated chemically by increasing carbonate and magnesium concentrations or microstructurally by inducing a finer dispersion of monosulfate.