The use of calcium-based stabilizers such as calcium oxide (lime) in sulfate-bearing clay soils has historically led to structural distress because of the formation of a mineral called ettringite and possibly thaumasite. In trying to control the damage associated with such formations, engineers have attempted to determine a threshold level of soluble sulfates-a quantity that is relatively easy and quick to measure at which significant ettringite growth and, therefore, structural distress occurs. This is indeed a complex problem related to not only soil composition but also construction methods, availability of water, ion migration, and the ability of the void structure to accommodate the expansive mineral growth. Unfortunately, experience alone and rules-of-thumb based on experience are not sufficient to deal with this complex issue. Thermodynamic geochemical models of the lime-treated soil can be used as a first step toward establishing thresholds for problematic levels of soluble sulfates for a specific soil. A foundation for the model development is presented, and two soils are compared to illustrate their sensitivities to ettringite growth on the addition of lime. Because the model predicts ettringite growth on the basis of site-specific properties, the model can be used to assess the potential amelioration effects of soluble silica.