Ettringite Formation in Lime-Treated Soils: Establishing Thermodynamic Foundations for Engineering Practice (05-2239)**
Dallas N. Little, Texas A&M Transportation Institute
Bruce E. Herbert, Texas A&M University
Sachin Kunagalli Natarajan, Texas A&M University System

The use of calcium-based stabilizers such as calcium oxide (lime) in sulfate-bearing clay soils has historically lead to distress due to the formation of a mineral called ettringite and possibly thaumasite. In trying to control the damage associated with such occurrences, engineers have attempted to determine a threshold value 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 not only to soil composition but also construction methods, availability of water, ion migration, and whether the expansive mineral growth can be accommodated by void structure. Unfortunately, experience alone and “rules-of-thumb” based on experience is not sufficient to deal with this complex issue. This paper describes how thermodynamic geochemical models of the lime treated soil can be used as a first step toward establishing problematic threshold levels of soluble sulfates for a specific soil. A foundation for the model development is presented, and two different soils are compared to illustrate their very different sensitivities to ettringite growth upon the addition of hydrated lime. Since the model predicts ettringite growth based upon site-specific properties, the paper also shows how the model can be used to assess the potential amelioration effects of soluble silica.