Sensitivity of Selected Colorado Soils to Form Ettringite/Thaumasite When Treated with Calcium-Based Stabilizers and When Soluble Sulfates Are Available

The sensitivity of five Colorado soils, which were selected to represent the variety of soils likely to be treated with calcium-based chemical stabilizers such as hydrated lime or Portland cement, to the formation of potentially expansive minerals is evaluated using thermodynamic principles based on Gibb?s free energy and direct measurements. Phase diagrams are used to predict the threshold levels of soluble sulfates that favor the formation of the expansive mineral ettringite, which is most widely blamed for deleterious expansion in sulfate bearing soils treated with calcium-based stabilizers. The results of the thermodynamic, phase diagram evaluation are checked against direct measurement of ettringite using differential scanning calorimetry (DSC), and the results agree well. The conclusions derived from this research are that, as expected, mineralogical differences among the soils affect the threshold level of soluble sulfates that trigger the development of ettringite and that the presence of soluble silica and the form of alumina present have a dominant effect. The research validates previous research that a safe lower limit of soluble sulfates is approximately 3,000 ppm as long as additional soluble sulfates do not migrate into the soil. A methodology is presented for using the DSC to directly assess the threshold level of sulfates for a particular soil. However, refinement of the method is a key goal of on-going research. The DSC method to assess the potential of a specific soil to react deleteriously with a calcium based stabilizer when sulfate content in the soil is above a threshold level should be implemented by the Colorado Department of Transportation (CDOT). This will require the purchase of a Q-2000, TA Instruments or equivalent DSC by the CDOT Materials and Geotechnical Branch.