Hydrated Lime Stabilization of Sulfate-Bearing Soils in Texas (0-4240-2)

Sulfate-bearing subgrade soils treated with calcium-based stabilizers often experience heaving problems (three-dimensional swell) due to chemical reactions with the sulfate and/or sulfide minerals. This project focuses on determining the sulfate content at which the deleterious chemical reactions occur and on evaluating the effectiveness of alternative construction practices aimed at reducing swell in high-sulfate soils. These practices include extended mellowing, double lime application, and increasing field moisture contents. To determine what concentrations of sulfate are too high for stabilization with lime in Texas, a soil from the Vertisol order that contained no detectable sulfates was selected for three-dimensional swell measurements. Two different sulfate compounds were added to the soil: sodium sulfate (Na2S04) and gypsum (CaS04'2H2O). Samples containing sulfate concentrations of 0, 1000, 2000, 3000, 5000, 7000, and 12,000 parts per million (ppm) were then subjected to a three-dimensional swell test for a minimum of 45 days. Results of these systematic swell experiments revealed the following: (1) sulfate contents up to 3000 ppm can safely be treated with traditional lime stabilization; (2) coarse-grained sulfates take longer to swell and form deleterious reaction products; (3) mellowing effectively treats sulfate concentrations up to at least 7000 ppm; (4) higher molding moisture contents (2% above optimum) reduce swell better than optimum moisture; and (5) single application of lime reduces swell better than double application. This study, using systematic laboratory experiments, confirmed empirical field observations of sulfate limits presented by other investigators.