Hydrated Lime Stabilization of Sulfate-Bearing Vertisols in Texas

Abstract

Sulfate-bearing subgrade soils treated with calcium-based stabilizers often experience heaving problems (three-dimensional swell) caused by chemical reactions with the sulfate or sulfide minerals. Two research questions were addressed: ( a) what sulfate content results in deleterious chemical reactions using traditional (no mellowing) lime stabilization and ( b) how effective mellowing, double lime application and increased moisture content are at reducing swell in high-sulfate soils. To determine what concentrations of sulfate are too high for stabilization with lime in Texas, a soil from the Vertisol order was selected for three-dimensional swell measurements. This soil did not contain any detectable sulfates. Two different compounds were added to the soil: sodium sulfate (Na2SO4) and gypsum (CaSO4.2H2O). The sulfates were added to individual samples at concentrations of 0; 1,000; 2,000; 3,000; 5,000; 7,000; and 12,000 parts per million (ppm). The samples were then subjected to a three-dimensional swell test for a minimum of 45 days. Results of systematic swell experiments revealed that sulfate contents up to 3,000 ppm could safely be treated with traditional lime stabilization. Coarse-grained sulfates take longer to swell, but still swell and form deleterious reaction products. Mellowing is effective at treating sulfate concentrations up to at least 7,000 ppm, higher molding moisture contents (2% above optimum moisture) reduce swell better than optimum moisture, and single application of lime reduces swell better than double application. With systematic laboratory experiments, empirical field observations of sulfate limits presented by other investigators were confirmed.

Mitigating the Expansive Behavior of Chemically Treated Soils

“…The method has been reported to be successful in mitigating swelling in sulfate-bearing soils. This method was applied by Harris et al (2004) to eastern Texas soils doped with known amounts of sulfate, and treated with various percentages of hydrated lime. Mellowing periods were varied up to 3 days, in order to find the proper combination of mellowing period and sulfate content.…”

“…The applicability of this method in practice comes from practical and cost considerations, which limit the duration of the mellowing stage. The method has been successfully applied to eastern Texas soils doped with sulfate compounds, and shown to be cost-effective in field applications (Harris et al 2004). Hence it was decided to access its potential applicability to lime-treated coal mine spoils.…”

Assessment of Sulfate Concentrations in Water Used During Chemical Stabilization and Its Potential Impact on Sulfate Induced Heave

“…When the pH is [10.5, the excess moisture and sulfate, can facilitate the exchange of calcium and aluminum forming ettringite (Eq. 1, Harris et al 2004):…”

Performance assessment of stabilised/solidified waste-forms

“…At typical temperatures (25°C) the pH must be above 10, and a source of water has to supply as much as 26 moles. Additionally, a source of aluminum, sulfur and calcium is also required (Harris et al, 2004). Ettringite is hardly detectable in concretes stored in dry climates, but can be observed in the void space after a short time (6 months) when exposed to alternating cycles of wetting and drying.…”