Measuring Sulfate in Subgrade Soil: Difficulties and Triumphs

Abstract

The accurate analysis of sulfate sulfur in subgrade soil is essential for road construction that involves calcium-based stabilizers (lime or cement). The objective was to determine if other tests give more reproducible results in a more timely fashion than Texas’s current sulfate test method (Tex-620-J), a gravimetric method. Literature review and interviews with commercial soil testing laboratories revealed three techniques to compare with Tex-620-J: ion chromatography, conductivity, and colorimetry/spectrophotometry. Soils were manufactured with known sulfate (gypsum and anhydrite) concentrations and sent to laboratories that performed gravimetric analysis and ion chromatography. Conductivity and colorimetric testing were performed in-house. Testing showed that Tex-620-J is not very precise, which creates the need for an unrealistic number of samples to obtain an accurate estimate of the sulfate concentration. To define the 95% confidence interval for true sulfate content to within ±10% of the true known value for a concentration of 5,000 ppm, Tex-620-J requires 43 tests; ion chromatography requires 14 tests. The colorimeter achieved the desired results in only one test, based on sulfate standard solutions. Results of this study revealed the difficulty with obtaining accurate sulfate measurements in the laboratory and indicated a few inexpensive pieces of equipment that can be used in both the field and laboratory settings that may yield better results.

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