Mineralogy of Soil Susceptible to Sulfate Attack after Stabilization

The mineralogy of a soil susceptible to sulfate attack was investigated in detail, with particular attention paid to the sulfate and clay minerals. The soil from north Louisiana suffered sulfate attack when stabilized by Type I portland cement. A number of analytical techniques, such as x-ray diffractometry, thermal analysis (thermogravimetry and differential scanning calorimetry), Fourier transform infrared spectroscopy, scanning electron microscopy, and x-ray absorption near edge structure (XANES), were used. Thermogravimetry showed that 7.8% gypsum (CaSO4·2H2O) was present in the soil from the average of four samples. Quantitative x-ray diffractometry yielded a value of 12.9(PLUS-MINUS)1.1% for one soil. XANES indicated that all the sulfur in the soil was present as sulfate phases. Gypsum was found in all size fractions. The clay minerals, kaolinite (highest concentration), illite (intermediate concentration), and smectite (lowest concentration) could be identified with certainty only after the clay fraction was concentrated by removal of gypsum by acid dissolution followed by centrifugation. Kaolinite (5%) could be easily quantified by thermogravimetry but not other clay minerals.