Particle Size Analysis of Gypsic Soils

Gypsum is a common mineral in many dryland soils throughout the world. Soils with gypsum have unique properties that impact roads, building foundations, corrosion of metal, and concrete disintegration. One particularly challenging aspect of gypseous soils is determining their particle size distribution. Because of their softness (1.5-2.0 on the Mohs scale), gypsum particles can be broken during pretreatment using rolling pins and shakers. Because of their solubility (0.241g/100 ml), gypsum particles will dissolve during traditional particle size analysis involving sedimentation in water. In addition, the current particle-size method requires that gypsum be removed from the soil sample. Removal of gypsum from a soil with 90% gypsum, for example, will take 4 liters of water and 3 weeks of time, and leaves a sample of less than 1 gram. This is not a representative sample of what is found in the field. This research explores ways to prevent the dissolution of gypsum while conducting textural analysis by using low viscosity oil, saturated NaCl solution, saturated gypsum solution, and a combination of a saturated Na2SO4-gypsum solution. Although the low viscosity oil prevented gypsum dissolution, it was unsuitable because of problems with the drying of the oil-saturated sample, disposal, and cost. The NaCl saturated solution was unsuitable because it did not prevent gypsum dissolution as a result of ion pairing. The saturated gypsum solution was successful in preventing gypsum dissolution because of the common ion effect, but problems with clay flocculation can still occur. Current experiments are evaluating the combination of Na2SO4 and gypsum solution for preventing gypsum dissolution but not dispersing the clay.