Effects of sodium and calcium sulphates on volume stability and strength of lime-stabilized kaolinite

Lime treatment of sulphate-bearing clay soils has been shown to adversely affect their properties. In the current work, the effects of soaking untreated and lime-stabilized kaolinite samples in water and sodium and calcium sulphate on swelling and unconfined compressive strength are investigated and compared. Specimens were treated with 1, 3, or 5% lime and cured at 35 °C for 1, 7, or 28 days, followed by soaking in water or 10,000 ppm sodium or calcium sulphate solutions. The results show that soaking lime-treated kaolinite samples in sodium and calcium sulphate solutions promotes swelling, which is greatly influenced by the length of the curing period. Swelling is the result of the formation of ettringite with high affinity for water absorption, which results in a reduction in unconfined compressive strength. Ettringite forms in environments with high pH and active sulphates as small crystals which destroy the structure of stabilized soils due to expansion, particularly at the early stages of formation, given its high affinity for water absorption. SEM micrographs show ettringite crystals growing on and between clay particles.