Evaluation of Hydrated Lime Stabilization of Sulfate Bearing Expansive Soils

Calcium-based stabilizers such as calcium oxide (lime) have been used extensively to improve the soil properties of expansive soils. However, in recent years, it has been reported that the presence of sulfate in lime stabilized soil caused abnormal volume changes in expansive soils due to the formation of the secondary minerals: ettringites. Ettringites are known as the highly expansive crystalline minerals. The increasing sulfate heave problems in lime treated expansive soils prompted an important research need in such areas. This research focuses on the understanding of the behavior of the lime treated soils in the absence and presence of sulfate in the soil and to evaluate the effectiveness of ground granulated blastfurnace slag (GGBS) on the lime-induced heave of sulfate bearing clay soils. In this study, an expansive clay with low sulfate level (640 ppm) was used as a control soil and lime was used as an additive for the treatment of this soil. Three different sulfate concentrations: 2000, 5000 and 10000 ppm were used to evaluate the effect of sulfate on the lime treated soils. In order to eliminate the harmful effect of sulfate in the lime treated soils, ground granulated blastfurnace slag was used. Atterberg limits, linear shrinkage, swell, compressibility, consolidated undrained triaxial CU, and cyclic swell-shrink tests were conducted on the natural, and the lime treated soils with the absence and presence of sulfate. Three different curing times (0, 30 and 365 days), three different mellowing times (1, 2 and 3 days) and two different temperatures (25, 40C) were used to evaluate the behavior of the lime treated soils in the presence and absence of sulfate. Test results revealed that lime, in the absence of sulfate, is very effective in reducing the plasticity and the swell potential and increasing the strength of the expansive soil. However, the presence of 5000 and 10000 ppm sulfate concentration in the limetreated soils caused the plasticity and the swell potentials to increase abnormally due to the formation of the ettringite minerals. High concentration of sulfate eliminated the remedial effect of lime in expansive soil and caused the swell to increase. The swell potential of the lime-treated soil with 10000 ppm sulfate concentration became three times higher than the control soil’s swell potential. Test results showed that the formation of the ettringite minerals in the lime treated soil with 5000 and 10000 ppm sulfate concentration was further accelerated with the increase in temperature (40C). Consolidated undrained triaxial, CU tests, showed that the shear strength of the lime treated soils decreased with the increase in the sulfate concentration and curing time. In the absence of sulfate, the effective stress path of the lime treated soil followed the same trend as overconsolidated clay. However, in the presence of sulfate, the effective stress path of the lime treated soil with 10000 ppm sulfate concentration became similar to that of a normally consolidated soil. The shear strength of the lime treated soil subjected to 10000 ppm sulfate decreased dramatically at 365 days curing time. The statistical ANOVA analysis results also verified the experimental findings that with the increase in sulfate concentration and the curing time, the sulfate concentrations and the cell pressures had statistically significant effect on the shear strength. The detrimental effect of sulfate in the lime treated soils was eliminated by adding slag into the lime treated soil. Addition of 6% slag into the lime treated soils prohibited the formation of the ettringite minerals and the increase in the swell potential values was prevented. In the presence of 6% slag, the swell potential of the lime-treated soil with 10,000 ppm sulfate concentration decreased from 8% to 1% whereas the lime-treated soil with 5000 ppm sulfate concentration showed no swelling. In the scanning electron micrograph of the lime treated soil subjected to 10000 ppm sulfate solutions, the growth of the ettringite minerals was easily observed.