Effects of ground granulated blastfurnace slag (GGBS) on the swelling properties of lime-stabilized sulfate-bearing soils

The use of calcium-based stabilizers such as calcium oxide (lime) in sulfate-bearing clay soils has historically caused structural distress due to the formation of ettringite, an expansive mineral which develops in the presence of sulfate, calcium, and aluminum compounds of clay fraction. In this work, a research was conducted to study the effectiveness of ground granulated blastfurnace slag (GGBS), an industrial by-product, for providing better stabilization of sulfate bearing soils. Laboratory tests were first performed on the lime-treated expansive soil containing different concentrations of added sulfate and then the same tests were repeated on the lime-treated soil, containing the same concentration of sulfate but this time adding 6% slag. Three different sulfate concentrations (2000, 5000, and 10,000 ppm) were used in the study and the compaction relationships, Atterberg limits, linear shrinkage and swell potentials were investigated. Test results indicated that the presence of sulfate in the soil resulted in abnormal increase in the plasticity and the swell potential of the lime-treated soil with 5000 and 10,000 ppm sulfate concentrations. The swell potential of the lime-treated soil with 10,000 ppm sulfate concentration became three times higher than that of the natural soil. The scanning electron micrograph of this higher swell potential soil confirmed the growth of the ettringite minerals. On the other hand, test results indicated that the use of lime with GGBS in stabilizing sulfate bearing clays produced significant improvements in the plasticity as well as the swell potential. Addition of 6% slag into the lime treated soil eliminated the harmful effect of sulfate in the soils. 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.