Calcium-free geopolymer as a stabilizer for sulfate-rich soils

Abstract The expansion and heaving of sulfate-rich soils is a long-lasting challenge for soil stabilization with calcium based stabilizers, such as lime and cement. It is caused by the formation of an expansive mineral, ettringite, when calcium and water are introduced to a sulfate concentrated soil. This study aims to explore the feasibility of using calcium-free geopolymer, which is an inorganic aluminosilicate cementitious material, as a stabilizer for sulfate-rich soils. To this end, metakaolin based geopolymer (MKG) was used to stabilize a synthetic lean clay premixed with 1000, 5000 and 10,000 ppm gypsum (equivalent to the sulfate concentration of 565, 2825 and 5650 ppm, respectively). The clay samples were stabilized with MKG at 8 and 13 wt.%, and their mechanical properties and swelling potential were determined through unconfined compression test and water bath soaking, respectively. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffractometer (XRD) tests were performed to further examine the microstructural and mineralogical characteristics of the clay samples. In addition, control sample sets, including unstabilized clay, lime- and cement-stabilized clay, were prepared and tested for providing a reference baseline to evaluate if MKG is an effective stabilizer for sulfate-rich soils. The unconfined compressive strength, Young's Moduli and failure strain of the clay samples were significantly increased after the stabilization with MKG. The swelling strain of the samples stabilized with the geopolymer was much lower than that of 4% lime stabilized counterparts after soaking in water. Geopolymer gels, but no ettringite crystal, were observed in geopolymer stabilized clay samples, as confirmed with the SEM–EDX results and XRD spectra. The results from this study confirmed that calcium-free geopolymer can be an effective stabilizer for sulfate-rich soils without excessive expansions. Using higher concentrations of geopolymer and geopolymers synthesized from industrial wastes is worth being studied in the future to further develop geopolymer-based stabilization technique for sulfate-rich soils.