An Experimental Study on the Effect of Desulfurized Gypsum on Geopolymer-Solidified Soil

Soft soil foundations need to be reinforced because of their low bearing capacity and susceptibility to deformation. Ordinary portland cement (OPC) is widely used in foundation treatment due to its strong mechanical properties. However, the production process for OPC curing agents involves high energy consumption and significant CO2 emissions. Given these problems, this paper proposes a fly ash-slag-based geopolymer to replace OPC curing agents, which can solidify soil while reducing OPC consumption. Another issue is that variability in environmental conditions influences the strength of soil solidified with fly ash-slag-based geopolymer, leading to subpar mechanical properties. However, by adding desulfurized gypsum as an admixture, the rich SO42− can react with Ca2+ and active silicate in the geopolymer to form Aft, thereby improving the mechanical properties. In the experiment, desulfurized gypsum is added as an admixture to a fly ash-slag-based geopolymer curing agent, and the resulting solidified soil is investigated through various macroscopic and microscopic tests. These tests include unconfined compressive strength measurements, water stability tests, scanning electron microscopy analyses, and X-ray diffraction tests. The results of these tests are combined with the response surface method to optimize the alkali–solid ratio, the modulus of the alkali activator, and the amount of desulfurized gypsum to 0.6%, 0.809%, and 15.96%, respectively. On this basis, an optimal mixing ratio was proposed and applied to form a geopolymer-solidified soil. The compressive strength and microstructure of this soil were then investigated using the single-variable method. An unconsolidated undrained triaxial test was performed on geopolymer-solidified soils of different curing times to investigate their shear performance. The water stability test was carried out to explore the influence of soaking time on the strength of solidified soil. Through microscopic observation, it was found that the fly ash-slag-based geopolymer generated significant amounts of (N, C)-A-S-H and C-S-H in solidified soil. With the addition of desulfurized gypsum, soil particles become filled with Aft and the solidified soil becomes more brittle instead of plastic, resulting in a significant increase in compressive strength. In addition, the cohesion and internal friction angle increase with curing time. With the increase of soaking time, the softening effect of long-term water soaking reduced the strength of the solidified soil.