The Influence of Fly Ash-Based Geopolymer on the Mechanical Properties of OPC-solidified Soil

Silt is a common waste soil that is often treated by solidification with ordinary Portland cement (OPC). Geopolymer is a cementitious material formed by the polycondensation reaction of fly ash and other industrial waste materials in a non-high-temperature environment under alkali excitation. This study used fly ash-based geopolymer to partially replace OPC in silt solidification to reduce the use of OPC. The investigated replacement ratios of geopolymer for OPC were selected as 0% (control group), 5%, 10%, and 15%, and the water contents were selected as 28%, 30% (liquid limit of 29.6%), 32%, and 34%, respectively. To investigate the influence of curing time (7, 14, 28, and 60 d) on the mechanical properties of solidified silt, unconfined compressive strength tests and stress-strain curve study were conducted. To better understand how geopolymer and cement behave during the solidification of silt, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and mercury intrusion porosimetry were used to evaluate the morphology, elemental composition, mineral composition, and pore distribution of the solidified soils. The experimental results show that the silt solidified with geopolymer-cement had good performance. Under the same water content, as the proportion of cement replaced by geopolymer increased, the compressive strength of the solidified soil first increased and then decreased. When the silt had a water content of 30% and a curing period of 28 d, the strength of the solidified soil increased significantly (approximately 13.17%-51.42%). Microstructural analysis showed that more cementitious materials were produced in the soil solidified with geopolymer-cement. The geopolymer-cement wrapped around the soil particles and filled the voids, resulting in increased soil compactness and thus improving the silt strength.