Triaxial mechanical properties and microstructure of Tianjin clay stabilized with fly ash-based geopolymer

The triaxial mechanical properties and microscopic mechanisms of fly ash-based geopolymer-stabilized Tianjin clays with different fly ash (FA) contents, alkaline activator (AA) contents, curing times and confining pressures were investigated via triaxial and scanning electron microscopy (SEM) tests. Based on the triaxial test results, compared with the unstabilized clay, the stabilized clay exhibited a steeper stress–strain curve, a greater peak strength and pronounced strain softening behavior. A significant increase in cohesion (from 4.18 kPa to 64.5 kPa) and a slight reduction in the internal friction angle (from 30.3° to 28.6°) occurred after geopolymer stabilization. The stiffness, peak strength and residual strength of stabilized clay generally increased with increasing FA content, AA content, curing time and confining pressure. An FA/clay ratio greater than 0.1 and an AA/FA ratio greater than 0.6 were needed to achieve high strength at ambient temperature. The stabilized clay exhibited a significant strength improvement after 28 d and had a relatively high long-term strength. SEM results revealed that the chemical reactions between FA and AA led to the formation of sodium aluminosilicate hydrate (N-A-S-H) gel, which strengthened the bonds, filled the voids and reduced the porosity of the clay. As a result, the overall stiffness and strength of the stabilized clay improved. SEM analysis revealed that with a higher FA/clay ratio, a higher AA/FA ratio or a longer curing time, the geopolymerization process was more pronounced, leading to increased formation of the N-A-S-H gel and resulting in a more densely stacked and stronger bonded structure.