Microstructure of geopolymer accounting for associated mechanical characteristics under various stress states

•We clarify mechanical characteristics of geopolymer under various stress states.•The SiO2/Na2O controls geopolymer silicon Q4(mAl) coordination structures.•The Q4(mAl) dominates stress-strain curves and failure modes for geopolymer.•Geopolymer mechanical properties with Q4(4Al) proportion correlates positively.•Friction-induced strength drops as the failure mode changes from split to shear.

Geopolymer was prepared with various SiO2/Na2O mole ratios and mechanical tests and microstructural analyses are performed to investigate how the constituents affect its mechanical behavior in distinct stress states. Laboratory results reveal that the SiO2/Na2O ratio affects the polymerization by influencing the formation of silicon Q4(mAl) structures. The proportion of Q4(4Al) correlates positively with the mechanical characteristics of geopolymer, and the proportion of Q4(2Al) correlates negatively with the mechanical characteristics of geopolymer. The proportions of Q4(mAl) affect the stress–strain curve and the failure modes of geopolymer under various confining pressures. Three types of stress–strain curves with different peak strengths and plastic deformations are obtained. Incomplete polymerization generates a geopolymer with an imperfect microstructure, which determines the plastic deformation while unloading. Polymerization of a geopolymer affects its apparent cohesion and friction angle. However, the friction-induced strength declines drastically when the failure mode changes from the split mode to the shear mode.