Understanding the relationship between geopolymer composition, microstructure and mechanical properties

A mechanistic model accounting for reduced structural reorganization and densification in the microstructure of geopolymer gels with high concentrations of soluble silicon in the activating solution has been proposed. The mechanical strength and Young's modulus of geopolymers synthesized by the alkali activation of metakaolin with Si/Al ratio between 1.15 and 2.15 are correlated with their respective microstructures through SEM analysis. The microstructure of specimens is observed to be highly porous for Si/Al ratios ≤1.40 but largely homogeneous for Si/Al ≥1.65, and mechanistic arguments explaining the change in microstructure based on speciation of the alkali silicate activating solutions are presented. All specimens with a homogeneous microstructure exhibit an almost identical Young's modulus, suggesting that the Young's modulus of geopolymers is determined largely by the microstructure rather than simply through compositional effects as has been previously assumed. The strength of geopolymers is maximized at Si/Al = 1.90. Specimens with higher Si/Al ratio exhibit reduced strength, contrary to predictions based on compositional arguments alone. The decrease in strength with higher silica content has been linked to the amount of unreacted material in the specimens, which act as defect sites. This work demonstrates that the microstructures of geopolymers can be tailored for specific applications.