Mechanical properties of sodium and potassium activated metakaolin-based geopolymers

In this study, a set of mechanical properties of geopolymers, synthesized by alkali (NaOH or KOH) activation of metakaolin and SiO2 mixture, were characterized at ambient temperature. Samples with K/Al or Na/Al atomic ratios equal to 1, Si/Al atomic ratios in the 1.25–2.5 range and H2O/Al2O3 molar ratios of 11 or 13 are cured at 80 °C for 24 and 48 h before characterization, to determine effect of Si/Al ratio and curing time on the structure and mechanical properties of geopolymers. The structure of synthesized geopolymers characterized using XRD, NMR, SEM, and density measurements was correlated to their mechanical properties, including compressive strength, Young’s modulus, hardness, and fracture toughness. The results of this study suggest a strong effect of Si/Al ratios (in the 1.5–2 range), density, and microstructure on the maximum strength, Young’s modulus, and hardness of geopolymers. There were also notable differences in strength between samples cured for 24 and 48 h, suggesting that the degree of geopolymerization reaction also plays important role in mechanical properties of this new class of inorganic polymers.