The effect of alkali and Si/Al ratio on the development of mechanical properties of metakaolin-based geopolymers

Statistical analysis of a systematic series of geopolymers with varying alkali type (sodium and potassium) and Si/Al ratio after 7 and 28 days ageing has been used as a basis for observing the development of mechanical properties with time. Minimal change in the compressive strength of specimens was generally observed in specimens of different alkali or between 7 and 28 days of ageing. However, mixed-alkali specimens with high Si/Al ratio exhibited significant increases in strength, while pure alkali specimens displayed decreased strength. The development of Young's modulus of geopolymers between 7 and 28 days was observed to be dependent on alkali, with the Young's moduli of Na-specimens decreasing at low Si/Al ratio, but increasing at high Si/Al ratio, while K-specimens exhibited the opposite effect. Mixed-alkali specimens all exhibited nominal change in Young's moduli, without any significant effect of Si/Al ratio being observed.