Influence of mix design parameters on geopolymer mechanical properties and microstructure

This paper reports on the influence of the four mix design parameters (Si/Al, water/solids ratio (w/solids), Al/Na and H₂O/Na₂O) on the mechanical properties and microstructure of metakaolin geopolymers. A series of geopolymers test specimens measuring 50 mm cubes were prepared by adding different concentration and amounts of sodium silicate solution to metakaolin. The compressive strengths of hardened geopolymers suggest that Si/Al ratio should have the most significant impact on the mechanical properties but the contributions of w/solids and Al/Na cannot be segregated from the contribution of Si/Al. The compressive strength results showed that H₂O/Na₂O did not seem to have a significant impact. W/solids ratio has been found to influence the density, porosity and microstructure of the geopolymers. However, it has been suggested that it is not the chief factor affecting these properties. Micro-structural observations using SEM support the trend that above-mentioned observations as Si/Al greatly influence the microstructure of geopolymers and that the other three factors do have a certain minor influence on the morphology and porosity in the geopolymers. In order to quantify the impact of mix design parameters on compressive strengths, a statistical analysis was performed which also suggested that Si/Al is the most vital parameter affecting the geopolymer compressive strengths. In summary, all the four mix design parameters studied in this work had a certain influence on microstructure and mechanical properties of geopolymers although Si/Al appeared to be the most significant. It was also suggested that unlike OPC system, in geopolymer system water-to-solids ratio is not the chief factor governing compressive strengths.