Effect of preparation parameters on properties of metakaolin-based geopolymer activated by silica fume- sodium hydroxide alkaline blend

In this study, metakaolin was activated using an alkali activator containing silica fume and sodium hydroxide to obtain a metakaolin-based geopolymer. Silica fume was used to partially substitute metakaolin (0 to 25 wt% of metakaolin). Enough reactive silica was introduced using silica fume, which was added to sodium hydroxide solution before being mixed with metakaolin. The Na2O/Al2O3 molar ratio was adjusted in the range of 0.8 to 1.2 by varying the sodium hydroxide concentration, and the geopolymer paste specimens were cured at three different temperatures, i.e., 60, 70, and 80 °C. The specimens were analyzed by X-ray diffraction, Fourier transforms infrared spectroscopy, scanning electron microscopy, mercury intrusion porosimetry, energy dispersion spectroscopy, and compressive strength measurement techniques. The results revealed that geopolymer-zeolite composites were produced in the presence of less than 10 wt% silica fume and Na2O/Al2O3 molar ratios higher than 1. When the SF content was raised above 10%, a significant increase in the formation of the geopolymer phase developed, resulting in increased compressive strength. Mercury intrusion porosimetry measurements show the geopolymer composite with 10 wt% silica fume and a Na2O/Al2O3 molar ratio of 1. It contained large zeolite crystals embedded in its macroporous geopolymer matrix and had a pore size of 400 nm. Maximum compressive strength of 33 MPa was achieved at 15 wt% silica fume, Na2O/Al2O3 molar ratio of 1, and curing temperature of 70 °C.