Effect of Diatomite and Silica Fume as Silica Sources on Compressive Strength and Shrinkage Percentage of Fly Ash–Based Geopolymer

This paper presents the effects of silica sources (diatomite and silica fume) on the shrinkage percentage and compressive strength of fly ash–based geopolymer containing silica sources at 0%, 5%, 15%, 15%, 20%, and 25% by weight, a curing temperature of 60°C, 80°C, H2O/dry binder of 0.36, 0.44 in a fixed Na2O/Al2O3 molar ratio of 1. Fourier transform infrared spectroscopy, X-ray diffraction, mercury intrusion porosimetry, scanning electron microscopy, and energy dispersion spectroscopy techniques were employed to investigate the physical and microstructural properties. The results demonstrated that the maximum compressive strengths of 28.7 and 22.3 MPa were attained through the addition of 15% by weight of silica fume and diatomite, respectively, in the alkali-activated fly ash–based geopolymer specimens at a curing temperature of 60°C. Increasing the incorporation of silica fume to 20% and 25% by weight results in noticeable shrinkage, but diatomite offers distinct advantages, including shrinkage resistance and enhanced moisture retention. This study compares three mixing methods: (1) wet mixing of two different silica sources in alkaline solutions with sodium hydroxide solution, (2) ultrasonic mixing of silica sources with sodium hydroxide solution, and (3) dry mixing of silica sources with fly ash. Ultrasonic mixing and dry mixing of silica sources in the production of geopolymer-based fly ash reduce shrinkage percentages and enhance compressive strength. Integrating silica sources via dry mixing (planetary milling) enhances the microstructure and achieves maximum compressive strengths of 40.42 and 36.8 MPa for 15% by weight silica fume and diatomite, respectively, at a curing temperature of 60°C, and reduces the shrinkage percentage of geopolymer specimens.