Influence of the addition of amorphous and crystalline silica on the structural properties of metakaolin-based geopolymers

Abstract

This paper analyzes the effects of crystalline and amorphous silica additions (15 wt%) on the geopolymers structures. The precursors were metakaolin with or without replacement of 15 wt% by an amorphous (rice husk ash) or crystalline (finely ground quartz sand) silica source. The reactivity of the raw materials (pozzolanicity index) was evaluated by the Modified Chapelle test and thermal analysis. Transmission electron microscopy (TEM), Infrared spectroscopy, and X-ray diffraction with Rietveld refinement were used to characterize the geopolymers. Both metakaolin and rice husk ash showed high pozzolanicity, whereas the ground quartz sand was a non-pozzolanic material. TEM analysis detected the formation of an amorphous binder (N-A-S-H gel) in the pure metakaolin-based geopolymer. The addition of the reactive rice husk ash raised the content of the Si-O-Si bond in the geopolymer binder. Using the ground quartz sand reduced the estimated amorphous content of the geopolymers. However, the samples displayed similar compressive strengths (from 52 to 56 MPa at 28 days), indicating that the chemical reactivities of the raw materials had little effect on the geopolymerization for silica addition of 15 wt%. It could be attributed to a non-reacted fraction of rice husk ash in such conditions. Still, the filler effect provided by the unreacted amorphous silica and quartz particles could contribute to the geopolymer properties.