Comparative study of silica fume and sodium silicate as replacement of active reactive silica in bauxite residue based geopolymer mortar

This study investigates the effects of using silica fume (SF) and sodium silicate (Na2SiO3) as replacements for active reactive silica in the production of geopolymer mortars made from bauxite residue. The geopolymers were produced by combining 0.45 ratio of binder liquid and varying concentrations (8 M and 10 M) of NaOH, while maintaining a constant ratio of binder to fine aggregate. The phase composition and molecular structure of the produced geopolymers were examined using X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) techniques. The XRD analysis revealed the existence of crystalline phases including iron oxide(alpha), titanium oxide(beta), sodium aluminum silicate(S), sodium aluminum silicate hydrate(2), and silicon oxide (q), among other compounds. The FTIR spectra confirmed the development of a geopolymeric gel and the existence of distinctive functional groups. The compressive strength tests demonstrated that the addition of silica fume and sodium silicate substantially improved the mechanical characteristics of the geopolymer mortars. Mixtures containing 10 M NaOH and higher slag percentages demonstrated superior compressive strengths. Among these mixtures, Mix B10 achieved the highest average compressive strength of 18.83 MPa. The average compressive strengths were compared by using Mix B1 and B2 as benchmarks. The compressive strength of mixtures B3, B5, B6, and B7 demonstrated significant improvements, particularly B7 which exhibited a remarkable increase of 151.23 % compared to B1. Blend B4, B8, B9, and B10 also exhibited substantial enhancements, with B10 displaying a 130.56 % rise compared to B2. The standard deviations demonstrated a significant level of consistency in the compressive strength measurements.