Impact of reinforcement additives on physical and electrical properties of fly ash-based geopolymer materials

The utilization of carbon-based additives, generated from waste managed materials, to synthesize fly ash (FA)-based geopolymers with enhanced mechanical and electrical properties offers benefits in environmental protection and waste reduction. This study focused on preparing FA-based geopolymers at ambient conditions through alkali activation with a combination of NaOH-activated quartz (AQ) and water glass (Na₂SiO₃ solution). The weight ratio of FA:AQ in the FA/AQ geopolymer was kept at 1:1 (wt:wt). Carbon-based additives, including carbon fibers (CFs) and thermally stabilized microcrystalline cellulose (SMCC), were separately mixed to FA/AQ geopolymer paste in two proportions (1% and 3% wt/wt) relative to FA. The formulated geopolymers were analyzed physically using Fourier transform infrared spectroscopy (FTIR), UV/Vis spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and EDX. SEM analysis reveals the presence of voids and cavities in the neat FA geopolymer. However, integrating AQ into the FA-based geopolymer leads to significant matrix densification and reduced porosity, restricting ion mobility, resulting in high mechanical strength, and low electrical conductivity. Additionally, the enhanced compatibility of a higher percentage of CFs and SMCC (CFs(3%)@FA/AQ and SMCC(3%)@FA/AQ) with the geopolymer matrix forms dense, amorphous sodium aluminum silicate hydrate (N-A-S-H) links. This is confirmed by the increased compressive strength (11.1 MPa and 18.1 MPa) and higher intensities of SMCC's XRD patterns. SMCC(3%)@FA/AQ demonstrates the lowest electric and dielectric properties (σ = 1.4 × 10^-7 S/cm and ε' = 7 × 10⁴), indicating superior insulating properties. In contrast, the CFs(3%)@FA geopolymer matrix exhibits higher values (σ = 4.4 × 10^-6 S/cm and ε' = 2.5 × 10⁶) compared to other matrices after shielding AQ that interrupt the conductive pathways.