Volcanic tuff as a sustainable precursor for geopolymer synthesis: optimization and microstructural insight

Abstract

The reactivity of precursors in geopolymers is strongly influenced by their mineralogical and chemical properties, in addition to the synthesis parameters. This study investigates the potential of Algerian volcanic tuff as a precursor for geopolymer synthesis, utilizing the Taguchi method to optimize key synthesis parameters for maximum compressive strength. An orthogonal L9 array was employed to optimize critical parameters, including NaOH concentration, silica modulus (Ms), curing temperature, and the solution-to-precursor (S/P) ratio. An orthogonal L9 array was employed to optimize critical parameters, including NaOH concentration, silica modulus (Ms), curing temperature (°C), and the solution-to-precursor (S/P) ratio. The Taguchi method, enhanced with Desirability Functions (DF), was used to determine the optimal conditions. While the highest compressive strength among the experimental trials (30.26 MPa) was achieved by mixture T9 (NaOH = 12 mol, Ms = 1%, 60 °C curing, S/P = 0.32), the DF model predicted a further optimum at 12 mol of NaOH, 1% of Ms, 80 °C curing, and an S/P ratio of 0.30, corresponding to a compressive strength of 35.23 MPa with a desirability score of 98.72%. This prediction was experimentally validated, yielding a compressive strength of 32.97 MPa, thus confirming the model’s reliability. Curing temperature emerged as the most significant factor, contributing 53.57% to the variability in compressive strength, with the Error/Residual term accounting for 5.5% of the total variation. Microstructural analysis of the optimal mixture (T9) revealed substantial N-A-S-H gel formation, resulting in a denser microstructure. Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS), and X-ray Diffraction (XRD) were used to validate the reaction mechanisms and structural evolution. Notably, an increase in both Ms content and curing temperature significantly enhanced compressive strength, particularly under elevated curing conditions. This study highlights the effectiveness of the Taguchi method in optimizing volcanic tuff-based geopolymer synthesis and offers valuable insights for the development of sustainable construction materials.