Sodium silicate-free geopolymers as coating materials: Effects of Na/Al and water/solid ratios on adhesion strength

Geopolymer-based coatings exhibit good fire inhibition, temper resistance, and anticorrosion properties. In this study, coal fly ash-based geopolymers were investigated as coating materials. Two variables, namely, Na/Al and water/solid ratios, were used to study the effect of the chemical composition of geopolymer on adhesion strength, setting time, microstructure, and thermal stability (up to 800 °C). Maximum adhesion strength (3.8 MPa) was achieved using Na/Al and water/solid ratios of 1.0 and 0.33, respectively. The coatings attained their maximum strength in 3 days at 60 °C, and additional curing time up to 6 months did not affect adhesion strength. No direct or indirect relationship was found between setting time and adhesion strength. The amount of water notably altered adhesion strength, setting time, and microstructure of the polymers. Microstructural investigations revealed formation of mesoporous polymeric structure, unreacted fly ash, and zeolite-like fibrous morphologies. A new and simple method for the quantitative analysis of geopolymer was developed using Fourier transform infrared spectroscopy (FTIR). FTIR peak-fitting method showed that geopolymer yield was between 50% and 70%, which was comparable to the result of acid-leaching method. The coating formulations were thermally inactive up to 800 °C with a mass loss of 11–12% mostly because of dehydroxylation. Na/Al and water/solid ratios had no effect on the thermal properties of the geopolymer.