Preparation of a geopolymer from red mud slurry and class F fly ash and its behavior at elevated temperatures

• Red mud slurry and fly ash-based geopolymers were successfully synthesized. • The RMFA geopolymer demonstrated strengths high enough for use in structural cement. • The RMFA geopolymer exhibited thermal stability up to 600 °C. • Recrystallization was found to begin above 600 °C, leading to decrease in mechanical strength. This study employed a synthesis method for a geopolymer sourced from red mud (RM) slurry and fly ash (FA) powder. Properties, including mechanical behavior, volume change, weight loss, and microstructural change, of the RMFA geopolymer in the temperature range of room temperature to 1000 °C were investigated. The geopolymer was successfully synthesized at 50 °C for seven days, followed by curing at room temperature and 40% relative humidity for an additional seven days. After curing for fourteen days, the RMFA geopolymer exhibited compressive strengths over 17 MPa. Mechanical tests found the material to retain its strength up to 600 °C. Although the mechanical properties remained relatively stable under 600 °C, weight loss and thermal shrinkage were observed in thermogravimetric analysis and dimensional measurements. These behaviors were attributed to the loss of free and structural water as well as the dehydroxylation reaction under heating. Above 600 °C, the mechanical properties, including compressive strength and Young’s modulus, dropped quickly, and the material showed rapid volume expansion. This observation was associated with the start of the breakdown of the geopolymer matrix, as well as recrystallization, as evident in the X-ray diffraction pattern. The RMFA geopolymer may be explored as an alternative material to ordinary Portland cement, thermal barrier material on substrate metal, thermal insulation material for large equipment, with environmental benefits including red mud and fly ash repurposing.