Fresh properties, compressive strength and microstructure of fly ash geopolymer paste blended with iron ore tailing under thermal cycle

• A geopolymer is proposed based on fly ash blended with iron ore tailing.
• IOT decreases fluidity and prolongs setting time of geopolymer.
• 20% IOT leads to denser microstructure and higher surface hardness after thermal cycle.
• Less than 30% IOT improves the thermal resistance of geopolymer.

This study aims to broaden the application of iron ore tailing (IOT) in geopolymer. Fresh properties, residual strength, mass loss and microstructure evolution of geopolymer prepared by using fly ash as starting material blended with IOT and activated by sodium silicate and sodium hydroxide solutions were investigated after exposure to 3 and 7 heat-cooling thermal cycles at different target heating temperatures of 200 °C, 400 °C and 800 °C when fly ash was partially replaced by IOT at levels ranging from 0% to 30% with an interval of 10%, by weight. The experimental results uncover that IOT addition influences the workability and setting time of geopolymer especially when more than 20% of IOT is added. Under the effects of thermal cycles, the compressive strength decreases significantly especially after 7 thermal cycles. The loss in compressive strength increases as the cycle target temperature increases from 200 °C to 800 °C. The presence of IOT evidently improves the thermal resistance of geopolymer with the replacement level of IOT less than 30%. The reduced Ca(OH)2 and the associated formation of additional C-S-H evidences the role of IOT in the formation of C-S-H and supports the results of the increasing compressive strength. Replacing of fly ash with 20% IOT leads to a reduction of the porosity and microcrackings which results in much denser microstructure. Surface Vickers-hardness of geopolymer has also been optimized by IOT addition regardless of exposure target temperature.