Microstructural and non-destructive investigation of the effect of high temperature exposure on ground ferronickel slag blended fly ash geopolymer mortars

Abstract

It is important to understand the high temperature response of a newly developed construction material in order to assess the effect of fire exposures. The effects of different percentages of ground ferronickel slag with fly ash and different temperature exposures on geopolymer mortars were evaluated by the changes in mass, cracking behaviour , compressive strength , microstructure and ultrasonic pulse velocity after exposure to temperatures up to 1000 °C. Higher residual strength and lower voids and cracks were found in geopolymer mortars containing 50% ground ferronickel slag. The improvement is attributed to the production of sodium magnesium aluminosilicate hydrate ( N - M -A- S -H), which was confirmed by the EDS and XRD analysis. The residual compressive strengths of geopolymer mortar with 50% ground ferronickel slag were 76, 65, 51, 30 and 29 MPa after 2 h of exposure at 200, 400, 600, 800 and 1000 °C, respectively. The XRD results showed a significant increase in the number of crystalline peaks and a decrease of amorphous content at 1000 °C. The amorphous content of the geopolymer with 50% ground ferronickel slag decreased from 79.6% at ambient temperature to 75.5%, 73.7% and 53.1% at 200, 600 and 1000 °C, respectively. The differential variation in higher and lower frequency components of the ultrasonic signal was associated with the residual strength of these specimens. They revealed that geopolymer mortar with 50% ground ferronickel slag had the least amount of pores and cracks. Overall, geopolymer mortar containing 50% ground ferronickel slag showed better performance against high temperature exposure compared to the other mixes. • Ambient-cured GFNS-fly ash geopolymer mortars were exposed to high temperatures. • Higher residual strength was found for the use of 50% GFNS with fly ash. • GFNS improved the residual properties by producing N - M -A- S -H gel. • Crystalline peaks increased and amorphous contents decreased significantly at 1000 °C. • Ultrasonic tests showed the least amount of cracks in the mortar with 50% GFNS.