Abstract

Alkali-activated fly ash-slag (AAFS) cured at ambient temperature has been considered as a promising eco-friendly alternative to traditional cement-based binders, offering reduced carbon emissions and enhanced durability. However, understanding how these materials respond to extreme conditions, particularly at high temperatures, remains a critical area of study. Therefore, it is essential to investigate this material for ensuring their reliability in fire-prone and high-temperature environments. This paper presents a systematic experimental study on microstructural and damage evolution in AAFS paste exposed to elevated temperatures (from 20 °C to 800 °C) in terms of changes in phase assemblage and pore structural characteristics as well as crack initiation and development, using a series of advanced characterisation techniques including backscattered electron microscopy (BSEM) coupled with energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Mercury Intrusion Porosimetry (MIP). Experimental results indicate that at temperatures up to 200 °C, minor shrinkage cracks appear due to water evaporation, while significant crack widening occurs between 200 °C and 600 °C as phase decomposition and thermal expansion mismatch accelerate damage. At 800 °C, viscous sintering leads to partial densification, reducing microcrack connectivity despite the extensive degradation of amorphous reaction products. EDS and XRD results reveal the decomposition of C-A-S-H and transformation of gels into more cross-linked N-A-S-H gels, accompanied by crystallisation of nepheline and gehlenite. This significantly changes the microstructural integrity of the matrix in AAFS paste. This study establishes a direct correlation between microstructural changes and damage evolution in AAFS paste, offering critical insights into its high-temperature/fire resistance applications. These results hold significant implications for advancing the use of sustainable, cement-free materials in fire-resistant infrastructure and heat-intensive environments.

Keywords

geopolymer, fire-resistance, microstructure, pore structure, damage mechanisms.

DOI

10.5703/1288284318147

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Microstructure Characterisation of Alkali-Activated Fly Ash-Slag Paste at Elevated Temperatures

Alkali-activated fly ash-slag (AAFS) cured at ambient temperature has been considered as a promising eco-friendly alternative to traditional cement-based binders, offering reduced carbon emissions and enhanced durability. However, understanding how these materials respond to extreme conditions, particularly at high temperatures, remains a critical area of study. Therefore, it is essential to investigate this material for ensuring their reliability in fire-prone and high-temperature environments. This paper presents a systematic experimental study on microstructural and damage evolution in AAFS paste exposed to elevated temperatures (from 20 °C to 800 °C) in terms of changes in phase assemblage and pore structural characteristics as well as crack initiation and development, using a series of advanced characterisation techniques including backscattered electron microscopy (BSEM) coupled with energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Mercury Intrusion Porosimetry (MIP). Experimental results indicate that at temperatures up to 200 °C, minor shrinkage cracks appear due to water evaporation, while significant crack widening occurs between 200 °C and 600 °C as phase decomposition and thermal expansion mismatch accelerate damage. At 800 °C, viscous sintering leads to partial densification, reducing microcrack connectivity despite the extensive degradation of amorphous reaction products. EDS and XRD results reveal the decomposition of C-A-S-H and transformation of gels into more cross-linked N-A-S-H gels, accompanied by crystallisation of nepheline and gehlenite. This significantly changes the microstructural integrity of the matrix in AAFS paste. This study establishes a direct correlation between microstructural changes and damage evolution in AAFS paste, offering critical insights into its high-temperature/fire resistance applications. These results hold significant implications for advancing the use of sustainable, cement-free materials in fire-resistant infrastructure and heat-intensive environments.