Analysis of Changes in the Microstructure of Geopolymer Mortar after Exposure to High Temperatures

Abstract

The inorganic structure formed at the stage of setting of the geopolymer binder ensures high durability of the material under high-temperature conditions. However, changes in the microstructure of the material are observed. The purpose of the study was to analyze changes in the structure of geopolymer mortar after exposure to high temperatures T = 200, 400, 600, 800, and 1000 °C. Mortars with a binder based solely on fly ash (FA) and mixed in the 1:1 ratio with a binder containing fly ash and ground granulated blast-furnace slag (GGBFS) were tested. The descriptions of their microstructures were prepared based on digital microscope observations, scanning electron microscope (SEM) observations, EDS (energy dispersive spectroscopy) analysis, and mercury intrusion porosimetry (MIP) porosity test results. Changes in the material due to high temperature were observed. The differences in the microstructure of the samples are also visible in the materials that were not exposed to temperature, which was influenced by the composition of the materials. Porosity increases with increasing annealing temperature. The distribution of individual pores also changes. In both materials, the proportion of pores larger than 1000 nm increases with the temperature increase. Moreover, the number of cracks and their width also increases, reaching 20 µm in the case of GGBFS. Furthermore, the color of geopolymers has changed. The obtained results extend the current state of knowledge in the field of changes in the microstructure of geopolymers subjected to high temperature.

Influence of Sintering Temperature of Kaolin, Slag, and Fly Ash Geopolymers on the Microstructure, Phase Analysis, and Electrical Conductivity

“…The crack healing ability of the slag geopolymer caused the reduction of crack lines in conjunction with the partial melting. This result was supported by the findings of Dudek et al [ 24 ].…”

The Influence of Sintering Temperature on the Pore Structure of an Alkali-Activated Kaolin-Based Geopolymer Ceramic

“…The kaolin-based geopolymer surface became glassy and glossy when sintered at 900 °C ( Figure 5 b). This microstructure change was attributed to moisture hydration and phase transformation, as reported by Dudek et al [ 17 ]. It can also be seen in Figure 5 b that the kaolin-based geopolymer ceramic samples sintered at 900 °C had a higher porosity, alongside cracks and voids.…”

Chloride Ions’ Penetration of Fly Ash and Ground Granulated Blast Furnace Slags-Based Alkali-Activated Mortars

“…Investigations on the rheological, mechanical and physical properties of AAM were conducted by the authors in their recent publications [ 18 , 19 ]. A microstructure analysis was also conducted [ 20 ], thereby enabling a thorough analysis to be performed on the evolution of properties with time. In this study, the durability aspects are investigated.…”

“…The porosity results for those materials were investigated in authors’ previous studies, and citation to those results were added into the text in order to provide more data for analysis. The detailed influence of GGBFS on porosity changes and microstructure (measured by mercury intrusion porosimetry) is presented by one of the paper co-authors in [ 20 ].…”