An application of alkali-activated fly-ash materials with low-compressive strength: Thermal stability at elevated temperatures

Abstract

Herein, alkali-activated fly-ash materials (FA-AAMs) with low-compressive strengths were employed as thermally stable materials after being exposed to 300, 600, 800, and 1000 °C. The phase transformation of the FA-AAMs is an important factor contributing to their thermal properties. Therefore, herein, the thermal properties of FA-AAM samples with different NaOH activators were investigated under standard curing conditions, considering their compressive strength and surface structure properties. X-ray diffraction, Fourier-transform infrared spectroscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy analyses were performed to study the phase transformations (crystal phase and amorphous gel) in the samples before and after high-temperature treatment. The results indicated that the residual compressive strength of the FA-AAMs increased significantly after high-temperature treatment. A high Na/Si ratio of 0.5 was found to be optimal for the N-A-S-H gel, and the long-chain network structure resulted in higher compressive strengths of the samples after treatment at 300 °C. However, with increasing temperature, the N-A-S-H gel transforms into a crystalline phase, destroying the sample microstructure. Owing to Ca-ion exchange, the large interlayer space of C-A-S-H provided a channel for the chemically bonded water to escape from the samples with low NaOH contents.