Resistance of fly ash–Portland cement blends to thermal shock

Abstract

Thermal-shock resistance of high-content fly ash–Portland cement blends was tested in the following ways. Activated and non-activated blends with 80–90% fly ash F (FAF) were left to set at room temperature, then hydrated for 24 h at 85°C and then for an additional 24 h at 300°C, and tested in five thermal-shock cycles (600°C heat – 25°C water quenching). X-ray diffraction (XRD) and thermal gravimetric analyses, along with calorimetric measurements and scanning electron microscope-energy-dispersive X-ray tests demonstrated that the activated blends form more hydrates after 24 h at 300°C, and achieve a higher short-term compressive strength than do non-activated ones. Sodium meta-silicate and soda-ash engendered the concomitant hydration of ordinary Portland cement (OPC) and class F fly ash (FAF), with the formation of mixed crystalline FAF–OPC hydrates and FAF hydrates, such as garranite, analcime and wairakite, along with the amorphous FAF hydration products. In sodium sulfate-activated and non-activated blends separate OPC (tobermorite) and FAF (amorphous gel) hydrates with no mixed crystalline products formed. The compressive strength of all tested blends decreased by nearly 50% after five thermal-shock test cycles. These changes in the compressive strength were accompanied by a marked decrease in the intensities of XRD patterns of the crystalline hydrates after the thermal shock. There was no significant difference in the performance of the blends with different activators.