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A thermodynamics-guided framework to design lightweight aggregate from waste coal combustion fly ash



Mohammad Balapour, Thiha Thway, Rathin Rao, Grace Hsuan, Yaghoob Franam, Newell Moser, Edward Garboczi


In this study, a systematic thermodynamics-based framework was applied to recycle waste low and high-calcium coal combustion Fly Ash (FA) into synthetic lightweight aggregates (LWA) through sintering. The process to successfully manufacture synthetic LWA was investigated in which a delicate balance among three phenomena was required: (i) sufficient liquid phase formation during sintering, (ii) appropriate viscosity for the liquid-solid phase, and (iii) sufficient amount of gas emission to form pores in the LWA. Thermodynamics modeling was used to quantify the formation of the liquid phase during sintering while the fluxing agent and the temperature change. The Browning and Krieger-Dougherty models were used to quantify the viscosity of the liquid and liquid-solid phase, respectively. A lower bound of 100 Pa·S for the viscosity was found to ensure spherical shape of the LWA. Using thermogravimetric analysis, it was shown that the LWA had a notable gas release potential, owing to the presence of anhydrite and hematite, which could create gas-filled pores in the LWA macro-microstructure. X-ray computed tomography (X-CT) observation revealed the formation of a porous structure for the produced LWA, where high calcium FA LWA generally had larger pores compared with low calcium FA LWA. By correlating the X-CT observations and thermodynamic modeling results, it was found that a minimum of 40 % liquid phase content (% by mass) is necessary for the formation of gas-filled pores in FA-LWA.
Resources Conservation and Recycling


Off-spec fly ash, liquid phase, viscosity, gas release, thermodynamics modeling, X-ray tomography


Balapour, M. , Thway, T. , Rao, R. , Hsuan, G. , Franam, Y. , Moser, N. and Garboczi, E. (2021), A thermodynamics-guided framework to design lightweight aggregate from waste coal combustion fly ash, Resources Conservation and Recycling, [online],, (Accessed April 15, 2024)
Created November 16, 2021, Updated February 7, 2023