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Engineering properties and pore structure of lightweight aggregates produced from off-spec fly ash



Edward Garboczi, Mohammad Balapour, Thiha Thway, Grace Hsuan, Yaghoob Farnam, Newell Moser


This study characterizes the engineering properties of lightweight aggregate (LWA) manufactured from low calcium (F-FA) and high calcium (C-FA) waste fly ash that were designed using a previously-developed thermodynamics-guided process. LWA properties that were characterized include specific gravity, vacuum absorption, water absorption over time, water desorption, porosity, pore size distribution, and permeability. The LWA studied had a low oven dry specific gravity ranging from 1.22 to 1.45. The vacuum absorption and total open porosity of F-FA LWA decreased as the fluxing agent (NaOH) concentration increased while the values for C-FA LWA exhibited some random variations with increasing amounts of fluxing agent. All LWA passed the ASTM C1761 requirements for application to the internal curing of concrete, which are: greater than 5 % water absorption (by mass) over 72 h and release of more than 85 % (by mass) of the absorbed water at 94 % relative humidity. X-ray computed tomography (XCT) and dynamic vapor sorption analyzer (DVSA) techniques were used to characterize the LWA pore structure. It was found that for F-FA LWA the normalized porosity, counting only pores smaller than 50 nm (gelpores + mesopores), ranged from 4 % to 11.5 %, while for C-FA LWA, this same pore size range had porosity ranging from 1.7 % to 2.8 %. Pore size distribution measurements of F-FA LWA using XCT showed that the pores became larger and the predicted permeability coefficient increased as the fluxing agent concentration increased. For C-FA LWA, the predicted permeability increased with increasing NaOH concentration. Since the pore size distribution had random variations and did not follow a specific increasing trend, the increase in C-FA permeability could only be due to an increase in pore connectivity.
Construction and Building Materials


Waste fly ash, lightweight aggregate, sorption properties, pore size distribution, X-ray computed tomography


Garboczi, E. , Balapour, M. , Thway, T. , Hsuan, G. , Farnam, Y. and Moser, N. (2022), Engineering properties and pore structure of lightweight aggregates produced from off-spec fly ash, Construction and Building Materials, [online],, (Accessed April 19, 2024)
Created September 19, 2022, Updated February 8, 2023