A one-step thermal process at 1050 degrees C and 1150 degrees C including bloating and sintering reactions produced lightweight aggregates (LWAs) with an apparent particle density of 2.08 and 1.18 g cm(-3), respectively, from pellets made of the sediment of a local reservoir. The roles of Fe compounds occurred in the sediment in bloating mechanism were determined with X-ray absorption spectroscopy (XAS) technique. About 59 at% of the total Fe is in forms of Fe2+ in the raw sediment: whereas most Fe was oxidized to Fe3+ in all LWAs, except in core part of the LWA produced at 1150 degrees C. The bloating reactions occurred in the core of the 1150 degrees C LWA is suggested to be mainly associated with the decomposition of FeSO4 into FeO with a concomitant release Of SO2, SO3, and O-2; the valence state of Fe was not changed. The generally accepted mechanism - the chemical reduction of Fe2O3 Component to FeO with a release of O-2 is responsible for the bloating phenomenon - is not observed in present study. (C) 2009 Elsevier B.V. All rights reserved.