Effect of temperature on microstructure and performance of lightweight aggregate prepared from steel mill sludge and fly ash

The effect of sintering temperature on the characteristics of lightweight aggregate made from steel mill sludge and fly ash by one step was studied. Lightweight aggregates made from fly ash and steel mill sludge mixed at a ratio of 8:2 sintered at 1050, 1100, 1150 and 1200 o C were characterised by scanning electron microscope-energy dispersive spectrometer and X-ray diffraction to analyse the microstructure evolution mechanism and its effect on performance. The attained lightweight aggregate has a compressive strength of 4.0 MPa, a bulk density of 721 kg m −3 and water absorption of 12.6%. With the increase of sintering temperature, the surface of aggregate becomes denser, and there are abundant closed pores inside. This structure ensures the performance of a high compressive strength, low water absorption and low bulk density of aggregate. The major mineral phases are quartz (SiO 2 )，anorthite (CaAl 2 Si 2 O 8 ), mullite (Al 6 Si 2 O 13 ) and hematite (Fe 2 O 3 ) when temperature was between 1100 and 1200 o C. The needle-shape mullite interlaced into a network structure on the anorthite matrix, which greatly enhanced the strength of the lightweight aggregate. This study demonstrates that the sintering temperature is one of key factors to determine the performances of lightweight aggregate, and the best sintering temperature is 1150 o C.