The electrostatic precipitator dust collected from a cement kiln was studied as received from the factory and when fired between 1000 and 1250°C. Two calcines (70:30 and 50:50) were preared by blending the raw dust and kaolin, fired between 1000 and 1250°C, and the resulting phases were also studied. These investigations were undertaken by chemical analysis, DTA and X-ray diffractometry, as well as weight losses. The results revealed that the dust consists of dolomitic limestone, minor and amounts of alkalies and quartz, together with 2 C 2S·CaCO 3 and 2 C 2CaSO 4. The dissociation of these phases occurred at 1000°C while volatilization of alkalies occurred at ⩾ 1200°C. The 70:30 (dust:kaolin) calcine is found to be more suitable, being composed mainly of gehlnite, while the other calcine is mainly composed of β-dicalcium silicate. The appropriate calcination temperature was 1150°C for 2 h, as negligible variations in phase composition could be detected above this temperature.