Hydration characteristics of hydrothermal treated cement kiln dust–sludge–silica fume pastes

Cement kiln dust (CKD)–sludge–silica fume pastes were hydrothermally hardened at a pressure of 8 atm of saturated steam for different autoclaving ages. Hydration characteristics of the autoclaved CKD–sludge–silica fume pastes were studied by the determination of compressive strength and chemically combined water contents at different autoclaving ages. The phase composition and morphology of the formed hydrates were studied using X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). The replacement of silica fume in CKD–sludge mixtures results in a marked increase in strength values of the autoclaved specimens at all stages of the hydrothermal process. The results of X-ray diffraction analysis and SEM-micrographs of autoclaved specimens for various mixtures indicates that the main hydration products identified are calcium silicate hydrates and minor amounts of CaCO3.

Research highlights
► Autoclaved cement kiln dust (CKD)–sludge pastes possess a considerably compressive strength at all different autoclaving ages. This is mainly attributed to the hydrothermal interaction between the lime released from CKD with sludge to give calcium silicate hydrates.
► The replacement of sludge by silica fume results in a marked increase in the compressive strength and the combined water contents of the formed hydrates at all the autoclaving ages.
► X-ray diffraction analysis and SEM micrographs show that the main hydration products are calcium silicate hydrates (CSH I, II) with a minor amounts of CaCO3.