Hydration characteristics of waste sludge ash utilized as raw cement material

Abstract

In this study, the hydration characteristics and the engineering properties of three types of eco-cement pastes, including their compressive strength, speciation, degree of hydration, and microstructure, were studied and compared with those of ASTM type I ordinary Portland cement. The results indicate that it is feasible to use sludge ash and steel-making waste to replace up to 20% of the mineral components of the raw material of cement. Furthermore, all the tested clinkers met the toxicity characteristic leaching procedure requirements. The major components of Portland cement, C 3S (i.e., 3CaO·SiO 2), C 2S (i.e., 2CaO·SiO 2), C 3A (i.e., 3CaOAl 2O 3) and C 4AF (i.e., 4CaO·Al 2O 3·Fe 2O 3), were all found in the waste-derived clinkers. All three types of eco-cements were confirmed to produce calcium hydroxide (Ca(OH) 2) and calcium silicate hydrates (CSH) during the hydration process, increasing densification with the curing age. The thermal analysis results indicate that the hydration proceeded up to 90 days, with the amount of Ca(OH) 2 and CSH increasing. The chemical shift of the silicates, and the resultant degree of hydration, and the increase in the length of the CSH gels with the curing age, were confirmed by 29Si NMR techniques. Compressive strength and microstructural evaluations confirm the usefulness of eco-cement.