Co-processing of grinding sludge as alternative raw material in cement production

Abstract

Utilization of industry wastes as alternative raw materials in the cement industry is a reality in several countries around the world. Waste streams from the industry are usually employed in the Portland cement production in order to preserve non-renewable resource, save energy cost, destroy toxic substances, and accrue economic incomes through waste management service. This research aims to investigate the potential of partial substitution of cement raw meal with grinding sludge in Portland cement clinker production. Research methodology involves determination of characteristics of cement raw meal and grinding sludge and optimal burning condition for clinker synthesis. It also addresses the effect of grinding sludge on properties of the synthesized clinker and leaching behavior of heavy metals from synthesized Portland cement mortar. One reference and three modified mixtures, containing 1%, 2%, and 3% of dry weight of grinding sludge were examined. XRF analysis results showed that CaO, SiO2, Al2O3, and Fe2O3 in ordinary cement raw meal were within typical concentration ranges. The grinding sludge had high concentrations of iron, cadmium, chromium, and nickel and thus it was classified as a hazardous waste. High Fe2O3 made the grinding sludge a good candidate for alternative raw material. Optimal burning condition for synthesized Portland cement clinker was found at 1400 °C and 60 minutes. The maximum substitution of grinding sludge was found to limit at 2 % of grinding sludge. The higher replacement of grinding sludge was found to improve burnability, reduce formation of C2S and C3A, lower silica and alumina ratios, but promoted the formation of C3S and C4AF. Microstructural analysis of the synthesized Portland clinker revealed that the grinding sludge promoted formation of secondary C2S as well as caused color change of C3S. SEM analysis also showed incorporation of C4AF into C3S and heavy metals into C2S. Incorporation of heavy metals including chromium, copper and nickel were more than 90% of total content. Regulatory leaching procedure was performed on the synthesized clinker and mortar samples and the heavy metals in the leachate were found below the concentration limits. Sequential extraction revealed strong chemical bonding between heavy metals and silica oxide in the hardened samples that might limit their releases into the environment. The leaching behavior of salts in cement mortar with 2 % of grinding sludge was controlled by diffusion mechanism while the leaching behavior of calcium, cadmium, chromium, and nickel were surface wash-off and diffusion mechanism. The metals with highest leaching potential from synthesized the cement mortar was nickel and the lowest was cadmium.