Rapid-Setting Cement Manufacture using FGD Sludge

摘要

Only 7.5% of the approximately 2 million metric tons of flue gas desulfurization (FGD) sludge isproduced in the U.S. is used for beneficial purposes. Sludge that has been converted to sulfatethrough forced oxidation has been utilized to make wall boards and portland cement. However,the sludge that is predominantly calcium sulfite is not appealing for direct utilization. Exploitingits compositional make-up, the mainly sulfite FGD residues can be utilized in manufacturing arapid-setting cement that already has a substantial market demand. This special calciumsulfoaluminate-based cement is widely used as rapid-setting cement and shrinkagecompensatingcement. Currently, rapid-setting cements are being used in many applications,including highway pavement, and the demand is rapidly increasing.Two FGD sludges have been investigated; one is predominantly a hydrate of calcium sulfate andthe other is a hydrate of calcium sulfite. The performance of Type I portland cement preparedwith the sulfate sludge was similar to or better than that prepared with standard “Terra Alba”gypsum. These results clearly indicate that sulfate sludge is a good source gypsum in themanufacture of portland cement. It is also economically attractive to the cement manufacturers.Sulfite sludge, on the other hand, is not suitable for making portland cement. However, itscompositional make up is compatible for making rapid-setting cement. Rapid setting clinkerwas produced in a pilot-scale rotary kiln system utilizing 13% sulfite sludge, based on dryweight. The clinkering temperature was approximately 1260°C, which is 200°C lower than thatrequired to manufacture portland cement clinker. Furthermore, as the rapid-setting clinker wasmuch softer, substantial energy can be saved in the grinding press.A one-to-one comparison with a commercial rapid-setting cement clearly indicates the sulfitederived,pilot kiln-produced rapid-setting cement has superior flow properties and comparablestrength development (over 15 MPa in two hours and over 30 MPa in 24 hours). Thesecharacteristics clearly suggest that FGD sludge can be a potential sulfur source in themanufacturing of rapid-setting cement.