The world cement industry is responsible for .5% of the total anthropogenic carbon dioxide emissions blamed for causing global warming. The production of cement clinker minerals by precipitation from a molten salt solvent offers a potential route to energy reduction in cement manufacture. Molten salt synthesis of the major cement compounds b-dicalcium silicate (b-Ca 2 SiO 4 , b-C 2 S) and tricalcium silicate (Ca 3 SiO 5 , C 3 S) has been attempted in fused sodium chloride (NaCl). The synthesis of b-Ca 2 SiO 4 was carried out by the reaction of CaCO 3 with SiO 2 in molten NaCl (
A material with potential for use as a sorbent has been prepared by carbonating damp Portland cement in a 100%CO 2 atmosphere at a pressure of 3 bars. Analysis by XRD showed the main crystalline phase present, after eight 1 h cycles of carbonation, to be calcite. A series of batch sorption experiments involving single metal solutions of Cd, Co, Cu, Ni, Zn, at concentrations varying from 200 to 1000 mg L 21 , were carried out employing the carbonated media. The solid residues obtained, were then used to examine the desorption of metals over a 24 h timescale. The zeta potential and conductivity of suspensions prepared from the residues were also recorded. The results showed the sorbent could remove significant quantities of metal ions from solution, and up to 176 mg g 21 was recorded for Cd. The desorption experiments showed metal release rates ,1% by mass of metal adsorbed. Electrophoretic analysis gave different values for zeta potential and conductivity for each sorbent-metal system, indicating that sorption might take place via different mechanisms including inner-and outersphere complexation.
The continuous growth of anthropogenic CO2 emissions into the atmosphere and the disposal of hazardous wastes into landfills present serious economic and environmental issues. Reaction of CO2 with alkaline residues or cementitius materials, known as accelerated carbonation, occurs rapidly under ambient temperature and pressure and is a proven and effective process of sequestering the gas. Moreover, further improvement of the reaction efficiency would increase the amount of CO2 that could be permanently sequestered into solid products. This paper examines the potential of enhancing the accelerated carbonation of air pollution control residues, cement bypass dust and ladle slag by applying ultrasound at various water-to-solid (w/s) ratios. Experimental results showed that application of ultrasound increased the CO2 uptake by up to four times at high w/s ratios, whereas the reactivity at low water content showed little change compared with controls. Upon sonication, the particle size of the waste residues decreased and the amount of calcite precipitates increased. Finally, the sonicated particles exhibited a rounded morphology when observed by scanning electron microscopy.
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