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Secondary aggregate can be made from industrial waste by treatment with carbon dioxide gas (accelerated carbonation). Trials were conducted on bauxite waste, biomass ashes, metal dusts, quarry fines, steel-making waste, and the ashes arising from municipal incineration, paper recycling, sewage sludge incineration, and coal fired power stations. The wastes were formed into pellets using a combined pelletising and accelerated carbonation process with patent pending. The manufactured pellets conformed to the bulk density requirements for lightweight aggregate specified by British Standard. Specification for lightweight aggregate is limited, and two commercial lightweight aggregates were tested alongside the manufactured pellets for comparison. Compressive strength, water absorption and apparent density of the materials were comparable to that of the commercial aggregate. Furthermore, the finished product contained up to 11% of its own weight in carbon dioxide gas, permanently bound as solid minerals. In a series of on-site pilot scale demonstrations, aggregates have been produced in bulk and used in commercial product trials. The aggregates produced by accelerated carbonation appear to be suitable for use in a wide range of applications including concrete, construction blocks, and substrates for green roofs.

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.