Production of lightweight aggregate from industrial waste and carbon dioxide Peter J. Gunning * , Colin D. Hills, Paula J. Carey Centre for Contaminated Land Remediation, University of Greenwich, Chatham Maritime, United Kingdom article info Article history: Accepted 28 May 2009 Available online 4 July 2009 abstract The concomitant recycling of waste and carbon dioxide emissions is the subject of developing technology designed to close the industrial process loop and facilitate the bulk-re-use of waste in, for example, con- struction. The present work discusses a treatment step that employs accelerated carbonation to convert gaseous carbon dioxide into solid calcium carbonate through a reaction with industrial thermal residues. Treatment by accelerated carbonation enabled a synthetic aggregate to be made from thermal residues and waste quarry fines. The aggregates produced had a bulk density below 1000 kg/m 3 and a high water absorption capacity. Aggregate crushing strengths were between 30% and 90% stronger than the proprie- tary lightweight expanded clay aggregate available in the UK. Cast concrete blocks containing the carbon- ated aggregate achieve compressive strengths of 24 MPa, making them suitable for use with concrete exposed to non-aggressive service environments. The energy intensive firing and sintering processes tra- ditionally required to produce lightweight aggregates can now be augmented by a cold-bonding, low energy method that contributes to the reduction of green house gases to the atmosphere. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction New treatment methods that can convert waste into products reduce the amount of materials sent to landfill disposal and protect virgin raw materials for the future. Annual waste production in the UK amounts to 335 million ton- nes (DEFRA, 2007). Mining and quarrying wastes comprise 29%, whilst industrial processes are responsible for a further 13%. Land- filling has been widely used, but as space is due to run out in less than a decade. Despite increased recycling, large amounts of waste are not effectively reused. These include incineration residues which are often classified as hazardous on account of their alkalin- ity and heavy metal content. Typically, these ashes must be land- filled, and are subject to landfill taxation which is set to rise from £40 in 2009, up to £48 in 2010 (DEFRA, 2008). Similarly, the need to improve carbon management has prompted industry to consider the mitigation of emissions using Carbon Capture and Storage schemes (CCS) at both the larger and smaller scale, but concerns have been raised over long term envi- ronmental impacts (I.P.C.C., 2005). Finding a safe, permanent stor- age solution for carbon dioxide remains a major challenge. One option, particularly suited to smaller CO 2 point-source emissions involves the use of Accelerated Carbonation Technology (ACT) to bind carbon dioxide into solid carbonate using naturally reactive materials such as wastes arising from thermal processes. Treatment by ACT can be applied to calcium and magnesium rich thermal residues, e.g. municipal and paper incineration ashes, wood ashes, pulverised fuel ashes, steel slags, etc. (Li et al., 2007; Fernández-Bertos et al., 2004; Johnson, 2000). Carbonation is achieved by exposing the waste to an elevated concentration of carbon dioxide gas in a controlled environment. The reaction takes place in the presence of moisture and carbon dioxide gas hydrates to form carbonic acid (1). In saturated conditions, carbonation is suppressed by the slower diffusion of carbon dioxide in water com- pared to air (Van Balen, 2005). The carbonation of hydrated Port- land cement, converts Portlandite (2) and Calcium Silicate Hydrate (3) into carbonate. Unhydrated calcium oxide and silicates also form carbonates (Sulapha et al., 2003; Jiang et al., 2000). These phases are typically found in many thermal residues (Johnson, 2000) and can combine with significant amounts of carbon dioxide gas in a short timescale CO 2 þ H 2 O () H 2 CO 3 ð1Þ CaðOHÞ 2 þ H 2 CO 3 ! CaCO 3 þ 2H 2 O ð2Þ 3CaO  2SiO 2  3H 2 O þ H 2 CO 3 ! CaCO 3 þ 2SiO 2 þ 6H 2 O ð3Þ Leachates generated from carbonated alkaline media have pH’s that are typically lowered by several units, and the formation car- bonate results in an increase in density and mass. The hardened, densified product can have potential for re-use in engineering applications (Rendek et al., 2006; Johannesson and Utgenannt, 2001). Combining ACT with a method of making agglomerates al- lows waste to be reconstituted into a lightweight aggregate (Gun- ning et al., 2008; Padfield et al., 2004). 0956-053X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2009.05.021 * Corresponding author. Tel.: +44 (0)2083317570. E-mail address: gp45@gre.ac.uk (P.J. Gunning). Waste Management 29 (2009) 2722–2728 Contents lists available at ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman