The beneficial re-use of discarded materials is an essential part of a circular economy. The recycling of process waste–based products directly into the materials supply chain results in considerable sustainability gains and drives innovation. The process presented here involves the use of both solid and gaseous waste in combination to produce aggregate for use in concrete.

The application of accelerated carbonation technology has enabled Carbon8 to stabilise and solidify industrial residues into hardened manufactured aggregates that are a direct substitute for natural stone (Gunning et al., 2009). In the UK, thermal residues are commercially aggregated by carbonation, and incorporated into concrete construction blocks. The technology has however, wider possibilities (Gunning et al., 2011a, b), as a variety of wastes can be carbonate-cemented into products suitable for a number of engineering applications.

The route to commercialising this innovative use of waste CO₂ involved clearly demonstrating the transition from hazardous waste feedstock to safe usable product. This was both difficult and complex and involved rigorous independent validation before ‘end-of-waste’ designation by the Environment Agency was possible.

In early 2012, Carbon8 commissioned a bespoke zero-emissions commercial plant in Suffolk, East Anglia, which now produces 60,000 tonnes of manufactured carbonated lightweight aggregate/year (Figure 1). A second plant (100,000 tonnes/year) is nearing completion in Avonmouth (Figure 2), and 3 more UK plants of a similar size or larger, are expected to be operational by 2018.

Waste is brought to site by powder tanker and is pneumatically conveyed and stored in silos before being delivered into the multi-stage carbonation process. Rainwater is harvested for use in the plant and stored CO₂ (captured and delivered from a local point source) is fed directly into the process in such a way that none is lost to atmosphere. Furthermore,  renewable energy is used to power the plant. The Carbon8 process results in a carbon negative manufactured aggregate, as it contains more imbibed carbon than is generated by its production (see Figure 3).

Consequently, the concrete construction blocks incorporating the aggregate can also be carbon negative. Independent block maker Lignacite produces such blocks, under the name ‘Carbon Buster’ (Figure 4).

In addition to diverting wastes from landfill, the amount of carbon dioxide that can be locked up as carbonate salts i.e. limestone rather than emitted to the atmosphere, is potentially significant. As production increases, Carbon8 will be mineralising tens of thousands of tonnes of CO₂ in its manufactured aggregates in the UK (Figure 5). Worldwide, the potential of common waste streams (Gunning et al., 2010), including: pulverised fuel ash, steel slag and kiln dusts to imbibe CO₂, could amount to hundreds of millions of tonnes.

Legislative and Commercial Challenges

The European waste legislation and its implementation presented numerous challenges to the development and commercialisation of the carbonation process in the UK. At each stage of scaling-up, the support of the regulator was required, and accredited laboratory testing and validation of the aggregates and blocks to European Standards was necessary.

In accordance with waste legislation, it was necessary to demonstrate that (a) the aggregate did not pose an environmental risk, (b) had a clear end use and (c) was a suitable replacement for natural aggregate. The submission was fully supported by third party accredited testing of the physical and chemical properties of the aggregate product and the resulting concrete blocks (to BS EN 771), so a clear end use for the material and confirmation that there were no detrimental effects were demonstrated. Thus, by working closely with the Environment Agency, ‘End of Waste’ for the aggregate was achieved and a commercial plant was permitted and was operational in 2012.

Despite satisfying the considerable demands to achieve ‘End of Waste’ in the UK, this still presents a challenge elsewhere in Europe, as the Waste Framework Directive (2000) is interpreted very differently in different Member States, e.g. France and Norway. The lack of a route to achieve product status is a significant barrier to the commercial development of innovative technologies and undermines the potential of this technology to contribute to Europe’s objective for the development of a Circular Economy.

Quality assurance of carbonated products

The carbonation process operated by Carbon8 utilises a strict quality system in compliance with ISO14001, OHSAS18001 and ISO9001. Daily checks on the physical and chemical properties of the incoming waste and outgoing aggregate product are carried out to ensure that the latter meets the agreed specification set out in the ‘End of Waste’ documentation. 

The future

The Carbon8 process in Suffolk relies upon CO₂ that is captured from the production of bio-ethanol. The CO₂ is delivered by tanker from a short distance away, but it remains an expensive product due to its purity and this currently limits what wastes can be processed economically.

As CO₂ use gains a value, it is likely that Carbon8 will sequestrate more of this gas in its products whilst also increasing the number of wastes it can treat. This shift will also facilitate the direct capture of CO₂ from point sources, as has been shown is technically possible during trials at a landfill site and cement plant. Apart from making the aggregated product more carbon negative by increasing the amount of CO₂ that is mineralised, the option to capture more significant amounts of CO₂ from small and medium-sized emitters (that fall outside the scope for CCS) then becomes a possibility.

As the greater possibilities for carbonated products and their application become more obvious, and the economics of using waste CO₂ as a feedstock improve, a new industry based upon carbonation engineering is a realistic outcome. However, for new mineralised products and processes to become available to the market a level regulatory ‘playing field’ is also required. Only then will Europe’s current lead in this area be fully consolidated.

References

Gunning, P.J., Hills, C.D., Carey, P.J. (2009). Production of lightweight aggregate from industrial waste and carbon dioxide. Waste Management, 29, 2722–2728

Gunning, P.J., Hills, C.D. and Carey, P.J. (2010). Accelerated carbonation treatment of industrial wastes. Waste Management, 30, 1081–1090.

Gunning, P.J., Antemir, A., Hills, C.D., Carey, P.J. (2011a). Secondary aggregate from waste treated with carbon dioxide. Proceedings of the Institution of Civil Engineers: Construction Materials, 164, 231-239.

Gunning, P.J., Hills, C.D., Carey, P.J., Antemir, A. (2011b). Novel approaches to the valorisation of ashes using aggregation by carbonation. In: Proceedings of the 2nd International Slag Valorisation Symposium, Leuven, Belgium.

Waste Framework Directive http://ec.europa.eu/environment/waste/framework/list.htm. Accessed 25.09.2015.