Where do recycled aggregates go?

The UK extracts just under 150 million tonnes of crushed rock, gravel and sand per year (as of 2022)^1–3.  Much of this supply of aggregates is used in construction.  This means building designers have a responsibility to understand the environmental impacts of this extraction and to consider how negative impacts can be mitigated through design decisions.

In a previous study, we explored the impacts of raw material extraction of aggregates and the available supply chain data.

Extracted (“primary”) aggregates come from land quarries or are dredged from the seabed and are transported by boat, train, or lorry to concrete batching plants or aggregate suppliers, before eventually reaching a construction site.  Although the embodied carbon of this process is small, close to half of UK quarries are near sensitive landscapes such as Areas of Outstanding Natural Beauty (AONB) or National Parks⁴.  Extraction causes habitat loss, and dewatering and runoff can affect groundwater, streams, and wetlands.  Other polluting effects include dust, noise, vibration, and increased traffic.  One positive upside is that former quarries can be turned into nature reserves – there is currently 110km² of new habitat planned according to the Mineral Products Association (MPA)⁵.  Marine sites take years to recover from the habitat loss due to dredging.  Fortunately, the area of dredged seabed in the UK is a small fraction of the UK’s total area of seafloor⁶.

The industry guidance to “reduce, reuse, and recycle” is just as applicable to the consumption of aggregates – reducing the demand for new concrete will reduce the demand for extracted aggregates.  This is reinforced by the fact that aggregates aren’t renewable (on a human scale at least, as they are formed over millions of years).  So, it is important for designers to avoid excessive or unnecessary specification of material.

70 million tonnes of aggregate are diverted from landfill in the UK each year.  Of which, 57 million tonnes are from within the construction industry and 13 million tonnes are from “secondary” sources, by-products from other industries⁷.  When looking into this supply chain, however, we found that there is no data on where these recycled and secondary aggregates (RSAs) are used.  This is a strange omission – how can we increase uptake of RSAs if we don’t have a clear picture of where they are being used today?  RSAs represent about a third of the UK’s demand in total, one of the highest rates in Europe largely due to the landfill tax and aggregates levy⁷. 

Primary aggregates tend to have better mechanical properties than RSAs, so if not used carefully, RSAs can increase concrete’s cement content and, as a result, its embodied carbon.  Even so, new secondary aggregate sources are being explored with examples including oil palm shell, coconut shell, rice husks, and finely ground waste plastics that may have future potential as part of a broader shift toward circular material use in construction^8–10 but their practical application requires further development.

The UK already does well in supplying RSAs, but the extraction of primary aggregates is still needed and comes at a cost to the natural world.  As designers we must use RSAs where the benefits outweigh the compromises and use primary aggregates sensibly and sparingly where needed.  In practice, therefore, RSAs should be used in lower-value applications, such as backfilling, to help ensure that primary aggregates are saved for efficiently designed concrete where the mechanical properties are required.  Regularly compiled national statistics on RSA use effectively ended in the late 2000s^7,11 but we think this data should be available to industry to help increase uptake of this low impact, circular resource.