Recycled content in concrete
It is common practice to include some recycled or secondary material content in the manufacture of concrete, including its cementitious binder, aggregate and reinforcement.
Examples include:
- Recycled content in the manufacture of CEM I, including recycled plasterboard
- GGBS and fly ash (by products of other manufacturing and industrial processes) as supplementary cementitious materials (SCMs)
- Recycled steel reinforcement
- Crushed concrete aggregate (CCA) as coarse aggregate
The extent and type of secondary material used depends on a many issues, but in general are limited by factors other than technical standards for the manufacture of concrete. Limiting factors include availability of appropriate supply; quality control, impact on mix design and ready availability of established supply chains for local, low carbon, natural aggregate.
Recycled aggregates in concrete
British Standard BS 8500 permits CCA to comprise up to 20% of the coarse aggregate in most designated concrete, without declaration – in other words, unless the concrete is explicitly required to exclude it. According to the standard, it is possible to use higher percentages. In general concrete designated GEN 0 to GEN 3 – that is, non-reinforced concrete used for footings and floor slabs in domestic applications – up to 100% is allowable.
Exposure class may be a limiting factor – in other applications, CCA is permitted provided it can be demonstrated to be suitable for the exposure class conditions. In practical terms, this would mean testing to meet quality protocols. Additional precautions may be required where there is concerns over fire performance.
If concrete is not separated from other recycled construction materials, such as brickwork, the result is a more general recycled aggregate (RA). This has more limited uses within concrete itself, typically limited to mass concrete.
Guidance on the definitions and use of recycled aggregate in concrete can be found in the following resources:
Alternative recycled and secondary resources for concrete manufacture
There are numerous research initiatives and products in development seeking to use other secondary or recycled in materials in the manufacture of concrete. These include the use of recycled plastic, glass and crushed sea shells as aggregate, waste material to manufacture lightweight aggregate and recycled brick dust as cementious content. In addition concrete has been used successfully as a medium to embed recycled materials in its surface.
Waste as a resource in cement manufacture
In 2023 47% of the thermal energy used in the UK to manufacture cement was supplied from waste-derived fuels. These alternative fuel sources comprise hard to recycle materials diverting over 1.2 million tonnes of waste from landfill every year.
This co-processing plays a vital role in the nation’s waste reduction strategy and offers opportunities for upcycling minerals into new cement – combining simultaneous material recycling and energy recovery in a single thermal process.
Examples of waste derived fuels used in the manufacture of cement include:
- Otherwise hard-to-recycle, tyres, tyre chips and tyre fluff. The waste steel within the tyres is upcycled, contributing useful iron content for cement manufacture.
- Non-recyclable household and business waste. This is diverted from land fill and processed to produce a clean, non-hazardous alternative fuel that is used at every UK cement plant.
- Pelletised sewage - this renewable energy source is a biofuel, produced from sewage sludge.
- Meat and bone meal (MBM). This 100% carbon neutral fuel has a significant calorific value. It was used alongside glycerin and hydrogen in a world-first demonstration of a cement kiln main burner successfully using a 100% net zero fuel mix.
- Polyurethane (PUR)- waste rigid polyurethane foam containing impurities is recycled through co-processing into cement.
For more information, please refer to Circular Economy (mineralproducts.org)