This week Holcim announced that it has successfully created the first clinker made entirely of recycled minerals and concrete. The circular economy can be truly closed in a circle if concrete can be reused to make clinker and in turn cement. Holcim believes the process will improve decarbonisation of the industry and help to reach net-zero emissions. However, there are many arguments against the recycling concrete for new buildings and cement manufacture.
Holcim's breakthrough research at Altkirch, France, was a test project to recycle concrete and use it as a basis for forming clinker with other materials, such as recycled water, wood ash and recycled rebar. It will be in the 3Q22 that 100 per cent recycled concrete is available for delivery from the Swiss multinational.
Other major cement producers have also been working on similar projects. HeidelbergCement has been working on a concept called ReConcrete-360˚ to reconstitute products from recycling concrete. The idea is to reuse demolition concrete by separating it into its components including sand, gravel, and hardened cement paste. ReConcrete-360˚ is then developed as a limestone substitute, which can also use carbon-containing exhaust gases from cement production to decarbonise the industry.
Vicat has also carried out pioneering work in this field with the Secondary Raw Materials for Concrete (SeRaMCO) project in 2017-21. The programme upcycled inert waste from demolition sites before incorporating fines in the cement manufacturing process at the Créchy plant in France, while clinker was milled in Chambéry, France. The EUR7.2m EU-funded project saw recycled construction and demolition waste create precast products in five northwest European countries. Four concrete mixes, over 1000t of cement and more than 10 prototype projects, including 130m2 of paving stones were created, and a new way of producing cement using recycled sand was established. The final precast concrete products were sold through Prefer in Belgium and Béton Betz in Germany.
Recycling concrete
The reuse of concrete has initially been steered in the direction of building products, road construction and land reclamation. Recycled aggregate is used in granular subbases, soil cement and new concrete. Recycled aggregates (RA) have a separate distinction from recycled concrete aggregates (RCA). The performance characteristics of RCA are better than RA and consequently there are fewer restrictions on the use of RCA in concrete, claims the Mineral Products Association, UK.
Approximately 28 per cent or 57Mt of the UK aggregate market is made up of recycled and secondary aggregates, which is the highest for all European countries. In the USA, 38 states recycle concrete as an aggregate base and 11 recycle it into new Portland cement concrete, according to the Federal Highway Administration 2004 study. The recycling of concrete as part of the circular economy still has a long road to travel, but cement producers and construction companies are seeing the value and cost savings in developing new products from construction waste and recycled concrete.
Should recycled concrete be used in new construction?
While the performance characteristics of recycled concrete aggregate in new concrete have been argued in many cases to be equal to using natural aggregate, there are arguments for not reusing concrete in new buildings and structures. Concrete that has been demolished may have been exposed to the elements for many years and its strength may have reduced, as well as compressive strength which can be decline by 10-30 per cent, says iMix Concrete. There are also performance characteristics such as workability and water absorption that must be considered. Water absorption can increase by 3-9 per cent in recycled concrete, claims iMix Concrete.
Evolving regulations for end-of-life materials
The use of aggregates in concrete is covered in BS EN 12620:2013. The European-wide 12620 regulations are supplemented by BS8500 for durability and working life as well as delivery, conformity testing, product control and transport for concrete producers. Since 2008, there is also Building Research Establishment's BES 6001, a responsible sourcing benchmark that certifies 92 per cent of the concrete for sourcing and traceability.
Arguably there will be increasing guidelines and regulations for the future use of recycled concrete. Safety remains the highest necessity in developing construction and building materials. The challenge for researchers is to devise a consistently performing series of products from recycled concrete. Separation technology to better recycle concrete is also recommended by CEMBUREAU. Moreover, a great deal of information must be known about the elements and history of the end-of-life concrete before it can be safely made into a new product where it can be tested to meet the best standards and be tested over time.
The pressure on natural resources will naturally encourage further development and reuse of end-of-life materials in concrete. But regulations are required for reconstituted types of cements and concretes to exclude certain recycled materials being used in structural concrete. New products must meet the highest regulations regarding concrete and aggregate strength and durability.