Closing the loop – 100% circular concrete

A technique for recovering the sand, the gravel and the cement from concrete

Concrete is the most commonly used building material all over the world. In the production process of concrete, cement is needed. During the high-energy treatment process to make cement, large quantities of CO2 are released in the atmosphere. In total, 1kg CO2 is released to make 1 kg of cement. In 2017, the cement industry alone produced 5,000 million tonnes of cement in 2017 and – as a by-product – also 5,000 million tonnes of CO2. Concrete recycling technologies will enable the concrete chain to be circular and will mitigate climate change.

Sustainable concrete recycling - Closing the loop © Koos Schenk, Smartcrusher BV

It is not widely known that in the making of concrete, only the outer area of a cement particle is reached by the water. The inner part is ‘protected’ by the part that has been activated. As a result, only about 50% of the cement in concrete is ‘activated’ by the water – and in concrete and concrete waste, still half of the unused cement is present.

In traditional crushing methods, only a small amount of cement stone will be released, which is polluted with fine silicate particles and can no longer be used for making traditional cement. These traditional crushers are very heavy-duty machines, using tremendous force and pressure as they are designed to break through everything. In breaking down the concrete rubble and drying the cement stone after that, all the still unhydrated cement is definitely lost in the process.

SmartCrusher SC2 © Koos Schenk, SmartCrusher BV
Researchers working on the recycling of concrete are thinking of ways to free the unactivated cement in it. With an innovative technology, the researchers of Slimbreker (SmartCrusher) in the Netherlands Marc Ottelé and Koos Schenk provide a solution for circular concrete: The SmartCrusher does not break straight through the sand and gravel but only crushes the cement stone, the glue between sand and gravel. This way, the cement and hydrated cement is not polluted by fine broken sand and can be reused to produce new cement paste.

In the application of this innovative technology, sand and gravel retrieved from concrete rubble are perfoming even better in new concrete than new sand and gravel: an increase in strength by 25% can be proven in research experiments. Furthermore, the embodied energy of the concrete rubble can be fully sustained. By using the hydrated cement that is released by ‘SmartCrushing’ the concrete rubble and by using climate-neutral electricity from bio-fuels for the heating process, cement can be produced 100% CO2-neutral, resulting in circular and closed-loop recycling.

More information about the SmartCrusher technology here.