Concrete recycling meets CO2 capture

Jan Skocek discusses a Heidelberg Materials’ project that combines concrete recycling with CO₂ capture...

Global Cement (GC): Please could you introduce Heidelberg Materials’ work in concrete recycling?

Jan Skocek (JS): This project originated around five years ago from wide-ranging discussions within Heidelberg Materials on all aspects of cement and concrete decarbonisation. We realised that concrete recycling could be linked to decarbonisation, specifically using enforced carbonation. From there the project developed two main strands: 1. Study of how to recycle and optimise concrete recycling and decarbonisation; 2. Development of novel technologies to carry out this process.

GC: What are the steps in the recycling process?

JS: The process uses waste concrete from demolition sites. A primary crushing step reduces this from large lumps to <10cm. The steel is removed magnetically.

The second step uses a proprietary disintegration technology developed by Heidelberg Materials. This uses friction to rub the material which separates it into its original constituents: clean aggregates, clean sand and powdered hydrated cement. We call this recycled concrete paste (RCP). The RCP is used in the CO₂ capture step, while the aggregates and sand can be reused to produce fresh concrete.

GC: How does the RCP capture CO₂ from the stack?

JS: The process that takes place between the RCP and CO₂ is enforced carbonation, simply speeding up the natural carbonation that happens slowly in concrete naturally. This generates calcium carbonate from the reaction with the calcium ions, which is the key reaction.

However, that’s not the whole story. The second and third components in recycled cement are silica and alumina, which upon carbonation, combine to form an alumina-silica gel. This is actually a super-reactive pozzolanic material, which can be used as a supplementary cementitious material (SCM) in a similar way to fly ash.

GC: What equipment is used for the enforced carbonation step?

JS: The beauty of this reaction is that it is robust and able to operate in the presence of impurities that are found in cement plant exhaust streams. There are two main considerations. Firstly, the residence time needs to be in the order of tens of minutes. Secondly the process must be either wet, or at high relative humidity. This is because the carbonation takes place on the wet surface of the RCP particles.

So far, we have repurposed existing equipment at two cement plants as close proxies for the kind of technology that we are developing within the project. In 2020 we tested the process using a semi-dry circulating fluidised bed at our Brevik plant in Norway. This was followed-up more recently with a trial involving a wet scrubber at our Ribblesdale plant in the UK. This latter trial is one of the biggest CO₂ mineralisation trials ever undertaken. Around 20t of RCP absorbed more than 2t of CO₂ in around 15 minutes.

GC: What kind of methodology do you envisage in industrial-scale versions of this technology?

JS: The technologies that we have used to date in the field are not optimised to this process. We are therefore developing a dedicated reactor for enforced carbonation. Ideally this will be as dry as possible, as we want to use the product as an SCM later in the process. We don’t want to wet it just to dry it out again. We will mechanically suspend the RCP in the gas stream, combined with gentle grinding to expose fresh surfaces to the CO₂ as much as possible.

GC: When will this technology be ready?

JS: We intend to test the technology in a semi-industrial pilot in the middle of 2024. It will be on the scale of ~1t/hr to demonstrate the process, before ramping up the tonnages. It will be fairly simple to install such a system within a cement plant, as it is essentially another gas cleaning step. You just need to pass the gas through it between the kiln and the stack. On the other hand, there needs to be some additional equipment installed to collect, store and feed the carbonated RCP to the final blending step.

GC: What comes after that?

JS: We plan to have two commercially-viable pilots operating in the market by 2025. This will give us experience in the market and operating the equipment ‘in the real world.’ We will then roll the technology out across our global operations over the following five years to 2030. We see the broadest deployment being in the 2030s as most of the world does not really recycle concrete at the moment.

GC: What are the main potential hurdles that the process could face?

JS: I don’t believe there are any big hurdles waiting on the technology side. However, there may be issues securing enough waste concrete to produce appreciable quantities of RCP. In some markets, particularly larger cities in Western Europe or North America, the process already makes economic sense, but elsewhere there are fewer incentives to recycle demolished buildings.

Often, waste concrete goes to non-circular re-use, for example as fill material for earthworks, road bases and other low-grade applications. This way, its decarbonisation potential gets wasted. The process that Heidelberg Materials is developing goes beyond that, offering a unique combination of concrete recycling, CO₂ capture and a new SCM. It is a win-win-win for the industry and for the future!

GC: Thank you for your time today Jan.

JS: You are most welcome.