Concrete ‘carbonation’ could cut production emissions and improve strength

Putting carbon directly into concrete or cementitious material could help offset and reduce greenhouse gas emissions from production and improve concrete strength, a white paper by sustainability consultant Tunley Environmental has revealed.

Concrete carbonation is a process whereby hydrated salts, particularly calcium hydroxide, within hardened cement react with CO₂ in the atmosphere and absorb it to reform calcium carbonate, the major constituent mineral in limestone.

The process occurs naturally during concrete curing and continues throughout a building’s lifespan, but is slow and problematic when not properly controlled, potentially leading to issues like cracking or rusting steel rebar.

According to the white paper, new research shows how the controlled carbonation of cementitious material can absorb large amounts of CO₂ and prevent ongoing natural carbonation, mitigating against potential problems.

Portland cement has a theoretical maximum carbonation capacity of 50%, said researchers, which if achieved at a global scale could sequester 2Gt of carbon dioxide a year, based on current levels of production. That’s equivalent to 77% of the 2.6 Gt of CO₂ emitted annually by global cement production.

The paper highlights three methods of controlled carbonation. The first involves using accelerated carbonation chambers to pump gaseous CO₂ into precast materials at a specific CO₂ concentration, humidity, and temperature.

Control over these parameters is essential, said researchers, to determine the rate and depth of carbonation, and the structure of finished concrete. Carbonation chambers aim to maximise carbonation depth because when the exterior pores in the concrete become blocked further carbonation underneath is prevented.

A second method involves embedding CO₂-rich materials within the concrete mixture to release emissions during curing as they react with calcium hydrate salts to form calcium carbonate. This process mitigates the harmful consequences of late-stage carbonation which can cause shrinkage and crack formation, explained researchers.

Early results have demonstrated the potential to sequester 15 kg of CO₂ per tonne of concrete. Scaled up to the 30Gt of concrete produced every year, 0.45Gt of CO₂ could be captured annually, the white paper notes.

Furthermore, adding recycled concrete aggregate (RCA) from demolition waste into fresh concrete, in combination with controlled carbonation, can boost the process. Carbonated RCA can reduce water absorption and increase stability to deter leaching, said researchers, it also ‘offsets emissions from the displaced aggregate,’ increasing overall CO₂ reductions.

Dr Gareth Davies, carbon reduction scientist at Tunley Environmental, said accelerated carbonation chambers are currently the most commercially developed of the technologies investigated, but carbonated RCA is the most viable at scale.

‘This process could be performed at the demolition site of disused buildings using most of the infrastructure already in place at site,’ explained Davies. ‘This would produce a filler material readily available for the construction of the new building, which will both replace a portion of the existing aggregate requirement and permanently sequester CO₂, reducing the concentrations of CO₂ in the atmosphere.’

Around 8% of manmade global carbon dioxide emissions are generated by cement production and half of that comprises ‘unavoidable’ process emissions caused by the calcination of limestone, which releases CO₂ directly into the atmosphere. Finding ways to sequester those emissions is therefore vital to decarbonise production.

According to Davies, carbonation of concrete may become critical if new low carbon pozzolanic materials or binders, or emerging carbon capture, use and storage technologies cannot be developed in time.