Manufacturing of durable precast concrete blocks using carbon dioxide (CO2) emitted during cement production will be subject to industrial-scale pilot trials this winter.
Canadian company Carbon Sense Solutions Inc, based in Halifax, claims its technique to accelerate concrete curing using CO2 will not only reduce greenhouse gas emissions, but will replace conventional energy-intensive steam and heat accelerated curing techniques. This could decrease energy and water consumption by as much as 155MJ and 142l/t of concrete, respectively, says the firm.
Combustion flue gases will be redirected to the curing process. The resulting effluent is scrubbed of CO2 in under an hour. The gas is stored in the concrete as calcite with no further reactions occurring.
‘Calcite, otherwise known as limestone, is the process feedstock for cement. We are simply reverting it back to its natural and most stable state. You can call this cradle-to-cradle engineering,’ says Robert President of Carbon Sense Solutions. The material is said to store up to half the weight of cement as CO2.
Niven is guarded about revealing more about the process, but says, compared to previous efforts at concrete carbonation, this work involves ‘a new reactor design that achieves complete carbonation, faster processing and improved material properties [faster early strength development, lower permeability, reduced shrinkage cracking and efflorescence resistance]’.
He adds, ‘In contrast to alternative mineral carbonation, the process does not promote additional mineral feedstock mining or disposal, and uses as captured combustion flue gases’.
Taking the challenge
However, concrete and cement science expert Dr Charles Fentiman of Fentiman Consulting in Southwater, UK, is sceptical about the ability to achieve complete carbonation during curing. He reserves judgement until the work is taken out of the laboratory and shown to overcome the practical problems that have impeded academics and industry for decades.
He says, ‘This seems to be an idea of making concrete elements and giving a warm cure in CO2. [But] in my experience, as soon as cement hydration starts, the CO2 coats everything and blocks further hydration. It does accelerate hardening, but then ongoing strength development is low and the concrete remains porous because hydration is blocked’.
Fentiman explains that academics have previously tried to overcome this through supercritical carbonation after the concrete has cured and the cement hydrated. However, ‘this would greatly slow the manufacturing process and the extra cost would need to be covered by the end user’.
Carbonation during curing is therefore ‘a difficult step’, he notes. ‘If they can achieve it, it would be marvellous. But people have been working on this idea for a long time.’
The team at Carbon Sense Solutions appears ready to take on the challenge as it scales up production and undertakes a complete material testing programme, looking at concrete performance, greenhouse gas reduction across the lifecycle, and economic feasibility.
If proven at this level, Niven insists the technology will be easily applicable in industry. He adds, ‘The entire project is based on providing simple and economical plant retrofits to concrete manufacturers’.