The science of carbon removal and carbon sinks

This week Brimstone announced that it had achieved third-party certification of its carbon-negative cement that meets the ASTM C150 standard. Low-carbon and green cements are continuously being developed as alternatives to ordinary Portland cement (OPC). Decarbonising the cement and concrete sectors will need such innovation, but the science of carbon removal from cement and concrete and to what extent they can act as carbon sinks is still a debated subject.

While carbon capture, utilisation and storage (CCUS) will be a major solution for removing CO₂ from the cement-making process, it will not be universally employed. Additional measures will be required and, once the carbon is captured, there is the question of what to do with it. To reach net-zero, carbon removal from the atmosphere is needed, even if emissions fall to zero, as the planet is still warm with the existing CO₂ in the atmosphere.

CO₂ removal and carbonation
CO₂ removal is the process of capturing and locking away CO₂ from the atmosphere. CEMBUREAU has highlighted that sustainable biomass as feedstock for cement plants, the injection of carbon into the cement production process and the natural absorption of CO₂ by cement and concrete through its lifetime (so-called carbonation) should be considered as carbon sinks. CEMBUREAU claims carbonation can account for up to 23 per cent of the annual calcination emissions from cement plants. However, critics such as Carbon Market Watch have argued that concrete and cement are “unlikely to deliver real and durable carbon removals, at least not for the foreseeable future.” 

Developing carbon sinks
While cement and concrete act as carbon sinks in existing buildings through the process of enhanced carbonisation, faster carbon sinks would greatly enhance the potential to reach net zero quicker. Therefore, the creation of new carbon sinks is a steadily growing science.

Heidelberg Materials is a pioneer of this approach. Its CIRCO₂BETON^® project is designed to selectively separate demolished concrete into sand, aggregates and recycled concrete paste (RCP). RCP can then be carbonated with captured CO₂ from cement production to replace clinker in new low-carbon cements. RCP will be transported to Heidelberg Materials' Ranville cement plant in Normandy, France, where a reactor will carbonate the RCP, exposing it to CO₂-containing exhaust gases from the rotary kin. The carbonated RCP then acts as a carbon sink and replaces clinker in the new low-carbon cement. Heidelberg Materials estimates that this process will reduce CO₂ emissions at its Ranville cement plant by 20 per cent from 2026 when the system becomes operational.

Universities are also turning their attention to carbon sink solutions. Earlier this year, Massachusetts Institute of Technology (MIT) engineers discovered in lab tests that adding sodium bicarbonate (baking soda) to concrete during the mixing stage and before setting resulted in the mineralisation of 15 per cent of the CO₂ associated with cement production. Trials have demonstrated that concrete sets quicker and doubles the mechanical performance in the early-stage, increasing the efficiency of concrete construction. 

“Our new discovery could further be combined with other recent innovations in the development of lower carbon footprint concrete admixtures to provide much greener, and even carbon-negative construction materials for the built environment, turning concrete from being a problem to a part of a solution,” said Admir Masic, MIT.

A further initiative that was announced this week is US-based C-Crete Technologies binder that is claimed to be CO₂-free in its production and can absorb CO₂ from the air over time. C-Crete’s developers claim that for each tonne of C-Crete binder that replaces OPC prevents approximately 1t of CO₂ emissions.

Cement producers may also consider carbon offsetting or generating CDR credits by investing in solutions such as biochar plants. When biomass grows and absorbs CO₂, bimoass sequesters this CO₂ during its lifetime and these woody plants create carbon sinks that can contain up to 50 per cent of their mass. By producing biochar from waste biomass using a process called pyrolysis, biochar can retain CO₂ in the soil for thousands of years.

Humanity's challenge
Innovative ideas for developing carbon sinks are likely to increase greatly with new start-up companies. Whether storing captured CO₂ in training shoes, as showcased by Oco this week, or creating building material carbon sinks like the CIRCO₂BETON^® project, dealing with the captured CO₂ and existing CO₂ in the atmosphere is humanity's goal for this century. It will be a combination of solutions that redefine our management of CO₂ and we are just at the start of that process.