New research into the carbonation of cement could improve its environmental reputation, Kathryn Allen reports.
The recent study, Substantial Global Carbon Uptake by Cement Carbonation, headed by Professor Dabo Guan of the University of East Anglia, UK, claims that cement materials form a significant carbon sink.
Suggesting that only limited attention has been paid to the natural carbonation of cement materials when considering their environmental impact, the team used data on these materials to calculate estimated carbon dioxide (C02) uptake from 1930–2013. These estimations, at both regional and global levels, considered the life of cement materials including demolition and secondary uses.
Published in Nature Geoscience, the study found the estimated amount of carbon captured, from 1930-2013, offset 43% of the C02 emissions released from the production of cement. However, this does not include carbon emissions from fossil fuels used during production. In the same period, an estimated 4.5 gigatonnes of carbon was removed from the atmosphere through the carbonation of cement materials. Carbonation occurs when the calcium components of cement-based materials react with C02 in the air to form calcium carbonate.
When asked about the relevance of the findings, the team said they were important in the mitigation of climate change. Addressing the negative environmental impact of cement materials, the team pointed to the volume of cement being produced, as well as that already in existence, and its potential to absorb C02. The researchers claim 76 billion tonnes of cement was produced globally between 1930 and 2013, with 4 billion tonnes produced, mostly in China, in 2013 alone.
The team describes the vast volume of cement available to carbonate C02 as an overlooked carbon sink. Responding to this claim, Dr Charles Fentiman, Director at Shire Green Roof Substrates, noted that by using the term carbon sink the team ‘seems to be trying for a positive spin to suggest that concrete could somehow offset the C02 generated by burning of fossil fuels for other purposes, such as heating, cars and so on.’ In reality, the net emissions from cement materials is simply lower than previously thought.
Considering the potential to improve the environmental reputation of these materials, Dr Alan Maries, Visiting Professor in Environmental Technology at the University of Greenwich, UK, claimed that while this research may do so, ‘the global production of concrete already exceeds that of all other man-made materials combined by more than an order of magnitude in volume, [therefore] I doubt whether it will affect its use very much. What it will do, however, is take the pressure off cement manufacturers as villains.’
Maries also pointed out that ‘the extent of sequestration of carbon cement-containing materials has been calculated from compositional data and exposure conditions (rather than actually measured), then mathematically modelled making various assumptions. There appears to have been little field measurement to support the calculations.’ However, considering the distinguished authors of the study and the reliable sources of information, Maries suggests that the conclusions drawn are well supported. A similar concern was raised by Dr Andrew Dunster, Principle Consultant at BRE, whose view that the study contains assumptions on quality and exposure of concrete and the speed of carbonation of demolished materials makes him think potential over-estimations of carbonation levels have been made.
The team behind the study has, however, acknowledged that data is lacking in terms of how carbonation is affected by the environment, for example, by coatings and coverings of cement materials. They quote studies that show coatings such as paint can reduce carbonation by up to 10–30% as well as studies that refute this claim.
Carbonation correction coefficients, which are intended to reflect the possible effects of coatings, were used in this study to, in theory, produce accurate results. However, the reliance on estimations in this study has not gone unnoticed.
The researchers hope their findings can be applied practically to future developments. Buildings made of cement materials can be designed to maximize carbonation and recycling and reuse of cement materials will prevent the absorbed carbon from being released back into the atmosphere. Discussing potential applications of the findings, Guan said, ‘We suggest that if carbon capture and storage technology were applied to cement process emissions, the produced cements might represent a source of negative C02 emissions. Policymakers might also investigate a way to increase the completeness and rate of carbonation of cement waste.’