Abstract
Abstract. Because of the alkaline nature and high calcium content of cements in general, they serve as a CO2-absorbing agent through carbonation processes, resembling silicate weathering in nature. This carbon uptake capacity of cements could abate some of the CO2 emitted during their production. Given the scale of cement production worldwide (4.10 Gt in 2019), a life-cycle assessment is necessary to determine the actual net carbon impacts of this industry. We adopted a comprehensive analytical model to estimate the amount of CO2 that had been absorbed from 1930 to 2019 in four types of cement materials, including concrete, mortar, construction waste, and cement kiln dust (CKD). In addition, the process CO2 emission during the same period based on the same datasets was also estimated. The results show that 21.02 Gt CO2 (95 % confidence interval, CI: 18.01–24.41 Gt CO2) had been absorbed in the cements produced from 1930 to 2019, with the 2019 annual figure mounting up to 0.89 Gt CO2 yr−1 (95 % CI: 0.76–1.06 Gt CO2). The cumulative uptake is equivalent to approximately 55 % of the process emission based on our estimation. In particular, China's dominant position in cement production or consumption in recent decades also gives rise to its uptake being the greatest, with a cumulative sink of 6.21 Gt CO2 (95 % CI: 4.59–8.32 Gt CO2) since 1930. Among the four types of cement materials, mortar is estimated to be the greatest contributor (approximately 59 %) to the total uptake. Potentially, our cement emission and uptake estimation system can be updated annually and modified when necessary for future low-carbon transitions in the cement industry. All the data described in this study, including the Monte Carlo uncertainty analysis results, are accessible at https://doi.org/10.5281/zenodo.4459729 (Wang et al., 2021).
Highlights
According to the International Energy Agency (IEA) statistics, cement industry is the second-largest industrial CO2 emitter with a share of 27 % (2.2 Gt CO2 yr−1) in 2014 (IEA and World Business Council for Sustainable Development (WBCSD), 2018), and is estimated to account for approximately 7.4 % of the total anthropogenic CO2 emission in 2016 (Sanjuán et al, 2020)
There are two direct sources of CO2 emission originating from cement production: (1) the thermal decomposition of limestone (CaCO3) in the process of producing clinker; (2) the energy required for the decomposition, largely provided by combustion of fossil fuels
The traditional standardised ordinary Portland cement (OPC), which has been the dominant type of cement used by humans so far, has very high clinker content historically, i.e. high clinker-tocement ratio
Summary
According to the International Energy Agency (IEA) statistics, cement industry is the second-largest industrial CO2 emitter with a share of 27 % (2.2 Gt CO2 yr−1) in 2014 (IEA and WBCSD, 2018), and is estimated to account for approximately 7.4 % of the total anthropogenic CO2 emission in 2016 (Sanjuán et al, 2020). There are two direct sources of CO2 emission originating from cement production: (1) the thermal decomposition of limestone (CaCO3) in the process of producing clinker; (2) the energy required for the decomposition, largely provided by combustion of fossil fuels. For the latter, energy efficiency improvement and cement kiln technology advancement have gained noticeable progress in recent years (Shen et al, 2016; Xu et al, 2014; Zhang et al, 2015). Using this consistent framework and model, we could include regularly updated annual estimates of cement carbon uptake into annual assessments of the global carbon budget (GCB) (Friedlingstein et al, 2019) as an important anthropogenic carbon sink, which has not been thoroughly assessed or documented
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