I wanted to pick this thread up based on the recent Dezeen article that is quoting the cement industry flag waving about a 50% sequestration that is quoted in the IPCC AR6 report.
The full report AR6 WG1 is here: https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Full_Report.pdf
The quotes from the Dezeen article are in chapter 5: “Direct CO2 emissions from carbonates in cement production are around 4% of total fossil CO2 emissions, and grew at 5.8% yr-1 in the 2000s but a slower 2.4% yr-1 in the 2010s. The uptake of CO2 in cement infrastructure (carbonation) offsets about one half of the carbonate emissions from current cement production (Friedlingstein et al., 2020)”
The Friedlingstein paper is available here: ESSD - Global Carbon Budget 2020
It references papers by Cao et al. (2020): The sponge effect and carbon emission mitigation potentials of the global cement cycle | Nature Communications, Guo et al. (2020): https://essd.copernicus.org/preprints/essd-2020-275/essd-2020-275.pdf, and Xi et al. (2016): Substantial global carbon uptake by cement carbonation | Nature Geoscience
It is the 2016 paper by Xi that contains the carbonation model. The model calculates carbon sequestration during three stages in the ‘life cycle’: service life, demolition and secondary use of concrete waste. They assume that building service life is 35 – 70 years, which is somewhat concerning. The bulk of the carbonation in their model occurs after demolition, and they state that because buildings have shorter lives in China (35 years) this is a benefit to the carbon cycle. Clearly they do not consider that the carbonated concrete is usually replaced with something, typically this is more carbon emitting concrete. Reductio ad absurdum the authors are advocating for digging limestone up, calcining it to release emissions and then crushing it to absorb back 50% of the emissions, clearly this is not a sustainable outcome unless we are going to live in rubble. (At least one of the authors of this paper is from the Swedish Cement and Concrete Research Institute.)
The real flaw seems to be in the carbonation rates used - these are taken from literature and field reviews of atmospherically exposed concrete. However, it is well established that carbonation rates vary with relative humidity, see this RILEM report for instance: https://link.springer.com/article/10.1617/s11527-020-01558-w I would contend that demolished concrete is normally going to be in a saturated condition, and thus the carbonation rates will be negligible. Therefore the effective sequestration of CO2 is likley to be <5% of process emissions. Does anybody have any sources that could shed further light on this?