Calcination of carbonate rocks during the manufacture of cement produced 5% of global CO2 emissions from all industrial process and fossil-fuel combustion in 20131, 2. Considerable attention has been paid to quantifying these industrial process emissions from cement production2, 3, but the natural reversal of the process—carbonation—has received little attention in carbon cycle studies. Here, we use new and existing data on cement materials during cement service life, demolition, and secondary use of concrete waste to estimate regional and global CO2 uptake between 1930 and 2013 using an analytical model describing carbonation chemistry. We find that carbonation of cement materials over their life cycle represents a large and growing net sink of CO2, increasing from 0.10 GtC yr−1 in 1998 to 0.25 GtC yr−1 in 2013. In total, we estimate that a cumulative amount of 4.5 GtC has been sequestered in carbonating cement materials from 1930 to 2013, offsetting 43% of the CO2 emissions from production of cement over the same period, not including emissions associated with fossil use during cement production. We conclude that carbonation of cement products represents a substantial carbon sink that is not currently considered in emissions inventories1, 3, 4

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Cenospheres are hollow fly ash particles. When performing air void analysis on a contrast enhanced plane section, air inclusions in cenospheres are counted as air voids. In the present study, air void analyses for air entrained concrete mixtures with fly ash (up to 50% of binder mass) were corrected based on chord counting for non-air entrained paste samples with various contents of fly ash. The correction only lead to a small reduction of the total air content, but it increased the spacing factor up to 25%. The concrete mixtures were also exposed to salt frost scaling testing. The amounts of scaling were unacceptable for several mixtures with high dosages of fly ash. Inferior strength or inadequate air void structure could not explain this. Additional testing pointed to that chemical surface degradation aggravated the physical frost attack for concrete mixtures with high contents of fly ash.