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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Alkali-activated ground granulated blast-slag (AAS) is the most obvious alternative material for ordinary Portland cement (OPC). However, to use it as a structural material requires the assessment and verification of its durability. The most important factor for a durability evaluation is the degree of carbonation resistance, and AAS is known to show lower performance than OPC. A series of experiments was conducted with a view to investigate the carbonation characteristics of AAS binder. As a consequence, it was found that the major hydration product of AAS was calcium silicate hydrate (CSH), with almost no portlandite, unlike the products of OPC. After carbonation, the CSH of AAS turned into amorphous silica gel which was most likely why the compressive strength of AAS became weaker after carbonation. An increase of the activator dosage leads AAS to react more quickly and produce more CSH, increasing the compaction, compressive strength, and carbonation resistance of the microstructure.

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The objective of the present study is to ascertain the effect of fly ash (FA) on the lifecycle CO2assessment of reinforced concrete structures. The reliable lifecycle CO2assessment approach of concrete structures were established and then specified using the developed CO2a performance evaluation table. The system boundary studied was from cradle to recycling of concrete, which includes material system, concrete production, transportation, construction, use and recycling activity phases. The assumed time and regional boundaries for concrete mixes were 2012 and Seoul, South Korea, respectively. The carbonation depth of concrete and CO2uptake during use of structure and recycling phases were calculated based on the Yang et al.’s model. Using the performance evaluation table, the effect of FA on the lifecycle CO2assessment of concrete columns and beams in an office building was examined under the different concrete strengths. The parametric study clearly showed that high-strength concrete is favorable to the reduction of lifecycle CO2amount of concrete columns, whereas the reduction is not expected for concrete beams. The lifecycle CO2amount of concrete structures decreases with the increase in the substitution level of FA up to 20%, beyond which the decreasing rate is insignificant.