Not only is this a critical distinction, but if you look into the papers cited in the studies the methodology appears flawed.
Within AR6 Chapter 5 the following statement is made:
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 first critique of this statement is that the timeline for carbonation is very similar to the decay of CO2 in the atmosphere, it is highly likely that this sink is already factored into GWP calculations, and thus any attempt by the cement industry to incorporate this into LCIs is probably double counting.
Secondly, a thorough review of the referenced literature finds that this figure is highly questionable, and at best is an upper estimate of the carbonation of cement. This error arises from a misinterpretation of the literature, and erroneous assumptions regarding the carbonation of concrete after service life. The studies quoted in (Friedlinstein et al. 2020): (Cao et al. 2020, Guo et al. 2020, and Xi et al. 2016), all rely on the carbonation model published by (Xi et al 2016), which makes assumptions about rubble stockpiling that are not borne out in practice, and the assumptions regarding the ongoing carbonation of demolished concrete and cement which is typically landfilled or used as roadbase are unrealistic.
Construction waste rubble is typically buried below grade and therefore is subject to much higher relative humidities than atmospherically exposed cement, in this condition the carbonation rates are controlled by leaching of calcium from the rubble, rather than CO2 diffusion. This process is strictly limited and unfortunately does not sequester CO2as efficiently as the authors contend. All studies to date rely primarily on the figure for buried concrete carbonation rates from (Lagerblad 2005), however the source explicitly states that this is an assumed carbonation rate, and no empirical evidence is provided to support the carbonation rates. Indeed (Lagerblad 2005) makes the following statement:
A normal concrete submerged in percolating water without erosion of the surface will leach less than 10 mm in 100 years and carbonates can only be observed in the outer 2-3 mm (Lagerblad 2001, 2003). Like carbonation, leaching approximately follows Fick ìs second law and the rate diminishes with the square root of time. One can, however, presume that all Ca-ions that leach will eventually form calcium carbonate either at the surface of the concrete or in the water. In soil the decay of organic matter may result in high CO2 concentration but on the other hand the speed of the diffusion of CO2 gas or carbonate ions in the soil may be slow.
This condition is the one most relevant to the discussion of carbonation rates after demolition, and would thus correlate to a carbonation rate of 0.2 - 0.3 mm/yr0.5. Furthermore, the authors don’t consider the kinetic limitations of diffusion of carbon dioxide, both in an atmospherically exposed stockpile, or below ground. Below grade, deterioration of concrete (e.g., by H2S and sulphates) is considered likely to greatly reduce the alkalinity available for reaction with CO2. For the brief period of time that the rubble is stockpiled they assume perfect conditions of CO2 exposure, humidity, and that the cement is perfectly spherical, perhaps these are reasonable assumption, but I’ve found no empirical evidence to support the carbonation rates assumed. (Pade and Guimaraes 2007) surveyed recycled concrete aggregate practices in Nordic countries and found that at best 5% of RCA ends up unbound and above ground. (Xi et al. 2016) go so far as to state that stockpiled rubble is very rarely stored under cover, because cement is hygroscopic it is my opinion that the particles in the stockpile will very quickly become saturated following rainfall or exposure to moisture (e.g., during hydrodemolition), restricting carbonation to very low rates.
(Xi.et al 2016) also provide carbonation rates based on 1,300 samples taken from across China, but provide no details on the source of these measurements, it is noted that their figures are approximately 30% lower than those of (Lagerblad 2005). They also assume that CO2 concentration is 3000 ppm below grade, without providing any empirical source for this assumption. Mattias Achternbosch and Peter Stemmermann provide a more detailed critique of the means and methods used to arrive at the 50% offset figure in the quoted studies, it is well worth a read.
Finally the point has to be made that the research used to underpin this carbon accounting fallacy has been funded by the cement industry, in particular the Swedish Cement and Concrete Research Institute.
Respectfully, the carbonation of cement stated in the report is highly dubious and based on simplifying assumptions that are unrealistic.
References
Friedlingstein, P. et al. Global Carbon Budget 2020. Earth Syst. Sci. Data 12 , 3269–3340 (2020).
Cao, Z. et al. The sponge effect and carbon emission mitigation potentials of the global cement cycle. Nat. Commun. 11 , 1–9 (2020).
Guo, R. et al. Global CO2 uptake of cement in 1930–2019. Earth Syst. Sci. Data Discuss. 2 , 1–28 (2020).
Xi, F. et al. Substantial global carbon uptake by cement carbonation. Nat. Geosci. 9 , 880–883 (2016).
Lagerblad, B. Carbon dioxide uptake during concrete life cycle - State of the art . CBI Rapporter (2005).
Pade, C. & Guimaraes, M. The CO2 uptake of concrete in a 100 year perspective. Cem. Concr. Res. 37 , 1348–1356 (2007).
Achternbosch, M. & Stemmermann, P. The carbon uptake by carbonation of concrete structures – some remarks by perspective of TA. 1–31 (2021). (web link)