Editing Cement (section)

==={{chem|CO|2}} absorption===
Hydrated products of Portland cement, such as concrete and mortars, slowly reabsorb atmospheric CO2 gas, which has been released during calcination in a kiln. This natural process, reversed to calcination, is called carbonation.<ref name="pade2007">{{cite journal|last1=Pade|first1=Claus|last2=Guimaraes|first2=Maria|date=1 September 2007|title=The CO2 uptake of concrete in a 100 year perspective|url=https://www.sciencedirect.com/science/article/pii/S0008884607001317|journal=Cement and Concrete Research|volume=37|issue=9|pages=1348–1356|doi=10.1016/j.cemconres.2007.06.009|issn=0008-8846}}</ref> As it depends on CO2 diffusion into the bulk of concrete, its rate depends on many parameters, such as environmental conditions and surface area exposed to the atmosphere.<ref>{{Cite journal|last1=Xi|first1=Fengming|last2=Davis|first2=Steven J.|last3=Ciais|first3=Philippe|last4=Crawford-Brown|first4=Douglas|last5=Guan|first5=Dabo|last6=Pade|first6=Claus|last7=Shi|first7=Tiemao|last8=Syddall|first8=Mark|last9=Lv|first9=Jie |last10=Ji |first10=Lanzhu|last11=Bing|first11=Longfei|last12=Wang|first12=Jiaoyue|last13=Wei|first13=Wei|last14=Yang|first14=Keun-Hyeok|last15=Lagerblad|first15=Björn|date=December 2016|title=Substantial global carbon uptake by cement carbonation|url=https://www.nature.com/articles/ngeo2840|journal=Nature Geoscience|language=en|volume=9|issue=12|pages=880–883|doi=10.1038/ngeo2840|bibcode=2016NatGe...9..880X|issn=1752-0908}}</ref><ref name="cao2020">{{Cite journal|last1=Cao|first1=Zhi|last2=Myers|first2=Rupert J.|last3=Lupton|first3=Richard C.|last4=Duan|first4=Huabo|last5=Sacchi|first5=Romain|last6=Zhou|first6=Nan|last7=Reed Miller|first7=T.|last8=Cullen|first8=Jonathan M.|last9=Ge|first9=Quansheng |last10=Liu |first10=Gang|date=29 July 2020|title=The sponge effect and carbon emission mitigation potentials of the global cement cycle|journal=Nature Communications|language=en|volume=11|issue=1|pages=3777|doi=10.1038/s41467-020-17583-w|pmid=32728073|bibcode=2020NatCo..11.3777C|issn=2041-1723|doi-access=free|pmc=7392754|hdl=10044/1/81385|hdl-access=free}}</ref> Carbonation is particularly significant at the latter stages of the concrete life - after demolition and crushing of the debris. It was estimated that during the whole life-cycle of cement products, it can be reabsorbed nearly 30% of atmospheric CO2 generated by cement production.<ref name="cao2020" />

Carbonation process is considered as a mechanism of concrete degradation. It reduces pH of concrete that promotes reinforcement steel corrosion.<ref name="pade2007" /> However, as the product of Ca(OH)2 carbonation, CaCO3, occupies a greater volume, porosity of concrete reduces. This increases strength and hardness of concrete.<ref>{{cite journal|last1=Kim|first1=Jin-Keun|last2=Kim|first2=Chin-Yong|last3=Yi|first3=Seong-Tae|last4=Lee|first4=Yun|date=1 February 2009|title=Effect of carbonation on the rebound number and compressive strength of concrete|url=https://www.sciencedirect.com/science/article/pii/S0958946508001236|journal=Cement and Concrete Composites|volume=31|issue=2|pages=139–144|doi=10.1016/j.cemconcomp.2008.10.001|issn=0958-9465}}</ref>

There are proposals to reduce carbon footprint of hydraulic cement by adopting non-hydraulic cement, [[lime mortar]], for certain applications. It reabsorbs some of the {{chem|CO|2}} during hardening, and has a lower energy requirement in production than Portland cement.<ref>{{Cite news|url=https://www.theguardian.com/commentisfree/2007/oct/23/comment.comment|title=Response: Lime is a much greener option than cement, says Douglas Kent|last=Kent|first=Douglas|date=22 October 2007|work=The Guardian|access-date=22 January 2020|language=en-GB|issn=0261-3077}}</ref>

A few other attempts to increase absorption of [[carbon dioxide]] include cements based on magnesium ([[Sorel cement]]).<ref>{{Cite web|date=9 March 2011|title=Novacem's 'carbon negative cement'|url=https://ceramics.org/ceramic-tech-today/novacems-carbon-negative-cement/|access-date=26 September 2023|website=The American Ceramic Society|language=en-US}}</ref><ref>{{cite web|url=http://www.imperialinnovations.co.uk/?q=node/176|title=Novacem|archive-url=https://web.archive.org/web/20090803053655/http://www.imperialinnovations.co.uk/?q=node%2F176|archive-date=3 August 2009|website=imperialinnovations.co.uk}}</ref><ref>{{cite news|url=https://www.theguardian.com/environment/2008/dec/31/cement-carbon-emissions|work=The Guardian|location=London|title=Revealed: The cement that eats carbon dioxide|first=Alok|last=Jha|date=31 December 2008|access-date=28 April 2010|url-status=live|archive-url=https://web.archive.org/web/20130806151853/http://www.theguardian.com/environment/2008/dec/31/cement-carbon-emissions|archive-date=6 August 2013|df=dmy-all}}</ref>