Editing IPCC:AR6/WGIII/Chapter-11 (section)

==== 11.4.1.2 Cement and Concrete ====

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The cement sector is regarded as a sector where mitigation options are especially narrow ( [[#Energy%20Transitions%20Commission--2018|Energy Transitions Commission 2018]] ; [[#Habert--2020|Habert et al. 2020]] ). Cement is used as the glue to hold together sand, gravel and stone aggregates to make concrete, the most consumed manufactured substance globally. The production of cement has been increasing faster than the global population since the middle of the last century ( [[#Scrivener--2018|Scrivener et al. 2018]] ). Despite significant improvements in energy efficiency over the last couple of decades (e.g., a systematic move from wet to dry kilns with calciner preheaters feeding off the kilns), the direct emissions of cement production (the sum of energy and process emissions) are estimated to be 2.1–2.5 GtCO 2 -eq in 2019 or 14–17% of total global direct industrial GHG emissions ( [[#Lehne--2018|Lehne and Preston 2018]] ; [[#Bataille--2020a|Bataille 2020a]] ; [[#Sanjuán--2020|Sanjuán et al. 2020]] ; [[#Crippa--2021|Crippa et al. 2021]] ; [[#Hertwich--2021|Hertwich 2021]] ; [[#Lamb--2021|Lamb et al. 2021]] ) (Figure 11.4). Typically, about 40% of these direct emissions originate from process heating (e.g., for calcium carbonate (limestone) decomposition into calcium oxide at 850°C or higher, directly followed by combination with cementitious materials at about 1450°C to make clinker), while 60% are process CO 2 emissions from the calcium carbonate decomposition ( [[#Kajaste--2016|Kajaste and Hurme 2016]] ; [[#IEA%20and%20WBCSD--2018|IEA and WBCSD 2018]] ; [[#Andrew--2019|Andrew 2019]] ). Some of the CO 2 is reabsorbed into concrete products and can be seen as avoided during the decades-long life of the products; estimates of this flux vary between 15 and 30% of the direct emissions ( [[#Stripple--2018|Stripple et al. 2018]] ; [[#Andersson--2019|Andersson et al. 2019]] ; [[#Schneider--2019|Schneider 2019]] ; [[#Cao--2020|Cao et al. 2020]] ; [[#GCCA--2021a|GCCA 2021a]] ). Some companies are mixing CO 2 into hardening concrete, both to dispose of the CO 2 and more importantly reduce the need for binder ( [[#Lim--2019|Lim et al. 2019]] ).

One of the simplest and most effective ways to reduce cement and concrete emissions is to make stronger concrete through better mixing and aggregate sizing and dispersal; poorly and well-made concrete can vary in strength by a factor of four for a given volume ( [[#Fechner--2012|Fechner and Kray 2012]] ; [[#Habert--2020|Habert et al. 2020]] ). This argues for a refocus of the market away from ‘one size fits all’, often bagged cements to professionally mixed clinker, cementitious material and filler mixtures appropriate to the needs of the end use.

Architects, engineers and contractors also tend to overbuild with cement because it is cheap as well as corrosion- and water-resistant. Buildings and infrastructure can be purposefully designed to minimise cement use to its essential uses (e.g., compression strength and corrosion-resistance), and replace its use with other materials (e.g., wood, stone and other fibres) for non-essential uses. This could reduce cement use by 20–30% ( [[#Imbabi--2012|Imbabi et al. 2012]] ; Brinkerhoff and GLDNV 2015; [[#D’Alessandro--2016|D’Alessandro et al. 2016]] ; [[#Lehne--2018|Lehne and Preston 2018]] ; [[#IEA--2019b|IEA 2019b]] ; [[#Shanks--2019|Shanks et al. 2019]] ; [[#Habert--2020|Habert et al. 2020]] ).

Because so much of the emissions from concrete come from the limestone calcination to make clinker, anything that reduces use of clinker for a given amount of concrete reduces its GHG intensity. While 95% Portland cement is common in some markets, it is typically not necessary for all end-use applications, and many markets will add blast furnace slag, coal fly ash, or natural pozzolanic materials to replace cement as supplementary cementitious materials; 71% was the global average clinker content of cement in 2019 ( [[#IEA--2020a|IEA 2020a]] ). All these materials are limited in volume, but a combination of roughly two to three parts ground limestone and one part specially selected, calcined clays can also be used to replace clinker ( [[#Fechner--2012|Fechner and Kray 2012]] ; [[#Lehne--2018|Lehne and Preston 2018]] ; [[#Habert--2020|Habert et al. 2020]] ). Local building codes determine what mixes of cementitious materials are allowed for given uses and would need to be modified to allow these alternative mixtures where appropriate.

Ordinary Portland cement process CO 2 emissions cannot be avoided or reduced through the use of non-fossil energy sources. For this reason, CCS technology, which could capture just the process emissions (e.g., the EU LEILAC project, which concentrates the process emissions from the limestone calciner, see following paragraph) or both the energy and process-related CO 2 emissions, is often mentioned as a potentially important element of an ambitious mitigation strategy in the cement sector. Different types of CCS processes can be deployed, including post-combustion technologies such as amine scrubbing and membrane-assisted CO 2 -liquefation, oxycombustion in a low-to-zero nitrogen environment (full or partial) to produce a concentrated CO 2 stream for capture and disposal, or calcium-looping ( [[#Dean--2011|Dean et al. 2011]] ). The IEA puts cement CCS technologies at the technology readiness level (TRL) 6–8 ( [[#IEA--2020h|IEA 2020h]] ). These approaches have different strengths and weaknesses concerning emission abatement potential, primary energy consumption, costs and retrofittability ( [[#Hills--2016|Hills et al. 2016]] ; [[#Gardarsdottir--2019|Gardarsdottir et al. 2019]] ; [[#Voldsund--2019|Voldsund et al. 2019]] ). Use of biomass energy combined with CCS has the possibility of generating partial negative emissions, with the caveats introduced in Section ( [[#Hepburn--2019|Hepburn et al. 2019]] ).

The energy-related emissions of cement production can also be reduced by using bioenergy solids, liquids or gases (TRL 9) ( [[#IEA%20and%20WBCSD--2018|IEA and WBCSD 2018]] ), hydrogen or electricity (TRL 4 according to [[#IEA--2020h|IEA (2020h)]] ) for generating the high-temperature heat at the calciner – hydrogen and bioenergy co-burning could be complementary due to their respective fast-vs-slow combustion characteristics. In an approach pursued by the LEILAC research project, the calcination process step is carried out in a steel vessel that is heated indirectly using natural gas ( [[#Hills--2017|Hills et al. 2017]] ). The LEILAC approach makes it possible to capture the process-related emissions in a comparatively pure CO 2 stream, which reduces the energy required for CO 2 capture and purification. This technology (LEILAC in combination with CCS) could reduce total furnace emissions by up to 85% compared with an unabated, fossil fuelled cement plant, depending on the type of energy sources used for heating ( [[#Hills--2017|Hills et al. 2017]] ). In principle, the LEILAC approach allows the eventual potential electrification of the calciner by electrically heating the steel enclosure instead of using fossil burners.

In the long run, if some combination of material efficiency, better mixing and aggregate sizing, cementitious material substitution and 90%+ capture CCS with supplemental bioenergy are not feasible in some regions or at all to achieve near-zero emissions, alternatives to limestone-based ordinary Portland cement may be needed. There are several highly regional alternative chemistries in use that provide partial reductions ( [[#Fechner--2012|Fechner and Kray 2012]] ; [[#Lehne--2018|Lehne and Preston 2018]] ; [[#Habert--2020|Habert et al. 2020]] ), for example, carbonatable calcium silicate clinkers, and there have been pilot projects with magnesium-oxide-based cements, which could be negative emissions. Lower carbon cement chemistries are not nearly as widely available as limestone deposits ( [[#Material%20Economics--2019|Material Economics 2019]] ), and would require new materials testing protocols, codes, pilots and demonstrations.

Any substantial changes in cement and concrete material efficiency or production decarbonisation, however, will require comprehensive education and continuing re-education for cement producers, architects, engineers, contractors and small, non-professional users of cements. It will also require changes to building codes, standards, certification, labeling, procurement, incentives, and a range of polices to help create the market will be needed, as well as those for information disclosure, and certification for quality. Even an end-of-pipe solution like CCS will require infrastructure for transport and disposal. Abatement costs for these strategies vary considerably from case to case and for each a plausible cost range is difficult to establish, but they are summarised in Table 11.3 from the following literature and other sources ( [[#Wilson--2003|Wilson et al. 2003]] ; [[#Fechner--2012|Fechner and Kray 2012]] ; [[#Leeson--2017|Leeson et al. 2017]] ; [[#Moore--2017|Moore 2017]] ; [[#Lehne--2018|Lehne and Preston 2018]] ; [[#IEA--2019f|IEA 2019f]] ; [[#Habert--2020|Habert et al. 2020]] ).

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