Concrete and Cement

The concrete and cement sector has a disproportionate impact on greenhouse gas emissions and will continue to do so for many decades. Even if all of the existing methods to reduce the climate impacts of the sector were successfully implemented and scaled, additional funding would still be urgently required for public-private partnerships to accelerate the development, demonstration and commercialisation of decarbonisation strategies. Key strategies include: 1) chemical carbon reduction of concrete and cement production techniques; 2) carbon capture and storage at manufacturing plants; 3) design for disassembly and re-use of components; 4) novel (bio-based) concrete mixtures to reduce binder requirements; and 5) computer-assisted and additive manufacturing to reduce carbon emissions from transport and on-site construction waste.

Figure 5.2 Total in-use cement stocks, by region, 1931-2014

In-use cement stocks have surged in Asia but are flattening in Europe and North America.

Source: Cao et al. 2017.

Concrete Is Widely Used in Buildings and Construction But Is Not Always Needed

Concrete buildings of less than 12 storeys could shift to bio-based structural assemblies.

Concrete is by far the most widely used construction material in the world, due in part to its strength and durability. It is produced by mixing cement and water with an aggregate, typically sand or gravel. In 2020, 4,300 million tons of cement were produced globally (IEA 2022a). Concrete mixtures are used for both residential and non-residential buildings and for infrastructure (e.g., railways, bridges).
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Concrete has experienced 10-fold growth over the past 65 years, compared with a 3-fold increase in steel production and near-stagnant growth in timber production (Monteiro, Miller and Horvath 2017). Globally, in-use cement stocks – the amount of cement embedded in existing buildings and infrastructure – have surged in Asia, while they are flattening in Europe and North America (see Figure 5.2). Since the mid-2000s, China has built the world’s largest in-use cement stocks, mostly in its buildings (80 per cent) and to a lesser extent in its infrastructure (20 per cent) (Cao et al. 2017).

In many applications, including housing, concrete is used where lower-carbon materials could suffice, largely because concrete has a developed supply chain and infrastructure, with ease of use and calculation. Many concrete buildings of less than 12 storeys could shift to bio-based structural assemblies for everything but the foundation and elevator shafts, if sustainable materials were available.

Figure 5.3 Dominance of concrete and cement in the embodied emissions of newly constructed buildings

Within concrete production, the main emissions are from cement production, in particular limestone processing.

Note: Figure (a) compares the operational (heating/ventilation/cooling over lifetime) and embodied (construction and maintenance) greenhouse gas emissions from the existing global building stock versus new construction. Figure (b) shows that for new construction, the largest emissions from a typical multi-family concrete building come from concrete production. Figure (c) shows that within concrete production, the main emissions are from cement production, in particular limestone processing (d). Source: Habert et al. 2020.

Concrete Contributes to Rising Global Greenhouse Gas Emissions, Among Other Impacts

Cement production accounts for 7% of global greenhouse gas emissions.

In cement manufacture, raw materials are milled to a homogeneous powder before being heated at high temperatures into clinker. The clinker is blended with gypsum to produce cement (IEA 2018a). Cement is the binding material in concrete, typically comprising around 10-15 per cent of the total (Habert et al. 2020). However, cementitious materials are by far the most carbon-intensive to produce, with cement production accounting for around 7 per cent of global CO2 emissions (Hasanbeigi 2021).
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Cement production is considered to be one of the most difficult industrial processes to decarbonise (Davis et al. 2018). This is because the majority of the carbon emissions (55-70 per cent) are released in the chemical process of converting limestone to calcium oxide; another 30-45 per cent of emissions stem from fuel combustion during the production process (IEA 2018a; Cao et al. 2020). Overall, producing one ton of clinker in a modern cement plant can generate around 600 kilograms of CO2 (Fennell, Davis and Mohammed 2021). Electrification of cement production with renewable sources can substantially lower emissions.
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For new construction, the largest embodied emissions from a typical multi-family concrete building are from cement production, in particular the processing of limestone into clinker (see Figure 5.3). This points to the urgent need to reinvent the cementitious binders used in concrete mixtures. Traditionally, ordinary Portland cement has been used as the binder to produce concrete and mortar; however, it is the material responsible for the highest CO2 emissions in cement production.
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To address rising emissions from the sector, there is an urgent need to substitute conventional cement components with low-carbon alternatives, such as by-products from industrial, agricultural, forestry and end-of-use sources. In the near term, cement demand can be reduced using available means by efficiently optimising the ratio of cement in concrete mixes and reducing rampant waste in construction due to lack of oversight and certification.

Concrete has other negative environmental impacts across its life cycle. In urban areas, concrete, along with asphalt surfaces, absorb more heat than natural vegetation, disproportionately contributing to urban heat island effects (Mohajerani, Bakaric and Jeffrey-Bailey 2017) and to the rising global demand for carbon-intensive cooling and air-conditioning systems. The impervious surfaces created by concrete and asphalt contribute to surface run-off, polluting waterways and causing soil erosion and flooding.

Figure 5.4 Evolutionary stages of per capita in-use cement stocks, by country

Future growth in cement use will likely be highest in Africa and South America, followed by Asia.

Note: The colouring of countries follows the progressive stages of per capita cement use shown in the chart at bottom left.
Source: Cao et al. 2017.

Developing Countries Have an Opportunity to Leapfrog Global Trends in Concrete Use

Since the early 2000s, China and other Asian countries have dominated global cement demand, accounting for 80 per cent of cement production in 2014 (Rissman et al. 2020). The region’s high use of cement has surged to meet the infrastructure needs of an expanding middle class. This rapid growth is in line with a study across 184 countries that links per capita in-use cement stocks to levels of affluence. The study found that as countries develop economically, they go through five progressive stages: from little cement use per capita (A), to a take-off stage with high growth rates (B and C), followed by a slow-down stage (D) and eventually a shrinking stage (E) (see Figure 5.4).

Figure 5.5 Potential for regions to leapfrog towards more wealth and less carbon-intensive cement

Through material efficiency strategies and low-carbon production, countries can decouple cement use from income.

Source: Adapted from Schmidt 2017.

The figure shows that Africa and South America have the lowest per capita in-use cement stocks (green) followed by most of Asia (blue). China is in a phase of rapid growth (red), whereas Europe, North America and Oceania no longer have strong growth in cement stocks (yellow). Japan and Sweden, meanwhile, are seeing a decline (brown), which is attributed in part to successful material efficiency strategies that allow for the same building and infrastructure services but with less cement use. These historical patterns suggest that China’s rapid growth in cement use could reach saturation in the near future, and that future growth will be highest in Africa and South America, followed by the rest of Asia.
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However, it will be crucial to break the global pattern of rising cement use while simultaneously increasing the living standards and urbanisation rates of low-income countries. Ideally, these countries will implement a mix of material efficiency strategies, coupled with low-carbon cement production, that enables them to leap-frog towards higher affluence with relatively low per capita cement consumption (see Figure 5.5) (Schmidt 2017). A key enabling tool will be reduction of the clinker-to-cement ratio by using novel supplementary cementitious materials from forestry and agricultural by-products.

Even with a shift towards bio-based materials, the rapid growth in urban density and infrastructure in developing countries means that the high-carbon cement and concrete sector will continue to soar for the foreseeable future.

Alternative, Low-Carbon Cement Binders Can Replace Portland Cement and Reduce Emissions

With many regionally available options, most major cement-producing countries could substitute alternative low-carbon options.

Significant potential to decarbonise cementitious materials exists along their life cycle, with the largest opportunities occurring in the production stage (57 per cent), followed by manufacturing (23 per cent) and end-of-use (14 per cent) (Pamenter and Myers 2021). During production, the use of alternative, low-carbon materials for concrete binders presents the largest decarbonisation potential (see Annex 2). Unlike for fly ash and granulated blast furnace slag, there is no supply shortage for many alternative secondary cementitious materials, especially bio-based ones derived from agricultural by-products.
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The road to net zero concrete by 2060 will require replacing Portland cement with the many regionally available options being explored around the globe from agricultural, forestry and industrial by-products, as well as from end-of-life materials (see Annex 2). Most major cement-producing countries could generate enough of these alternative materials to substitute most of their demand for Portland cement, with the primary outlier being China, the world’s largest producer of Portland cement (Shah et al. 2022). The study estimates that the theoretically achievable lowest clinker-to-cement ratio is around 0.14 globally, reflecting a 61 per cent reduction in the use of Portland cement compared to the current average (0.75).

Figure 5.6 Whole life-cycle thinking in the cement and concrete sector

The whole-systems pathway results in emission reduction through more efficient use of cement and concrete.

Note: The figure illustrates the possibility to expand the levers for decarbonisation in the United States of America, China, and India through whole life-cycle stakeholder accountability. Each grey bar in the circular bar charts corresponds to the sum of cumulative net CO2 savings across the cement and concrete cycle by 2060 in the buildings and road sectors for the three countries. Source: Cao et al. 2021.

Whole Life-Cycle Thinking Will Enable More Efficient Methods and Use of Cement and Concrete

Engaging stakeholders across the value chain offers flexibility on the path to net zero.

With a focus on the three largest cement producers and consumers globally – the United States of America, China and India – Figure 5.6 describes two distinct pathways to achieve net zero emissions in the cement and concrete sector by 2060. These are: a production-centric pathway that relies entirely on the efforts of cement and concrete producers to mitigate emissions from the sector, and a whole-systems pathway that engages a broad range of actors – from producers to designers and recyclers – to implement more efficient methods and use of cement and concrete (Cao et al. 2021).

The whole-systems pathway results in an overall reduction of demand (and therefore emissions) through a more efficient use of cement and concrete in the built environment. As a result, it is less dependent on the need for maximum measures at the production level. In other words, engaging with all stakeholders across the value chain offers much-needed flexibility on the pathway towards net zero emissions by 2060. This includes a growing importance of the end-of-use stage, as the massive quantities of structures dating from the mid-20th century are due for replacement.
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Engagement of stakeholders across the life cycle is key to integrate both production-centric and whole-systems decarbonisation scenarios (Cao et al. 2021). For whole-systems approaches to be adopted, mechanisms for knowledge sharing and transfer need to be established among producers, architects, developers and building maintenance operators. However, even if all the existing levers are incentivised, immediate actions are needed to galvanise research and development of innovative methods. Merely capitalising on current opportunities will not be enough to achieve net zero emissions by 2060.
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More radical but still speculative methods for carbon capture during production show promise but require further analysis and development. Carbon capture and utilisation for concrete production (CCU concrete) has been projected to save between 0.1 to 1.4 gigatons of CO2 by mid century, and there are claims to extremely significant enhanced structural performance (ICEF 2016). However, there are conflicting opinions as to whether the benefits in increased strength and optimisation of materials will outweigh the carbon costs of capturing, transporting and incorporating the captured CO2 into concrete products. To scale these technologies, it will be critical to verify the enhanced compressive strength from CO2 curing and mixing, while ensuring that all electricity used in CO2 curing is supplied through renewables to produce a net CO2 benefit from CCU concrete (Ravikumar et al. 2021).

KEY STEPS TOWARDS

Decarbonizing Cement and Concrete

Shift to renewable electricity in cement production

The highest priority for cement decarbonisation is to electrify the grid and the means of production, using renewable energy resources such as solar and wind power.

Electric kilns should be the standard for any newly built cement plants (Global Cement and Concrete Association [GCCA] 2020; IEA 2022b).

Prioritise locally sourced alternative binders to reduce the clinker-to-cement ratio

Portland cement contains more than 90 per cent clinker (a clinker-to-cement ratio above 0.9).

Blending can reduce the clinker-to-cement ratio to around 0.75. In a net zero scenario, this could go down to 0.69 by 2025 and 0.56 by 2050 (Pamenter and Myers 2021; GCCA 2022; IEA 2022b).

Reducing the clinker-to-cement ratio to 0.5 and below could be achieved using (bio-based) secondary cementitious materials. Local protocols would be needed for testing and certification.

Use of calcined clay limestone (LC3) could reduce the clinker-to-cement ratio to 0.5 using existing technologies, and is close to commercialisation (Scrivener et al. 2018; Fennell et al. 2022)

Including lime clasts in cement mixtures could provide durability and extend the life of concrete applications, resulting in energy savings (Seymour et al. 2023). However, building codes and design practices will need to adapt to the variable material properties (Scrivener, John and Gartner 2018).

Avoid unneeded concrete use through training, building standards and design

Material scientists need to be educated on the plethora of new cement production technologies so they can optimise the material input and technology for the given context; this requires better communication among scientists, structural engineers and architects (Schmidt, Alexander and John 2018).

Regularly updating building codes to account for these technological advances will be key, ideally coupled with incentives for manufacturers to produce the most low-carbon cement and concrete.

Material efficiency should be a key consideration in building design, avoiding overspecification and using concrete only in those applications that require its outstanding structural properties.

Changes to building codes, alongside education of architects and engineers to use best available technologies, could save over 25 per cent of cement by reducing overengineering (IEA 2019).

Use digital methods to improve material efficiency and allow for pre-fabrication

“Design for circularity” and systems integration can revolutionise material flows through the use of digital methods and artificial intelligence. Industry must be supported to adapt and modernise.

Digitalisation across the cement life cycle (via improved process controls, next-generation measurement devices) can improve efficiencies and reduce emissions (Fennell, Davis and Mohammed 2021).

Moving inefficient and emission-intensive on-site construction to factory-controlled fabricated assemblies can reduce on-site pollution and increase the use of circular, recyclable components.

An industry-wide effort is needed to reduce material consumption, optimise structures, and design customised parts through pre-fabrication and digitised construction, which produces an inventory of circular components for future disassembly and re-use (see Box 5.1).

Standards need to rely on performance-based metrics rather than prescribing outmoded conventions, so that cement production can be adapted to local needs (Scrivener, John and Gartner 2018).

Improve concrete through carbon capture and storage, which has partial future potential

Capturing and storing carbon (either underground or within materials to enhance material strength) is critical to reduce emissions from cement production.

To achieve the International Energy Agency’s scenario for net zero emissions, around 95 per cent of CO2 emissions from cement would need to be stored by 2050, up from just 5 per cent by 2030 (IEA 2022a). Currently, less than 0.1 per cent of all global emissions are captured and stored.

Because the CO2 stream needs to be almost pure to store it cost effectively, research is urgently needed on more viable methods to scale up carbon capture and storage (see Box 5.2).

Carbon capture and storage cannot be the only answer. Relying solely on improvements in these technologies within the cement and steel industries will require a 14,000 per cent increase in carbon storage capacity by 2050; meanwhile, in the last 10 years, the world has witnessed a 30 per cent reduction, rather than increase, in carbon storage capacity (Global CCS Institute 2020).

Urgently innovate cement and concrete recycling

Currently, less than 1 per cent of concrete is made from recycled materials (Cao et al. 2020; Pamenter and Myers 2021).

Design for circular recycling and reuse has lagged in the cement and concrete sector, even as these materials have disproportionate impacts on operational carbon across many climates.

Figure 5.7 Creation of hollow-core concrete slabs at Botswana Innovation Hub

Design optimisation of modular components can greatly reduce the use of steel.

Source: From Left to Right: Scheme of a Termodeck slab (Spiroll); Pre-fabricated modular hollowcore concrete (Spiroll); Vortex Extruder on-site “mobil factory”; Botswana Innovation Hub (ShoP Architects; Buro Happold Consulting Engineers)

Box 5.1

On-site pre-fabrication of modular concrete components in Botswana

Design optimisation of modular components can greatly reduce the use of steel for environmental control systems, leading to energy savings and future circularity. Architects and engineers for the Botswana Innovation Hub have developed methods for creating prefabricated modular concrete components using an on-site “mobile factory.” These include hollow-core concrete slabs for buildings that greatly reduce the amount of steel and equipment needed for ducting and environmental control systems (see Figure 5.7). The slabs could lead to operational energy savings in buildings of 20-50 per cent and to reduced peak cooling loads of 70-90 per cent.

Box 5.2

Emerging research on storing carbon dioxide in concrete

Capturing carbon in concrete production is an active area of research around the world. However, the exact amounts of CO2 that could be absorbed by concrete are uncertain. This approach should be considered emerging and is not yet included in emission inventories overseen by the United Nations Framework Convention on Climate Change. At the University of California at Los Angeles, a research project is under way to upcycle carbon by taking CO2 directly from the exhaust stream of a coal plant and transforming it into concrete building blocks. In Canada, the company CarbonCure claims to have delivered 2 million truckloads of concrete injected with CO2, saving 132,000 tons of CO2 (Fennell et al. 2022).