With Concrete, Less Is More

Changes in demand may drive the future of zero-carbon concrete

As world leaders gather for the World Economic Forum’s annual conference in Davos to discuss global issues, including climate change, they will spend the day surrounded by concrete. Houses, skyscrapers, roads, highways, bridges, sidewalks, water systems, dams and more rely on concrete for its unmatched strength properties, durability, versatility and low cost. It is no surprise that global demand for cement, which hardens into concrete when mixed with water and minerals, is expected to increase by 48% from 4.2 billion to 6.2 billion tonnes by 2050.

In China’s recent massive urbanization, the nation used more concrete between 2011 and 2013 than the United States did in the entire 20th century. While China’s use of concrete is slowing, consumption in India, Africa and other developing countries will skyrocket amid economic development, with uses spread between residential, commercial and infrastructure. Although concrete is essential to building the structures that enhance our everyday lives, concrete is responsible for 8% of global carbon emissions; and 90% of these emissions come from the production of clinker, the primary strength-contributing ingredient in concrete.

In line with the Paris Climate Agreement, the global concrete industry must reduce emissions by 16% by 2030 and 100% by 2050 to stay within the 1.5°C carbon warming budget. This effort will require significant changes across the concrete value chain, but the easiest and most cost-effective measure is to use less material and at the same time meet the project requirements. Reducing demand for carbon-intensive clinkers will help put the concrete industry back on track to meet its climate goals.

Better design

The first step towards reducing the sector’s carbon footprint is to use less concrete in each application, even as applications grow. There are several existing and developing methods of ensuring concrete material efficiency that can lead to large carbon savings without changing the material itself. Traditional designs of buildings and other projects aim to minimize costs rather than carbon emissions and impose excessive design margins. However, recent and ongoing advances in automated design tools allow structural engineers and architects to quickly explore multiple structural options for a given project, taking greater consideration of material efficiency.

RMI publication Cost-effective decarbonisation of heavy transport and industrial heat delves into the concrete and steel savings that can be profitably achieved through better structural design. For example, such engineering methods saved New York’s Freedom Tower and Shanghai Tower in China 40% and 24% of concrete usage, respectively. As automated design software improves, lean design is expected to be cost-competitive (in design time) with current methods and become a major driver of demand reduction.

Another handle is not to use new concrete. In some cases, options such as recycling concrete elements from old structures can provide CO2 reductions. However, the right answer will vary from region to region and depend on a host of other factors, including the type of construction, design requirements, local availability of materials, and more.

At the same time, building codes and market preferences must be adapted to allow the use of low-carbon concrete. As carbon becomes an important consideration in building our infrastructure, we must close the innovation gap between designers’ needs and existing technology. Tools such as the EC3 tool effectively compare the embodied carbon of project design options, along with rigorous testing, will pave the way for the low-carbon structures of the future.

Low carbon concrete

In addition to reducing the total amount of concrete used in buildings, using less cement per concrete unit another effective way to reduce the clinker content and carbon intensity of concrete. To reduce binder intensity without increasing risk, we should move to bulk cement use such as ready-mixed concrete, where cement waste is reduced by up to 30%, mix specifications and mix preparation are more precise, and chemicals called admixtures can be added to improve concrete properties and reduce cement requirements.

In many countries, builders still prefer the use of cement in bags, which leads to both waste and overuse. Industrializing part of the world’s bagged cement market (currently 42%) will provide significant cement savings, but will require investment in ready-mix plants and cement trucks and significant changes in local market dynamics. In regions like the US and the EU, cement consumption is already largely industrialized, whereas the market share of bagged cement in India is almost 90%, offering a huge opportunity for carbon savings. Existing plants already incorporate a wide range of chemical additives such as dispersants that reduce the need for water and thus the amount of cement required. Other admixtures include accelerators, which strengthen concrete more quickly, and air entraining agents, which allow air bubbles to increase in volume and displace added solids for lower strength applications.

Innovation to create new admixtures as well as increase the usability, efficiency and cost competitiveness of existing solutions will allow for drastically lower clinker and cement content in concrete while maintaining performance in a given application.

Alternatives to traditional portland clinker cement have also been the subject of much research, but cost, material performance, availability of raw materials and energy input have resulted in limited use. While this leverage has some benefit, large-scale deployment of these niche solutions is unlikely in the near term, as they remain relatively customized products seeking niches in a highly commoditized world.

However, use of less common Portland cement has gained traction in another way – by substituting limited amounts of supplementary cementitious materials (SCMs) in cement mixes to partially displace clinker. SCMs exhibit similar behavior to clinker when mixed with water and contribute to the strength of the cement mixture, but cannot completely displace clinker in most cases. SCMs include industrial waste products such as ground granulated blast furnace slag (GBFS) and fly ash, calcined clay, natural pozzolans and ground limestone. The current ratio of clinker to cement is 0.72, but the Global Cement and Concrete Association (GCCA) aims for an 18% reduction in the average global clinker content of cement by 2050.

Choosing concrete with high clinker substitution rates can instantly reduce related carbon emissions from a traditional five-story building by 32% with less than a 0.5% increase in total construction costs. The ancient Roman Pantheon is constructed entirely of natural puzzolance cement, while more modern examples include The Spheres in Seattle and the iconic Tower in Cairo, both of which use Holcim’s low-carbon ECOPlanet cement. While many of these SCMs provide performance benefits, increased setting time and reduced early strength can delay project timelines and incur additional costs in some cases.

Improvement of the accelerating admixtures that improve the setting time and strength is essential for greater incorporation of the SCMs into mixed concrete. Further testing and updating standards are also essential to improve adoption. However, it is important to note that the supply of fly ash and GBFS is expected to decrease as their sources, coal power and steelmaking plants, respectively, are phased out and decarbonized, making the exploration and extraction of other SCMs even more critical to creating greener concrete. Given the global availability of limestone and calcined kaolinite clay, LC3 cement – consisting of 50% clinker, 30% calcined clay, 15% limestone and 5% gypsum – is seen as a promising approach for the future of low-carbon concrete. As new SCMs and improvements to the performance of current blended cements emerge, the clinker factor and carbon intensity of concrete will continue to decrease. However, time is of the essence in this crucial decade of climate action.

A net-zero future

Entities like the First Movers Coalition, ConcreteZero, the Industrial Deep Decarbonization Initiative and others are calling for low-carbon concrete now, and suppliers will have to act to match the growing demand. Innovation across the concrete value chain can reduce both emissions and costs, push the technical limits of carbon intensity and inform policy changes. We need a multi-pronged approach that targets reductions in concrete, cement and clinker. Structural engineers of the future will quickly explore an efficient frontier of design possibilities using state-of-the-art software, while ready-mix plants deliver less carbon-intensive concrete to projects. These demand reductions are only part of the equation, and supply-side measures such as alternative fuels, electrification and carbon capture will need to eliminate residual emissions.

Through the Mission Possible Partnership, RMI has partnered with the Energy Transitions Commission, Systemiq, World Economic Forum, European Cement Research Academy (ECRA) and GCCA to explore each of these decarbonisation arms in addition to supply-side decarbonisation arms in the soon-to-be-released Cement Sector Transition Strategy. The concrete and cement industry is on the cusp of a radical transformation on its journey towards the net-zero energy transition.

By Ben Skinner and Radhika Lalit, © 2023 Rocky Mountain Institute. Published with permission. Originally published on RMI Outlet.

Featured photo by Pawel Czerwinski on Unsplash

Related story, Snøhetta & Saferock transform mine waste into zero-carbon concrete




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