Lower carbon concrete is achieved not by any one action, but rather the cumulative effects of many material and design considerations.
Along the lifecycle of cement and concrete, there are generally five stages where carbon reductions are achieved, the responsibility for which is often shared among project stakeholders:
- Design and Construction
Clinker and Cement
Despite making up only 10 – 15% of a concrete mix, cement can represent up to 85% of the total carbon footprint. Cement manufacturing is an energy intensive process—specifically the manufacturing of clinker, an intermediate product in the manufacture of cement—contributing 7% of global greenhouse gas emissions. In Canada, the relative emissions from cement represent approximately 1.5% of our national emissions.
Recognizing its impact and responsibility, the cement industry in Canada has been improving its energy efficiency for decades, and rapidly implementing or exploring other key technologies levers to reduce the carbon footprint of its products, including:
- Lower carbon cements – We have introduced lower carbon cements like Portland-limestone cements (PLC) that reduces CO2 emissions by 10% compared to traditional cement, yet produces concrete of equivalent strength and durability. PLC is rapidly becoming the preferred standard for new construction projects, adopted by Canadian Standards Association (CSA A23.1/A23.2) and National Building Code. Once widely adopted across the country, PLC could reduce Canada’s greenhouse gas emissions by up to 1 megatonne annually.
- Transitioning to Low or Zero Carbon Fuels – We are working hard to overcome policy challenges so that we can accelerate our transition to low or zero carbon fuels, such as those derived from the waste stream, including: construction, demolition and agricultural waste and non-recyclable plastics. Low carbon fuel technologies are used commonly in Europe where some facilities have achieved reductions of up to 50% in the carbon intensity of their fuel mix. Achieving current global best-in-class fuel substitution rates could reduce cement industry GHGs by another 20% — about 2 megatonnes per year — in Canada.
- Investing in Carbon Capture, Storage and Utilization – We are deeply invested in longer term game-changing carbon capture, utilization and storage (CCUS) technologies that could transform concrete from a carbon emitter to a carbon neutral or even carbon negative building material. Several technologies are fast becoming commercially viable, and several Canadian cement facilities are well advanced in the implementation of carbon capture systems.
A product of its parts, concrete has an infinite number of possible compositions, which can be designed to meet very specific project requirements down to an elemental level. However, there is more than one mix that can meet the same performance requirements and there are numerous strategies available to reduce the carbon footprint of a given mix, depending on the conditions and tradeoffs that may be permissible. Here, the use of performance specifications can be particularly effective over more traditional, restrictive prescriptive specifications.
Some of the carbon reduction strategies a ready-mix producer may implement for a given project specification may include:
- Clinker Substitution – The aim of clinker substitution is to reduce the energy-intensive component of the cement product (i.e., clinker) while maintaining performance according to a given application’s requirements. Strategies for reducing the clinker content of cement include: the use of Portland-limestone cement and increased substitution rates for Supplementary Cementing Materials (SCMs). There are a wide variety of SCMs on the market that producers and designers are familiar with, and the benefits of the most commonly used SCMs are well known in terms of their impact on concrete properties. With climate change, however, there is an impetus to explore carbon reduction opportunities, and the pace at which new innovations are being developed for cement and concrete is accelerating beyond what has been typical historically.
- Innovative Concrete Utilization Technologies – Commercially viable technologies accelerate carbonation. This is accomplished either by injecting CO2 into concrete, curing concrete in CO2, or creating artificial limestone aggregates using CO2. For example, one company uses CO2 captured from industrial emissions, which is then purified, liquefied, and delivered to partner concrete plants in pressurized tanks. This CO2 is then injected into the concrete while the concrete is being mixed, which converts the CO2 into a solid-state mineral within the concrete. The minerals formed enhance compressive strength. The process reduces CO2 emissions in two ways: through direct sequestration of CO2 injected into the concrete mixture and by reducing cement demand since this concrete requires less cement to produce concrete at a specified strength.
- Use of Admixtures – Almost all concretes use some sort of admixture. Most affect the plastic properties to make concrete more workable, economical, shorten or lengthen set time, and so on, improving performance and efficiency. Since admixtures have an extremely low carbon footprint and represent only 1 per cent or less of the concrete materials, they also can dramatically improve concrete performance. While these materials are commonly thought of in terms of the significant workability enhancements they provide to concrete placement and finishing operations, they also have the ability to significantly reduce the environmental footprint of concrete as their possible use is considered for addressing the overall carbon reduction goals of the project.
Design and Construction
Design and construction teams play a key role in reducing the carbon footprint of their projects. They can unlock concrete’s carbon reduction potential through setting and communicating carbon reduction goals for their project, optimizing designs, and perhaps most importantly, enabling the strategies identified elsewhere in the production chain. In general, designers should endeavour to:
- Focus on performance rather than prescriptive mix designs, allowing the use of Portland-limestone cements, increased substitution rates for supplementary cementing materials (SCMs), utilizing the latest edition of the CSA standards for Cement and Concrete (CSA A3000, CSA A23.1/A23.2)
- Optimize designs according to each concrete element, ensuring the strength and strength gain time horizons are carefully considered to allow for more flexibility with concrete mix designs.
- Optimizing concrete volume to maximize material efficiency, utilizing voided floor slabs or increased spacing of columns to reduce concrete volumes and eliminate or reduce waste concrete.
- Leverage concrete’s carbon sequestration properties in their overall design, maximizing the exposed surface area of concrete and utilizing best practices for end-of-life recycling.
When evaluating a project with the aim of reducing the embodied emissions in concrete, it is best to work directly with concrete producers and contractors, engaging them early in the process, as they understand what technologies and concrete ingredients are available locally. Local concrete producers will be familiar with concrete mix optimization strategies that can best meet your project goals.
Further, by requiring concrete to be supplied by a certified ready-mix facility, specifiers can be assured that their concrete supplier is not only using equipment that is kept in good operating condition, but that it is capable of producing quality concrete as per Canadian Standards Association A23.1 – Concrete Materials and Methods of Concrete Construction, and A23.2 –Test Methods and Standard Practices for Concrete. For more information on plant certification, please visit your Provincial Ready-Mix Concrete Association’s website.
For additional guidance, check out the links provided below:
Carbonation (Carbon Uptake)
Concrete’s ability to naturally sequester carbon from the atmosphere, permanently capturing it in a process known as carbonation (or carbon uptake), is among concrete’s least well-known advantages. The rate of CO2 uptake depends on many conditions and is difficult to predict. However, what is known is that rates of CO2 uptake are greatest when the surface-to-volume ratio is high, such as when concrete has been crushed and exposed to air. During the design phase of a project, an effective strategy for maximizing CO2 uptake is for architects and engineers to specify the use of exposed concrete wherever possible. At end of life, current best practices for maximizing the benefits of carbonation involves crushing the concrete and leaving exposed to air for up to two years before re-use in other applications.