CONCRETE

CHOOSE LOW EMISSIONS CONCRETE

Choose Low Emissions Concrete

It is important for designers to understand what is possible on the supply-side so that they have complete knowledge to engage in whole supply stream collaboration. Concrete suppliers are limited by what is regionally available and by what contractors, engineers, and designers require. Understanding what ingredients are available and the effects of those ingredients should guide the designer towards efficient applications. The best supply-side opportunities to reduce emissions begin by 1) mixing concrete smarter and using cement wiser. New processes can be introduced to 2) reduce and replace clinker and 3) abate heat-related emissions. The final step to fully decarbonize is to 4) employ carbon capture and utilization/storage (CCUS) technologies.

1. Mix Concrete Intelligently and Use Cement Wisely

Reducing the amount of cement (and therefore clinker) used per unit volume of concrete is the most direct way to reduce emissions. Best practices to reduce demand for cement can save 9% of sector-wide emissions¹. Furthermore, improving the quality of concrete mixes reduces the range of error, which reduces total over-design and thus emissions¹.

Achieve these reductions by:

Enabling flexibility in mix design with performance specifications
Considering the mixing method
Considering recycled aggregates, and
Sourcing high-quality aggregates

Enable flexibility in mix design by using performance specifications: Using performance specifications provides flexibility for the contractor and supplier to find the lowest emission concrete mixtures that meet the performance criteria. Performance specifications outline the characteristics of fresh and hardened concrete, depending on the application and aspects of the construction process that are necessary. Frequently, prescriptive specifications restrict innovation by concrete suppliers and contractors. Performance specifications should clearly specify the test methods and acceptance criteria for verification and enforcement.

Performance specifications can remove the following misconceptions present in many prescriptive specifications:

Minimum cement and/or maximum Supplemental Cementitious Material (SCM) quantity can result in the use of more cement than necessary.
Imposing a low water-to-cement ratio limit can increase cement content and affect the ability to place and finish concrete.
Using a maximum water-to-cement ratio, where not required, increases costs and does not support sustainable development.
Specifying air content reduces strength, thereby requiring more cement. Specifying air content that is inappropriate for a structural member increases costs and materials while reducing performance and sustainability.

Consider the mixing method: Some mixing methods can create high-strength concrete with a lower volume of cement, and therefore less emissions. For example, the aggregate process of scattering-filling – which involves vibrating the mixture while it is poured, paved, or placed – can result in 10-30% less cement being used than is typical, while increasing compressive strengths⁷. This method is sometimes called “controlled particle size distribution.”

Consider using recycled aggregates, where appropriate: Recycled aggregates are relatively low-cost and have lower embodied emissions than new aggregates. Use of recycled aggregates may be appropriate for certain applications that are not sensitive to aggregate absorption, such as pavements. However, recycled aggregates typically have a high variability in shape and texture and have been found to have higher absorption values, therefore requiring more water and ultimately more cement.

Source quality characteristics of aggregates: Aggregates form the bulk of the mass and they are inert. Their characteristics, however, play a significant role in water demand, and therefore cement demand. Aggregates can affect cement use by up to 17%. Detailed assessments of the characteristics of aggregates is key to understanding the emissions impact. Frequently, recycled aggregates and secondary aggregates can be utilized, but careful analyses and tests must be performed to understand the emissions impact and performance of proposed aggregates².

Whenever possible, specify strong aggregates to reduce the required cement quantity and create concrete with a high resistance to abrasion and a longer lifespan. Weak, lightweight, soft, and porous aggregates can require more cement and have low wear resistance reducing the material’s lifespan.

2. Reduce & Replace Clinker

Reduce the impact of clinker by:

Improving the clinker-binder ratio
Replacing clinker in cement with SCMs, and
Replacing cement with alternative binders

Improve the clinker-binder ratio: The clinker-binder ratio, sometimes called the clinker-cement ratio, is the proportion of cement that is made up of clinker. Improving this ratio so there is less clinker will reduce emissions. Currently, the clinker-binder ratio averages 0.63, ranging anywhere from 0.53-0.96. Reducing the ratio to 0.52 can reduce costs by 5-15% and emissions up to 13%. This has no substantial impact on performance and it paves the way for the use of more SCMs.

If Ground Granulated Blast-Furnace Slag (GGBS) is used as an SCM, a clinker-binder ratio as low as 0.2 is possible.

Replace clinker with Supplementary Cementitious Materials (SCMs): Supplementary Cementitious Materials (SCMs) can be used to reduce the proportion of clinker in portland cement. They come from a wide range of naturally-occurring materials and industrial byproducts, which supports circularity, lowers the cost of cement by up to 15% by 2050, and reduces emissions by up to 18% by 2050. Many SCMs also improve the durability of the concrete product. The use of SCMs is regulated by cement and concrete standards¹. Owners, designers, and builders must seek to procure the best options available, being aware that commercial availability of cementitious materials may vary over time².

EFFECTS OF SUPPLEMENTARY CEMENTITIOUS MATERIALS ON CONCRETE PROPERTIES

Replace cement with alternative binders: The clinker in cement has two primary constituents, alite and belite, which are calcium silicates and responsible for early strength gain and later-age strength gain, respectively. Alternative binders can be used to replace some of the cement in order to reduce cement emissions.

Alternative binders vary in their hardening processes. Some use calcium silicates, but require less alite (and therefore less limestone). Others are entirely different chemistries and/or processes for the hardening of concrete. Each alternative binder represents different challenges and emissions reduction opportunities.

3. Abate Heat-Related Emissions

35% of the concrete sector’s emissions come from heating limestone to create clinker. Coal and petcoke account for 82% of the heat energy used in the industry¹. This means that a full 2% of the world’s GHG emissions come from the coal and petcoke used to heat limestone for clinker. Using less clinker should be the first step to reduce heat-related emissions. Next steps include:

Investigate what kiln type is use
Explore sourcing from cement plants that have electric kilns that run on a prominent renewable energy supply, and
Consider plants that use bio waste or industrial waste if no renewable-powered electric kilns are available

Investigate which kiln type is used: Request that cement comes from the least energy intensive kiln that is locally available. The different kiln types used for heating limestone, listed in increasing order of energy intensity, are: dry with preheater and precalciner, dry with preheater, long dry, and wet. Dry with preheater and precalciner kilns use an average of 85% less energy than wet kilns⁸.

Explore sourcing from cement plants that have electric kilns powered by renewable energy: Electrifying clinker production has increasing benefits as the proportion of renewables in the electricity supply increases.

Opt for plants that use bio waste or industrial waste as fuel: Using bio waste or industrial waste increases overall circularity and reduces emissions and costs in both the concrete and waste sectors. Furthermore, some ashes from the combustion of waste can be used as SCMs. If favorable regulations were deployed globally, waste-use for clinker production would increase from 6% today, to 40% in 2050. If the waste is industrial, then carbon capture would be required to bring emissions to zero and control non-CO2 emissions.

4. Implement Carbon Capture and Utilization/Storage

Carbon capture utilization and/or storage (CCU/S) is an existing and developing technology that captures the carbon during industrial processes (including clinker production) and then utilizes the carbon in other processes and products, or stores it underground. Ideally, many industrial facilities that intend to capture their emissions with CCU/S will be clustered in one area so that they can benefit from sharing a single CCU/S infrastructure system. At the moment, this technology faces economic feasibility challenges, but should be supported wherever available.

With concrete, in particular, it is possible to inject the CO₂ back into the concrete during mixing. However, these methods are currently limited and expensive.