CONCRETE

Carbon Impact of Concrete

Concrete is the most widely used construction material in the world, and is responsible for 6-10% of global anthropogenic carbon dioxide (CO₂) emissions. Portland cement is the primary ingredient in concrete and is responsible for the majority of concrete’s carbon emissions. CO₂ is released at two points during cement production: roughly 40% of the CO₂ generated is from the burning of fossil fuels in the manufacturing process, and the remaining 60% is from naturally occurring chemical reactions during processing. Technology is available to capture the CO₂ emitted during chemical reactions and re-inject it into the concrete mix, thereby sequestering, instead of emitting, this CO₂¹. The proportion of ingredients in a concrete mixture can greatly influence its carbon impact.

Using less cement is the most impactful way to reduce the carbon footprint of concrete.

Carbon Smart Attributes

Kiln type matters for cement

The different kiln types used for cement production, listed in descending order of energy intensity, are: wet, long dry, dry with preheater, and dry with preheater and precalciner. Dry with preheater and precalciner kilns use on average 85% less energy than wet kilns¹. Understand what type of kiln your concrete supplier uses for cement production, and request cement that comes from ‘dry with preheater and precalciner’ kilns whenever possible.

Less cement = less carbon

After reducing the carbon impact of cement production, additional carbon reductions can be made by reducing the amount of cement used per unit volume of concrete. Substitute cement with supplementary cementitious materials (SCMs) (including, but not limited to, fly ash, slag, calcined clays, agricultural byproducts, silica fume, etc.), and/or use larger sized aggregate (e.g. 1” vs ¾” coarse aggregate) where appropriate.

Consider new mixing methods

New methods for mixing concrete are being developed that can create high-strength concrete with a lower volume of cement. For example, the scattering-filling aggregate process adds an additional “10-30% (by the volume of the finished concrete) of coarse aggregate while the concrete is being poured, paved or placed, then vibrating the mixture to form a consolidated concrete”². This method results in 10-30% less cement that conventional concretes, reducing the carbon emissions while increasing the compressive strength of the mix.

Utilize carbon sequestration (CO₂ injection)

New technology captures waste carbon dioxide emitted from industrial processes and injects it back into the concrete mix during mixing, thereby converting the CO₂ to a mineral and permanently sequestering it in the concrete. Encourage concrete suppliers to use carbon sequestration/CO₂ injection methods.

Specify hard, clean, and strong aggregates

Weak aggregates require the addition of more cement to achieve the necessary mix strength. Soft, porous aggregates can also result in weak concrete with low wear resistance, reducing the life-span of the material. Whenever possible use hard aggregates to reduce the required cement quantity and create concrete with a high resistance to abrasion and a longer life-span³.

Consider using recycled aggregate, where appropriate

Aggregates and recycled materials from construction and demolition waste are relatively low-cost and have lower embodied carbon than new materials. 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. Use of recycled aggregates may be appropriate for certain applications that are not sensitive to aggregate absorption, such as pavements.

Design Guidance

Specify different mixes for different uses and plan ahead

Concretes with high supplementary cementitious materials, meaning less cement, often require longer set times than conventional concrete to achieve the desired strength. Identify building components that don’t need high early strength and plan ahead to allow for these components to have longer cure times. For example, footings and mat slabs are good targets for low cement mixes.

Consider bay size (column spacing)

Use shorter bay sizes to reduce the need for post-tensioned slabs and beams, which generally require more cement in order to reach their desired early strength. However, if more columns will be needed to make up for shorter bay sizes, a careful analysis should be done to ensure carbon emissions will be reduced overall.

Incorporate voids to reduce concrete volume

Use voids (such as air-filled recycled plastic spheres) in concrete slabs where structurally appropriate to reduce the amount of concrete needed. This method has been shown to yield slabs that use an average of 35% less concrete than traditional slabs, while performing like solid reinforced concrete. This method also eliminates up to 95% of the needed traditional formwork, reducing waste material.