Low-carbon concrete: what contractors need to know

Concrete is one of the largest sources of embodied carbon in a building, largely because of the cement in the mix, whose manufacture releases carbon dioxide both from the fuel burned to heat the kiln and from the chemical reaction that turns limestone into clinker. New mixes that replace part of that cement with other materials can cut a mix’s embodied carbon significantly, and clients are starting to ask for them by name rather than leaving the mix design entirely to the contractor.

What the replacement materials are

The most common cement replacements are ground granulated blast-furnace slag, a by-product of steelmaking, and fly ash, a by-product of coal combustion, both of which can replace a substantial share of the cement in a mix while still meeting the required strength class. Newer options include calcined clay blends and mixes that use recycled aggregate alongside a reduced cement content. Availability varies a lot by region, since both slag and fly ash depend on nearby heavy industry, so what counts as a standard low-carbon mix in one area can be a special order elsewhere.

What changes on site

Many low-carbon mixes gain strength more slowly, particularly in cold weather, because the chemical reactions involving slag and fly ash generally proceed at a more gradual pace than the reaction with ordinary cement. Plan for longer curing before formwork is struck, adjust the programme for elements on the critical path such as transfer slabs, and keep more test cubes than usual for early-age strength checks so the site team has real data rather than an assumption about when it is safe to load a new pour.

Hot weather can partly offset this, since the slower-reacting mixes are often less prone to the early cracking that fast-curing standard concrete can suffer in high temperatures. That makes low-carbon mixes an easier sell on some summer pours than the cold-weather caveats might suggest.

Specification tips

Ask suppliers for an environmental product declaration for the specific mix being proposed, not a generic figure for the product range, since embodied carbon can vary significantly between plants and batches depending on the exact proportions used. Compare mixes on strength class, exposure class and delivered carbon together, not on price or carbon figure alone, because a mix that needs a thicker section to hit the same strength can end up with a similar total carbon footprint to a standard mix despite a lower figure per cubic metre.

Working with the design team

The earlier a low-carbon mix is agreed, the more the design can be adjusted to suit its properties, such as allowing extra time in the programme for early-age strength gain on load-bearing elements. Bringing the concrete supplier into design conversations before the specification is fixed, rather than after, avoids late substitutions that undermine both the carbon savings and the programme.

Cost

Premiums are shrinking as supply grows and more plants can produce these mixes as standard, and some projects now see no difference in price at all between a low-carbon mix and a conventional one. Where a premium does exist, it is usually smallest on mixes using locally available slag or fly ash, and largest on newer, less established blends that still carry a novelty cost.