Low-Carbon Concrete – Challenges and Innovations

Concrete Connect Q&A: Low-Carbon Concrete – Challenges and Innovations

With Anthony Smith, head of product management, Sika UK

As pressure mounts to reduce embodied carbon in concrete, manufacturers, specifiers and contractors are all seeking solutions that meet sustainability targets without compromising performance or compliance. In this Concrete Connect Q&A, we speak to Anthony Smith, head of product management at Sika UK, about the practical challenges of reducing CEM I content, the role of supplementary cementitious materials (SCMs), and how admixtures are helping unlock new opportunities for low-carbon concrete under current British standards.

1. There’s a growing push to reduce the carbon footprint of concrete, particularly through lower CEM I content. What are the key technical challenges specifiers face when trying to make these reductions without compromising on performance?

One of the major challenges is whittling down the wide array of cement blends and mix designs now allowed under updated British standards to options that are a) locally available, b) technically feasible, and c) actually result in a measurable reduction in embodied carbon.

Generally speaking, as we reduce the carbon-intensive cement clinker in a mix, we lower the CO₂ footprint — but also reduce the rate of early mechanical strength gain. This can lead to practical issues onsite, such as delayed formwork striking times or strength compliance needing to be demonstrated at 56 days instead of 28.

2. SCMs like GGBS and PFA are widely used as cement replacements. How do you see their role evolving in the UK market, and are there limits to how far we can push replacement percentages?

GGBS and PFA are the most well-established SCMs in the UK and have a long history of safe and effective use. They will remain dominant in the short to medium term due to availability and established supply chains. Alongside increasing use of limestone powder, they currently represent our best opportunity to reduce the carbon footprint of concrete at scale.

Longer term, we may see more uptake of alternatives like calcined clays and natural pozzolans, but domestic production of these materials is still at an early stage. Sika has developed admixture solutions tailored for these emerging SCMs, though variability in chemistry and processing methods makes a one-size-fits-all approach unlikely.

As for replacement rates, CEM II/C binders allowing up to 95% clinker replacement have been recognised in global standards for some time. The UK’s BS 8500 only allowed easier specification of CEM II/C-type cements at the end of 2023. CEM VI also holds promise, though it still requires a minimum of 35% CEM I, so the carbon savings are less than with CEM II/C.

3. How can admixture technologies help unlock higher levels of CEM I replacement without increasing total binder content? Can you give an example of this in practice?

Admixtures play a crucial role in enabling low-carbon binders to perform well. Although they make up only a small fraction of the mix by volume, they have a disproportionate impact on concrete's fresh and hardened properties — and offer more flexibility for innovation than core ingredients like cement or aggregates.

Our focus at Sika has been on making lower-carbon concrete workable and efficient onsite. For example, using a CEM II/C binder without admixtures might require a contractor to wait several days before formwork can be stripped — delaying progress. By contrast, our admixtures can restore or even improve performance despite high CEM I replacement rates.

In one industrial flooring project, we used SikaRapid-800 ECO, a liquid accelerator, to enable 50% GGBS replacement while achieving similar setting and finishing times to a standard CEM I mix. In another case, SikaRapid-950, a powdered accelerator, was used in piling trials to achieve 90% CEM I replacement while still meeting strength targets.

4. There’s a perception that low-carbon mixes mean slower strength gain or longer turnaround times. What advances have been made to address this?

Sika has developed a new generation of strength-enhancing admixtures that challenge that assumption. SikaRapid-800 ECO can be used in lower doses to accelerate set and early strength, or at higher doses to improve late-age strength — even at 80% cement replacement.

This was followed by SikaRapid-4 UK, designed to enhance early-age strength in mixes with up to 80% replacement, and now SikaRapid-950, a powdered product that enables up to 95% replacement with either GGBS or PFA. Crucially, these admixtures improve strength gain at both early and later stages, helping projects meet 28-day targets without increasing binder content or delays.

5. How do current British standards support — or potentially limit — innovation in sustainable concrete? Are there areas where updates or greater flexibility would be beneficial?

Revisions to BS EN 197-5 (2021) and the UK National Annex to BS 8500 (2023) have greatly expanded the number of allowable binder combinations. Additionally, BSI Flex 350 v2, published in 2024, provides a framework for testing and using alkali-activated materials (AAMs), which are entirely cement-free.

In theory, these changes make it easier to specify lower-carbon concrete. In practice, however, the industry is slow to adopt them. There’s still a need for greater awareness — and a stronger demand signal from specifiers and clients — to encourage producers to invest in alternative materials. Suppliers like Sika also need to keep innovating to remove practical barriers to wider uptake.

6. Finally, what’s your view on adoption at site level? Are you seeing real uptake of low-carbon mixes in commercial and infrastructure projects?

Demand is definitely growing, from both clients and contractors. But in reality, widespread adoption is still slow — partly due to the conservative nature of structural design, where durability and risk aversion rightly take precedence.

There are also practical hurdles: concrete producers may need new silos for alternative binders, additional dosing lines, or entirely new mix designs. Cost is another factor, especially in 2025’s climate of squeezed budgets and slow project starts. As with any new technology, there may be small cost premiums for early adopters — but the reward is being among the first to demonstrate viability and build a strong sustainability story around your projects.

Ultimately, it’s up to all of us in the sector to keep pushing the boundaries and finding ways to lower concrete’s carbon footprint without compromising on quality or buildability.

For more on Sika’s low-carbon concrete technologies, visit:
sika.co.uk/sustainable-concrete