Explainer: How to accelerate the transition to low-carbon materials

Research published by the UN Environment Programme in 2022 found that carbon dioxide (CO₂) emissions from building operations reached record highs in 2021. Despite making some progress on cutting these emissions, the sector will need to reduce them more quickly in the coming years. 

Much of this comes down to the materials it uses. A 2023 study found that half of the total 43Mt CO₂e embodied carbon used by UK construction in 2018 came from concrete, while steel accounted for 22%. The use of only two materials, then, is responsible for almost three-quarters of the sector’s emissions.

Using lower-carbon versions or alternative substances will therefore be crucial if the sector is to hit its emissions targets and the nation is to become a net-zero economy by 2050.  

The Low Carbon Concrete Routemap, published in 2022 by the ICE and the Green Construction Board’s Low Carbon Concrete Group (LCCG), has a stark message: “If the UK is to achieve net-zero emissions by 2050, current behaviours need to change.” 

A key component of concrete is conventional Portland cement. Manufacturing this material accounts for up to 90% of the greenhouse gas emissions associated with concrete. The heating and decomposition of limestone in the production process emits about 860kg of CO₂ for every tonne of cement made. The carbon footprint imposed by the UK’s annual consumption of cement is equivalent to the emissions of 7.2 million cars.

The challenge here lies in finding viable alternatives to Portland cement. To effectively gauge a concrete’s carbon intensity, it’s standard practice to measure reductions in carbon against reference values for each strength class (see graph, below). These are based on mixes using Portland cement without secondary cementitious materials (SCMs).

The LCCG offers the graph as an "acceptable first iteration" of a benchmark for rating fresh concrete's carbon, while urging users to appreciate its limitations and consider the boundaries between ratings as approximate.

The most commonly used SCM is ground granulated blast-furnace slag (GGBS) – a by-product of iron and steel mills that can significantly reduce concrete’s cement content and, hence, its CO₂ emissions. 

In 2023, for instance, Laing O’Rourke switched to lower-carbon concrete featuring GGBS for all of its UK projects, predicting a 28% year-on-year drop in emissions.

However, GGBS isn’t a long-term solution. Ironically, as steel manufacturing seeks to cut its own carbon emissions, it will produce less GGBS. Other alternatives – for instance, calcined clay, silica fume and limestone powder – may become more prominent in the UK over the coming years. Meanwhile, ongoing research into new cement components is finding promise in materials such as synthetic SCMs, graphene and biochar.

Noushin Khosravi, sustainable construction manager at the Mineral Products Association and a member of the LCCG, highlights the BS8500 concrete standard revision. This incorporates two lower-carbon ternary cements: CEM II/C-M and CEM VI.

Another solution is more straightforward: use less concrete. Its usage can be reduced substantially by optimising designs, re-using existing structures or incorporating alternative lower-carbon materials. In some cases, cement-free concrete (non-structural fill or low-strength infill) can be used.

The LCCG and the ICE will update the Low Carbon Concrete Routemap periodically to give further recommendations for materials to reduce emissions. The industry also published its own net-zero strategy in 2020: Decarbonising UK Concrete and Cement.

Refining iron into steel traditionally requires carbon-intensive coke and coal. For every tonne of steel it produces, the process releases about 1.8 tonnes of CO₂. Steel production accounts for about 8% of global emissions – a bigger carbon footprint than that imposed by air travel. 

Yet it is possible to make lower-carbon steel. About 5% of global production uses the direct reduced iron (DRI) method, which eliminates the need for coke. This emits up to 61% less CO₂ than the traditional process. What's more, DRI plants using hydrogen (H₂) have the potential to cut emissions by 97%, which means that they could produce a tonne of steel and emit only 50kg of CO₂.

With more than 70% of the world’s steel-producing blast furnaces set to come offline this decade, the industry is gradually adopting greener methods, including electric arc furnaces and H₂-based DRI. In 2021, Swedish manufacturer SSAB used the latter process to make the world’s first fossil-free steel batch for Volvo.

There are also low-carbon alternatives to steel-reinforced concrete. One is fibre-reinforced concrete, where fibrous materials are added to enhance structural integrity. These range from polypropylene and polyester to natural materials such as coir or jute.

Two others are glass-fibre-reinforced polymer (GFRP) rebar and basalt-fibre-reinforced polymer (BFRP) rebar, which uses fibres derived from basalt rock. There is also unreinforced concrete which, as its name suggests, doesn’t contain any reinforcing bars or mesh. This lacks tensile strength, but it can still be suitable for compression-loaded structures and non-structural elements. 

Steve Caucutt, sustainability and environment adviser at civil engineering contractor BAM Nuttall, also highlights the importance of recycling. Steel is already one of the most recycled materials globally, with more than a quarter of its production derived from reclaimed scrap. 

From this perspective, Paul Moody, director of operations at Morrisroe Demolition, calls on engineers to consider the end-of-life recovery process at the design stage. He also stresses the potential of directly re-using salvaged structural steel. 

As the graphic above shows, it can take approximately 5,400 tonnes of cement and 1,400 tonnes of steel to construct a 30-storey high-rise building that is about 100m tall. Producing these materials releases 5,830 tonnes of CO₂. That could be brought to below zero through four steps:

• Using fewer materials
• Switching production processes
• Using low-carbon heat sources
• Carbon capture and storage

While the construction industry acknowledges the pressing need for lower-carbon materials, risk-averse attitudes remain. 

As the Low Carbon Concrete Routemap reports, traditional views and business-as-usual models in construction are creating barriers to progress. It cites an industry survey by the LCCG which found that, while 70% of respondents had considered using low-carbon concrete, there were also “concerns about the availability of low-carbon technologies (22%) and the ability of concrete producers to provide a low-carbon alternative (35%)”.

Each construction project will involve several stakeholders, making it hard for a single entity to adopt a novel technology without broad consensus.

Sam Draper, co-founder and CEO of Seratech, which produces a carbon-negative cement replacement, notes that contractors facing tight schedules and slim profit margins may gravitate towards familiar concretes. If even one party resists, introducing new materials becomes a daunting task. Caucutt adds that not all clients prioritise or even share the same carbon reduction goals.

There is an onus on engineers, therefore, to challenge received wisdom and move away from the status quo. By fostering early collaboration and knowledge exchange among project teams and suppliers, they can debunk the perceived problems limiting the adoption of low-carbon materials, harmonise views and set clear expectations.

Liv Andersson, co-founder and CEO of BioZeroc, a specialist in carbon-neutral building materials, suggests starting with low-risk applications for real-world testing.

As decarbonisation progresses, what’s deemed a low-carbon material today might become a high-carbon outlier tomorrow. There’s a shift towards performance-based specification, as opposed to the traditional prescriptive standards. Khosravi points to the performance-oriented BSI Flex 350 standard under development to aid the introduction of novel technologies.

BAM Nuttall has begun using basalt fibre reinforcement in places as a substitute for steel in concrete and the firm has expressed interest in using graphene. While Caucutt accepts that it won’t be suitable for every project, he’s also an advocate of precasting. He stresses that material selection will often hinge on design specifications and client requirements.

As steel manufacturing works to reduce its carbon footprint, the availability of by-product GGBS – a key component of some types of low-carbon concrete – is dwindling. Draper, who believes that the industry must recognise that GGBS isn’t the ultimate solution, favours the judicious use of this declining resource. Moody agrees, adding that other efficient forms of construction, such as post-tensioned slabs, should be explored.

Seratech offers an alternative to GGBS that can replace up to half of the Portland cement in concrete. The production of its SCM, made from magnesium silicates such as olivine, also consumes carbon dioxide, rendering the process net carbon-negative.

Other carbon-negative synthetic SCMs and alkali-activated cementitious materials are moving from lab tests to site trials, with UK-sourced alternatives including calcined clay, silica fume (as with Seratech), limestone and vegetable ashes. But, for these materials to be available on a large scale, the appropriate infrastructure must be in place to recover, manufacture, deliver and batch them.

Other innovations in the field include:

• BioZeroc, which uses bacteria to bind sand and aggregates at room temperature, eliminating cement. This has achieved 85% reductions in CO₂ emissions. 
• The University of Manchester’s Concretene, an enhanced concrete using graphene, which eliminates the need for steel reinforcement.
• Cemfree, a cement-free concrete used by BAM Nuttall for a 300m³ continuous pour at Chatham station, Kent, for Network Rail.
• Storing carbon in concrete. This is a form of carbon capture and storage in which CO₂ is injected into concrete while it’s being mixed. The quantities involved are relatively small (about 0.2% by mass of cement), so the greater carbon saving is still likely to be in cement reduction.

The industry’s experiments with entirely new materials that could replace or reduce the need for concrete and steel, such as cross-laminated timber, are promising but remain niche.