Toxic concrete concerns

The government’s desire to be seen to be doing things at pace is already leading to poor and costly decisions. There is also a real risk that the public will be put in danger. Concerns have already been raised that the “low carbon pathfinder” status of National Highways’ £16 billion Lower Thames Crossing (LTC) will encourage contractors to use toxic incinerator ash.

Some waste management and construction firms are pushing for incinerator bottom ash aggregate (IBAA) – the residue left after burning municipal waste in an incinerator – to be used in “low carbon” concrete.

However, one environmental consultant has warned that this could mean future generations will have to treat every concrete structure to be demolished as potentially contaminated. There are also safety concerns when the concrete is made.

Neither National Highways nor the contractors building the LTC, who are required to keep their carbon emissions below a “highly ambitious” target level, have ruled out using IBAA on the project. Given the LTC will use nearly a million tonnes of concrete (mostly to line the two 2.6 mile long tunnels), it could be a significant hazard.

About IBA and IBAA

Incinerator bottom ash (IBA) mainly consists of brick, rubble, glass, ceramics and stone, but it can also contain far more hazardous materials such as heavy metals and ‘forever’ chemicals¹. After processing, it can be used as an aggregate (IBAA) in construction², in two main ways: ”loose”, like gravel, in road sub-bases, or “set” in concrete.

Concerns have been raised that the material is “insidiously hazardous and underregulated”³ and that processes for cleaning IBAA can be shortcut.

Because of the potential for toxic chemicals to leach out of IBAA, the Environment Agency (EA) has issued a regulatory position statement (RPS) on using it, with rules that limit the potential for rain to wash through it and how close to water it can be used⁴.

In 2024, a “lower carbon concrete solution” containing IBAA was laid at Veolia’s Ling Hall IBA processing site in Warwickshire “with full permission from the EA as part of a collaborative effort to push the boundaries of sustainable construction”⁵. However, environmental experts have said burying the substance in structures such as roads represents “landfill” rather than recycling.

Concerns about the use of IBAA have been raised in the Netherlands. There, an official from the country’s Human Environment and Transport Inspectorate said that it is not possible to tell from the outside whether a block of concrete contains contaminated bottom ash, and its status is not recorded. Consequently, “future generations will have to treat every concrete structure to be demolished and reused as potentially contaminated”⁶.

About the LTC

The Lower Thames Crossing, approved in March 2025, will be 14.3 miles of new smart motorway passing through a tunnel linking Kent with Essex.

In February 2022, National Highways designated it as a pathfinder to explore carbon-neutral construction, with the aim of making it “the greenest road ever built in the UK”⁷ (ignoring the traffic it would generate). However, the project has frequently been accused of greenwashing⁸ both in terms of construction and emissions from the vehicles that will use it. In 2022, Highways magazine revealed that its claim to have cut construction emissions by a third involved comparing its plan to use a high proportion of GGBS (Ground Granulated Blast-furnace Slag), a low carbon cement replacement widely used in concrete production, against an unlikely scenario using “zero” GGBS⁹.

The company claimed in its planning application that contractors would be required to stick to carbon emissions caps for their share of the scheme. It has refused to disclose penalties for non-compliance¹⁰ but in January 2024 said it would not allow contractors to use offsetting to meet their targets¹¹.

With GGBS becoming harder to obtain, National Highways is exploring alternative low-carbon materials for the LTC. However, it has not yet clarified what these are and whether they include toxic IBAA.

When the LTC project announced the shortlist for its competition to find the low-carbon “footbridge of the future” in December 2024, none of the five shortlisted entries used concrete – suggesting a lack of confidence in the possibility of sourcing genuinely low carbon concrete for the LTC.

But in February 2025 National Highways said it had set itself an “ambitious new target” to cut carbon on the scheme by around 70% against its spurious baseline. It said this was made possible thanks to its delivery partners and suppliers who would only use low carbon steel and concrete¹².

In May 2025, the project’s director of environmental sustainability said decarbonising the cement industry was proving to be one of the hardest challenges on construction’s journey to net zero and the sector needs to move more quickly¹³.

This all raises the question of whether the contractors will include IBAA in concrete.

What has National Highways said?

A spokesperson for National Highways said it has “no plans to use Incinerator Bottom Ash Aggregate in concrete or cement” on the scheme, but did not rule out doing so or allowing its contractors to do so.

What have the contractors said?

A spokesperson for Skanska Construction UK Limited, which has the contract to build the Kent Roads, said that IBAA is not currently being considered for use, but that as the project is still at the very early stages, “decisions on these construction materials are still some way off”.

A spokesperson for Bouygues Travaux Publics – Murphy Joint Venture (BMJV), which has the contract to build the scheme’s Tunnels and Approaches, said it does not “intend” to use IBAA on the project.

Balfour Beatty Civil Engineering Ltd, which has the contract to build Roads North of the Thames, did not provide a substantive response.

This means that none of the three “delivery partners” have definitively ruled out using IBAA, but neither do they seem to have considered substantially reducing the construction carbon emissions.

Another issue is that if private finance is involved, investors will want to cut costs to maximise profits. With a lack of effective enforcement and no transparency about the penalties for contractors if they breach legally binding environmental commitments, it is a strong possibility that corners will be cut.

What’s the solution?

Given that the Lower Thames Crossing will have substantial amounts of its structure within or close to the water table, certainty about the safety of the materials being used will be essential. Using toxic materials needs to be ruled out from the beginning, with contractors not given a choice on the matter.

With their lamentable records on tackling water pollution, how can we trust National Highways or the Environment Agency to properly monitor construction materials for the LTC and other road schemes?

Ultimately, with doubts about whether the Lower Thames Crossing can be built with the vastly lower construction emissions promised and the certainty of increased road user emissions when it opens, the solution must be to scrap the it altogether, especially as there are better options available¹⁴.

1. Standard Rules Consultation No 29 – Proposals for new standard rules permit for using unbound incinerator bottom ash aggregate (IBAA) in construction as a deposit for recovery operation – Environment Agency – Citizen Space ↩︎
2. IBA Aggregate – MIBAAA ↩︎
3. Toxic Fallout – Waste Incinerator Bottom Ash in a Circular Economy – Zero Waste Europe ↩︎
4. Using unbound incinerator bottom ash aggregate (IBAA) in construction activities: RPS 247 – GOV.UK ↩︎
5. Industry collaboration produces first pour of pioneering IBAA concrete in England | Veolia UK ↩︎
6. https://www.ftm.nl/artikelen/as-vuilverbrander-aeb-belandt-op-straat ↩︎
7. National Highways to use Lower Thames Crossing as carbon neutral construction ‘pathfinder’ | New Civil Engineer ↩︎
8. Lower Thames Crossing Greenwashing Challenged – TAN ↩︎
9. Thames Tunnel carbon emissions up by 25%, Highways Magazine, 8 December 2022 ↩︎
10. Thames tunnel carbon pledges ‘not watertight’ – Highways Magazine ↩︎
11. Lower Thames Crossing facing carbon questions – Highways Magazine ↩︎
12. Bar raised with 70% carbon target as industry responds to net zero challenge – National Highways ↩︎
13. How construction is under pressure to decarbonise its cement more quickly – Construction Briefing ↩︎
14. Essex-Kent Superlinks Report ↩︎