The UK’s biggest projects are facing a shortage of concrete due to the dwindling availability of a key material.
Readymix incorporating fly ash or ground-granulated blast furnace slag (GGBS) is commonly used on most major civil engineering schemes, including Crossrail and the Thames Tideway, chiefly because of its strong technical and environmental credentials (see box below).
But supplies of fly ash have fallen recently due to the decline in use of coal by power plants.
Cemex technical director Steve Crompton said: “Fly ash is obviously a by-product of coal-fired power stations, but they have been switching to biomass or to gas – so as a result there is less coal being burned, and less fly ash.”
The materials firm has responded by using GGBS more in its concrete, but the inevitable result is “that slag is also now in short supply,” said Crompton. “We are certainly unable to get enough to fill the gap left by fly ash.”
The shortage is being felt by other manufacturers, confirmed Jerry McLaughlin, public affairs director at the Mineral Products Association. “All members are reporting a ‘significant’ shortage of fly ash,” he said.
A materials engineer for Bam Nuttall said the issue was being noticed at site level. “On one project, we were informed just ahead of a concrete pour by the supplier that no concrete with fly ash was available, and could we use another product,” said David Simons. “Fortunately, the exposure conditions were such that we were able to use a concrete with a traditional CEM I cement (not blended with fly ash or other materials). But the site team had to be extremely careful because of the risk of thermal cracking.”
He warned that a shortage of fly ash could be a “serious problem” for major projects.
Fly ash supplies can run low during summer, when power stations use less fuel anyway. Robert Carroll, technical director of the UK Quality Ash Association, said this year’s summer dip in production had been “unusually low”, though he was reluctant to say if it was a long-term trend.
Between 80% and 90% of all readymix concrete contains some level of fly ash or GGBS, Cemex has estimated.
Fly ash is prized for its technical qualities, and big projects see it as another way of reaching testing environmental targets.
Cemex is a fly ash producer, and Crompton said the firm was “looking at improved ash recovery techniques to help with this shortage”.
He has encouraged clients to switch to a different concrete made only with non-blended cements (CEM I). But this is not always suitable.
“We are working with our customers on their product selection,” said Crompton. “Traditional ‘Portland’ cement can be ok depending on the project. But this type of concrete is also more expensive to manufacture, and we obviously have to recover the extra cost.”
The problem is exacerbated, according to Chris Leese, Cemex’s vice-president for readymix, by contractors wanting to “commoditise” the product. “They think it all costs the same,” he said. “They need to recognise that readymix using fly ash and GGBS is a high value product.”
Fly ash and concrete
Fly ash and ground-granulated blast furnace slag (GGBS) are both widely used with cement in the production of concrete.
Last year, the total market for cementitious materials was 11.5M.t, or which fly ash and GGBS made up 1.9M.t - or 16.4%.
Fly ash - also known as pulverised fuel ash - is the fine ash produced at coal-fired power plants which develops cementitious properties when mixed with cement and water. GGBS is the slag from iron producing blastfurnaces which is ground into a powder.
Increasingly, fly ash and GGBS are viewed as a vital ingredient in blended cements (known as CEM II) and can comprise between 40% and 70% of the mix.
Fly ash is highly regarded for its technical benefits including increased durability, plus reduced shrinkage and permeability of the finished product.
Being a waste product, fly ash can significantly reduce the carbon footprint of a project.
Given the high technical performance and strong environmental credentials, major projects routinely order readymix containing fly ash-based cement. That may have to change.