Fly ash

A by-product of the coal-fired power industry, fly ash has been added to cement since the 1930s and it is now commonly used to reduce the need for clinker.

A by-product of the coal-fired power industry, fly ash is the dust-like particles captured from the flue gases of coal-fired furnaces or removed from the fluidised bed of coal and biomass boilers. Although its chemical make-up varies, depending on the type of coal that was burned, all fly ash is pozzolanic.

Its use in concrete was first reported in 1937 and it now a common supplementary cementitious material (SCM), used to reduce the clinker content of cement.

In addition to upcycling what would otherwise be a waste product, the use of fly ash brings a range of benefits to concrete. Longer setting times reduce the heat of hydration and therefore the risk of cracking, as well as resulting in higher ultimate strengths. It also decreases the permeability of concrete and provides higher resistance to chemically-aggressive environments.

As a result, fly ash concrete is likely be more durable over time – an important benefit in terms of the economic, social and environmental sustainability of the buildings it is used to construct.

In addition, the spherical nature of fly ash particles improves the workability of concrete, reducing the amount of water needed during production.

More challengingly, its slower setting time means its early strength is lower than traditional cement, making it unsuitable for use in the precast industry and potentially increasing construction times. It also requires more curing than standard concrete, while its resistance to carbonation is lower, risking corrosion of carbon (black) steel reinforcement.

Moreover, as a by-product of coal-fired power plants, its production is often seasonal and depends on the use of coal in a particular power market. With coal-fired power becoming increasingly unpopular in certain regions, as a result of its environment impact and competition from alternative power sources, such as renewables, the long-term availability of fly ash is in question. For now, however, it remains a well-understood and widely-used SCM.