Fly ash is a great supplementary cementitious material - it has the right properties, meaning that it reacts with lime to form cementitious compounds. It is by-product from coal-firing industries, but in some cases has ended up in landfills – especially up until 1929, when it was first used in concrete to minimize the use of cement when building the massive Hoover Dam on the Colorado River in the USA.   

Fun fact

More than 5 million barrels of Portland cement and 4.5 million cubic yards of aggregate went into building the Hoover Dam.

With the potential to replace up to 30% of traditional clinker, fly ash quickly became very attractive to the cement industry and a sought-after commodity. Today, as the green transition of powerplants and other heavy industry is accelerating - some countries are phasing out coal and turning towards green energy, natural gas, and / or biofuels, with the result being that fly ash is now in short supply.

But just as steel, paper and sugar industries are eager to minimise their environmental footprint, so is cement. And the use of fly ash is both a proven and effective ingredient. The shortage of fresh fly ash has led more and more industry stakeholders to turn their attention towards the centuries of landfilled fly ash.


To date, billions of tonnes of fly ash have been landfilled. ‘Harvesting’ fly ash from these landfills makes some industry experts confident that this waste-product could have a second wind in cement.


“As we strive towards fulfilling our MissionZero promise of enabling net zero cement production by 2030, we need every tool in the toolbox. Reducing the clinker factor is a key element to that. Fly ash is a proven and well-integrated SCM – to pursue the exploitation of landfilled fly ash would obviously boost our efforts.” To Thomas Petithuguenin, Head of Research and Partnerships for Cement at FLSmidth, every possible path to MissionZero needs to be explored. “I am not saying that fly ash harvesting is a quick-win, but from a product point of view, it is a known ingredient and gives confidence in terms of quality and performance. The challenge is the logistics and infrastructure, which we need to investigate with stakeholders across the value chain.”   

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