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Last edited 10 Jan 2021
Fly ash supplies dwindling
The march towards cleaner and more sustainable energy has impacted supplies of 'fly ash' to the construction industry. In this article, Nigel Cooke from the UK Quality Ash Association (UKQAA) proposes some alternatives.
Pulverised fuel ash (PFA) is the fine ash produced during the combustion process used at coal-fired power stations.
When finely ground coal is burned the fine ash is carried out with the flue gas and is sometimes referred to as 'fly ash'. The ash passes through electrostatic precipitators that remove these fine particles from the gas stream where it is stored in silos or hoppers.
It has played a major role in many construction products and construction applications for the past 50 years.
However, all coal fired power is expected to cease by 2025 and securing sources of PFA throughout the year is already becoming harder, resulting in the need for imports and extraction from temporary stock piles.
PFA is collected from the silos and can either be sold dry for use in concrete and applications such as aerated concrete blocks or wetted (called conditioning) for applications such as fill, grouts, etc. Any excess ash has traditionally been mixed with large quantities of water and pumped to lagoons or conditioned and sent to landfill. Both lagoon and landfill ash can be recovered and sold for fill, grouting and other applications.
Securing future supply and safeguarding existing assets is now becoming urgent.
Some may be aware of the pozzolanic benefits of using PFA in concrete, which enhances chemical resistance and durability as well as producing more pumpable mixes. However, some might also be less aware that some 1.5 million tonnes a year of PFA is used in the manufacture of aerated concrete blocks and grouts for mine filling and ground stabilisation.
Blast furnace slag can be substituted for PFA in concrete though prices of slag are increasing and sourcing is becoming more difficult with some slag being imported from as far away as China. Sand can replace PFA in the manufacture of aerated concrete blocks and for grouting though the performance of sand in such applications is not as good as PFA. Moreover, sand is also becoming a scarce commodity.
 UKQAA’s proposed solution
It is estimated that there are well in excess of 50m tonnes of PFA currently deposited in landfill sites in the UK and managed as a waste. UKQAA is seeking to redefine such deposits as future 'pozzolanic reserves' that could be adopted under future minerals plans. This would then allow the UK construction industry to secure such reserves as a 'national asset' for future extraction for use in cementitious and non-cementitious applications.
UKQAA recognises that a considerable amount of work is required to align all the various stakeholders to redefine PFA landfill deposits as pozzolanic reserves.
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