Fly ash is a by-product from coal combustion. It can be re-used in cement production and improves the durability and workability of concrete.
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Fly ash can be used as a partial replacement for the sand, limestone and cement content in concrete. By reducing the need for cement production (a highly energy-intensive process), the re-use of fly ash leads to reductions in greenhouse gas emissions of almost 1 tonne of carbon dioxide per tonne of cement. Fly ash also enhances the performance of concrete in regard to workability, shrinkage and durability.
While there are many potential uses for fly ash, there are currently limited recycling options available. In 2008, only 31% of the fly ash produced in Australia was effectively used for a beneficial purpose, while the surplus is typically placed into storage ponds for a future reuse opportunity. The cement and concrete industry have built significant markets for the beneficial use of fly ash but it remains a challenge to development other high volume uses for this by-product.
What Happens When It's Recycled?
When coal is burnt, two types of ash are produced. The fine ash recovered from the flue gas is called fly ash. Particles that fuse together and fall to the bottom of the furnace is called bottom ash.
In Australia, the majority of fly ash produced is suitable for cement production, because it reacts with lime in the presence of water to form a type of cement. The coarser furnace bottom ash is used as a sand replacement, aggregate for lightweight blocks, a road-base component, for agricultural drainage mediums and bulk fill.