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Fly ash in concrete
One of the leading uses for fly ash is in Concrete Products applications. Fly ash use in concrete improves the workability of plastic concrete, and the strength and durability of hardened concrete. Fly ash is also a cost effective ingredient in concrete because it contributes to the total cementitious component in the mix and therefore allows the producer to reduce the amount of cement in a typical mix design.
Benefits to plastic concrete
- Improved workability The small and spherical shape of fly ash particles provides a “ball bearing” effect within the concrete mix that produces a paste with superior plasticity.
- Decreased water demand The replacement of cement by fly ash reduces the water demand for a given slump and can even reduce drying shrinkage in certain situations.
- Increased pumpability The spherical shape of fly ash reduces internal friction and produces a mix that is easier to pump.
- Reduced segregation The improved cohesiveness of fly ash concrete provides added body to plastic state concrete which resists segregation.
- Reduces bleedwater The lower water content in fly ash concrete reduces bleeding.
Benefits to hardened concrete
- Increased strength Fly ash will continue to react with available lime and alkali in concrete, producing additional cementitious compounds that allow the concrete to continue to gain strength beyond 28 days.
- Reduces drying shrinkage The reduced water demand allows fly ash concrete to maintain workability and therefore reduces drying shrinkage.
- Decreased permeability The packing effect of the fly ash particles helps to reduce permeability. The chemical reaction between fly ash and lime forms additional bonds that block bleed channels and fill pore spaces.
- Improved durability - The decrease in free lime and the resulting increase in cementitious compounds, combined with the reduction in permeability enhance concrete durability.
- Resistance to sulphate attack Fly ash combines with free lime making it unavailable to react with sulphates that can cause expansion problems.
- Reduced heat of hydration Large masses of concrete typically produce high internal temperatures and thermal cracking. Less heat is generated when fly ash is used to replace a portion of Portland cement in the mix.
- Mitigates alkali aggregate Fly ash reacts with available alkalis in the hardened cement matrix making them less likely to react with the aggregate.
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