PFA has both pozzolanic and physical properties that enhance the performance of concrete.
When Portland cement hydrates it produces quantities of alkali hydroxide (lime). Pozzolans like PFA react with this lime to form stable calcium silicate and aluminate hydrates. These hydrates fill the voids within the concrete, removing some of the lime and thus reducing the permeability. This process improves the strength, durability, chloride and sulphate resistance of the concrete.
The pozzolanic reaction occurs relatively slowly at normal temperatures, enhancing strength in the longer term relative to normal Portland cement concrete. The finer fraction of PFA, i.e. those particles that pass through a 45µm sieve, act as particulate plasticiser. These particles are spherical and act like ball bearings within the concrete reducing the water requirement for a given workability. A reduction in the water content lowers the permeability and increases strength and durability. In addition the concrete is more cohesive, has a lower rate of bleeding and is less prone to segregation.
If there is 25% or more PFA within the cement content of a mix the effect of the high alkalinity from sodium and potassium is reduced. Any risk of alkali silica reaction are significantly reduced, if not completely eliminated.
Furthermore, if there is at least 25% PFA within the cement content of a mix then this will provide sulphate resisting properties to the concrete, even up to Class DC-4 exposure.
PFA can be used in site-mixed, ready-mixed and pre-cast concrete. In most cases where PFA is used as a constituent of concrete the material must be quality assured and comply with the European Standard EN450. It should be noted that following publication of a new version of the EN450 Standard during 2006 ( EN450-1&2:2005) all conflicting standards, such as BS3892 Part 1, have had to be withdrawn.
We provide EN450 quality PFA on a consistent and reliable basis. This ensures that the availability of suitable quality PFA for concrete will continue long into the future.