How much fly ash should be used in concrete?
The amount of fly ash, like most pozzolans, in a concrete mix is described as a “percentage of cementitious materials”. This percentage is obtained by dividing the mass of fly ash in a cubic meter of concrete by the sum of all cementing and pozzolanic materials the mix. 20% fly ash is a very commonly used amount of fly ash for most mixes utilizing fly ashes. Slightly higher percentages are common in lower and higher strength mixes and mass concrete often has fly ash percentages above 30%. Early strength application concretes typically use little to no fly ash. There have been demonstration projects done in which up to 50% and greater of the cementing materials in a cubic meter of concrete has been fly ash. Lehigh Cement does not promote or endorse such high uses of fly ash but encourages continued investigation into the appropriateness of higher uses of fly ash in highly engineered and monitored concrete applications.
