Fly ash or pulverised fuel ash is a by-product of the combustion of pulverised coal in thermal power plants. The recognition that fly ash frequently exhibits pozzolanic properties has led to its use as a constituent of concrete. In South Africa the carbon content (as measured by loss on ignition, LOI) and the particle size of classified ash is limited to 5 % LOI and less than 12, 5 % larger than 45 (m respectively. Furthermore, the maximum percentage of cement that can be replaced using ash is normally limited to 30 %. A literature review was undertaken to establish the relevance of these limitations based on international research findings. In this article results published by other researchers were reworked and are presented in graphical form to emphasise the effect of carbon content and particle size. Research results indicate that ash with high carbon content reduces the workability of mixtures thus increasing the water demand thereby reducing the strength of the concrete. No proof could be found that the particle size of ash has an effect on the strength of the concrete. The use of large percentages of ash replacement does result in reduced early strength but for specific applications there could be benefits in replacing high percentages of cement with ash. Research should be conducted to establish whether sources of ash previously deemed unfit for use in concrete could be used in future.