Concrete has played a major role in the construction industry for hundreds of years. As the material is so commonly used, more sustainable methods of producing the material are constantly being sought. The use of fly ash as a constituent in concrete has become more and more popular over the past 30 years due to its potential to improve the mechanical properties of concrete, whilst providing a more economically and environmentally friendly product. Fly ash is produced in abundance as a result of burning coal in coal fired power stations. Due to the substantial amount of fly ash produced each year, it currently isn’t being consumed to the same extent, resulting in large stockpiles of the material. This is an environmental concern on its own, however, even more concerning is the amount of carbon dioxide emitted into the atmosphere as a result of cement production. Approximately 0.9 tonnes of CO2 are produced for each tonne of cement1. With this in mind, it was set out to determine which of the two most common methods of substituting concrete constituents with fly ash provides the best results. The two methods are partial cement replacement and partial fine aggregate, or sand replacement. The partial cement replacement method is well documented and is known to have a delayed pozzolanic reaction, negatively affecting the early age compressive strength of the concrete while later ages are comparable or improved. Fine aggregate replacement is far less documented and is therefore being investigated and directly compared against partial cement replacement. Experiments found that fine aggregate replacement produced consistently higher compressive strength results, both across the replacement percentage range and in 7 and 28 day curing situations. However, when the replacement percentage is optimized, partial replacement of cement with fly ash showed potential to produce marginally better results than fine aggregate replacement and much higher compressive strength results at 28 days than the no ash mix. Early strength indicated by 7 day compressive strength results demonstrated that cement replacement is consistently lower, however, only marginally where the optimum replacement percentage is used. Cement replacement proved to be the most beneficial method over all due to the considerable environmental and economical benefits over fine aggregate replacement.