Fly Ash Route to Low Embodied CO2 and Implications for Concrete Construction

This paper addresses the need to reduce the amount of CO2 embodied in concrete mixes with regard to achieving durable and sustainable solutions and the implications this has for the construction industry. Shortcomings of the current methods of specifying concrete are discussed and an alternative offered that overcomes the drawbacks of mix limitation or prescription approaches. At previous WOCA conferences, the authors have demonstrated that recycled and secondary aggregates can be used to reduce demand on quarried materials. However, an adjustment to mix constituent proportions using fly ash is required to enable ‘equal’ performance to be maintained. In this paper, the authors progress this concept to enumerate the embodied (eCO2) of typical structural concretes. The UK has now agreed the various eCO2 levels for all relevant cementitious materials. Using this framework, the role of fly ash, of up to 55% of the total cement content, to displace Portland cement (PC) is demonstrated. These ‘low carbon’ concretes can enhance, or at least match, the properties of equivalent, ‘traditional’ all PC concretes. However, while resistance to chloride ingress is improved, with carbonation a trade-off between these is likely to be required. Furthermore, it is shown that this approach does extend the time to achieve early strength, which may have implications for formwork removal, prestressing and live load application. It is postulated that alternative cements that have a high early strength development can largely offset this problem and data will be presented to support this