A review on fly ash characteristics – Towards promoting high volume utilization in developing sustainable concrete

The use of fly ash in concrete dates back to the late 20th century and its advantages and disadvantages had been widely researched. Despite the broad based research carried out across the globe in utilizing fly ash as a cement replacement material in concrete, the level of replacement is still limited to a maximum of 35% of cement by mass. In view of increasing the level of fly ash replacement in cement to minimize the carbon footprint, this work summarizes the following: firstly, the current state of fly ash applications in concrete by considering about 200 papers published since 1980 to till date. Secondly, the analysis of form-structure-property of fly ash reported in various literature and its correlation with strength and durability characteristics. Thirdly, the contradictions reported in literature regarding the performance of fly ash, particularly, in the context of shrinkage, high temperature curing, water demand etc. Overall, this review brings to light that, apart from chemical composition, the influence of other factors such as morphology, crystallinity, size etc. have major influence in altering the hydration mechanism which in turn bring changes in mechanical and durability properties of fly ash concrete. The critical examination of properties of fly ash provides insight for wider utilization of fly ash, facilitating a higher replacement of cement possibly upto 60% in a scientific way rather than by trial and error basis. Further, this review recommends for the classification of fly ash apart from the existing ASTM classification of fly ash as Class F and Class C. Furthermore, amendments in existing codes are recommended for high volume utilization of fly ash.