Harnessing fluidised bed combustion fly ash as a potential green binder: Origin, characteristics, mechanisms, and products properties

Over the past decade, the production of fluidised bed combustion (FBC) fly ash has overtaken that of pulverised coal combustion (PCC) fly ash, which provides near future opportunity for its use as a binder material for construction industry. FBC fly ash however exhibits different characteristics (i.e. morphology and chemical and phase compositions) to PCC fly ash. This review aims to overview the generic features of PCC and FBC processes that contribute to their unique product characteristics. The review then discusses and summarises the important characteristics of FBC fly ash and the reported variations in the literatures due to the different coal types and origins, which determine their binding application suitability. Comprehensive discussions are then made on the requirements, mechanisms, and product properties of the four most widely studied routes of using FBC fly ash as binder material, i.e. (i) adding Portland cement to form blended Portland cement; (ii) adding bauxite to form calcium sulphoaluminate cement; (iii) adding granulated blast furnace slag to form supersulphated cement; and (iv) adding alkali activator to form alkali-activated binder. Despite the vast potential of FBC fly ash, there are two key challenges, i.e. (i) lack of standard specification that caters specifically to various FBC fly ashes and (ii) high variation in the final cured product properties. This review will pave a pathway towards wider adoption of FBC fly ash for sustainable construction application.