Coal bottom ash as constituent binder and aggregate replacement in cementitious and geopolymer composites: A review

Coal thermal energy is the primary source of energy for many countries. Throughout the years, energy consumption has increased, causing coal energy consumption to spike. The use and production of cement in the construction industry also showed an increasing trend with global population growth and development. These two phenomena contribute significantly to the increase in carbon dioxide (CO2) emissions and discharge of coal ash which are harmful to the environment. Coal thermal plants produce coal bottom ash (CBA) which can be used in mortar or concrete to produce greener building materials and promote recycling by-product material. Throughout the years, there have been several studies on the use of CBA as cement/aggregate replacement to prove the use of CBA can be feasible and reduce the use of natural resources. This review is intended to consolidate recent global research on the use of CBA as a fine aggregate or cement replacement material in concrete and encourage its use in the concrete industry to reduce natural resources extraction and consumption. It summarizes the advances in processing CBA, fresh properties, and hardened properties of CBA cementitious composites. Besides, a critical review and research gap analysis of past studies and researches related to CBA as aggregate and cement replacement is also provided. The review has established the suitability of coal bottom ash as a constituent binder and aggregate replacement in concrete.