A critical review and gap analysis on the use of coal bottom ash as a substitute constituent in concrete

Coal has been increasingly used in the past few decades as a fuel source in power plants and industrial facilities in many countries. The escalating demand for coal has been associated with undesirable combustion byproducts, such as coal bottom ash (CBA). There are attempts to utilize CBA as a replacement for fine aggregates or cement in concrete, or to produce geopolymers. This review critically collates the reported physical and chemical characteristics of CBA and discusses the impact of the source (locality) on its characteristics. A number of treatments have been commonly performed on CBA to enhance its pozzolanic reaction or mechanical properties for use in concrete and mortar. Treatments span sieving, grinding, and inclusion of superplasticizer or alkali activators. It was observed that CBA treatment type, rather than content, has a higher impact on the compressive strength. The achieved strength for concrete, mortar and cement paste vastly vary from 1 to 163 MPa, depending on CBA utilization and mix design. The microstructure as well as the properties of the fresh and hardened CBA concrete are discussed in this review. A gap analysis has shown that the long-term durability, leaching, abrasion resistance and chemical attack resistance of CBA concrete require further evaluation. Despite the source dependency, the review has shown the potential of CBA for a wide range of applications.