Upcycling of pond ash in cement-based and geopolymer-based composite: A review

Abstract

The coal based thermal power plants are the major source for generation of electricity throughout the world, and this in-turn has resulted in creation of large quantity of coal ash. Coal ashes include Fly Ash (FA) and Bottom Ash (BA) which when unused are pumped into the ponds and this ash in ponds is termed as Pond Ash (PA). Contamination of soil, air and water has been observed in the neighborhood of these ash ponds. To reduce negative environmental effects due to PA, many researchers have used PA as fine aggregate or as Supplementary Cementitious Material (SCM) or as precursor in geopolymers. However, most of the earlier studies are limited to local experiences on PA utilization. Further, in recent years, worldwide research on the use of PA in concrete has intensified. The goal of the present research is to review the earlier studies done all over the world on PA and identify research gaps for future research. Present review revealed that wet disposal of ash into ponds, the duration of ash ponding and the proportions of FA and BA within PA has a significant impact on its physical, chemical and microstructural properties. Presently various treatments like sieving, magnetic separation, milling, air classifying and alkali activation are performed on PA prior to its utilization in concrete. Further studies need to be carried out to establish the right sequence of these treatments to optimize the usage of PA as a raw material for concrete. Review on PA utilization as replacement to fine aggregate indicated that although there is detrimental impact on workability and compressive strength, PA can be used as fine aggregate in concrete and mortar for up to 50%. The review confirmed that PA subjected to proper treatments can be used as replacement to cement in cement concrete or as a binder in geopolymers. However, further research needs to be carried out on durability and dimensional stability of PA based concrete.