Processing and uses of fly ash addressing radioactivity (critical review)

Fly ash is the residue of coal combustion collected by electrostatic or cyclone separator. It is one of the largest quantities of waste disposed in the world. Fly ash represents mostly less than 100 micron in size spherical particles with pozzolanic and hydraulic properties depending on its composition. Utilization of fly ash depends on its chemical, mineralogical composition and morphology. Because of coal nature, fly ash represents a significant drawback with presence of radionuclides such as ²²⁶Ra, ²³²Th and ⁴⁰K. The fly ash can be used for various applications. The main amount of the fly ash is used for building materials production as cement additive and concrete production. Therefore, the determination of radiological properties both in the fly ash and final products are important parameters to consider. Radioactive isotopes cause release of alpha, beta, particles gamma rays and radon exhalation. However, fly ash addition doesn't increase the gamma dose substantially. Moreover, radioactive elements are generally immobilized within glass phase and therefore, radon emanation is not high. In this review the latest development of utilization of the various fly ashes with a different level of radioactive elements content for value added application are presented and a possible new direction of applications are discussed.