Preparation, properties and applications of fly ash-based porous geopolymers: A review

A very large amount of fly ash is generated from coal combustion, and the high value-added and efficient utilization conversion of this solid waste into a resource remains an issue to be solved. Among the many methods of waste resource utilization, the preparation of porous geopolymers is an effective way to improve the added value of fly ash. In this review, the processes for obtaining fly ash-based porous geopolymers are divided into two categories: bulk-type (direct foaming, sacrificial template, additive manufacturing, and fast microwave foaming method) and sphere-type (suspension solidification, direct molding, and calcium chloride solution gel method). The synthesis routes, main characteristics (pore structure, porosity), and performance (mechanical and thermal properties as well as adsorption etc.) of fly ash-based porous geopolymers are reviewed, and examples of applications such as heat insulation, filtration, sound absorption, and pH buffering are presented. Moreover, the future development prospects of this field are discussed.