Integrated gasification combined cycle (IGCC) slag and Si sludge, classified as general waste, were used as raw materials for making geopolymers. Geopolymers containing Si sludge have lightweight characteristics; however, they can easily collapse during the bloating process because of the formation of explosive hydrogen gas through the reaction between Si and the alkaline activator. Several methods, such as controlling the concentration of the alkaline activator, curing temperature, and mixing time, have been attempted, and the relationship between these parameters and the collapse rate has been investigated. Providing extra heat to the specimen during the bloating process would be an effective way to prevent collapse; however, it would not be economical because an additional cost would be required for extra heating during the bloating process. It was speculated that the most effective and economical way of preventing collapse or enhancing the collapse rate would be to supply extra mixing time during the bloating process. The optimum mixing time must be estimated under each set of geopolymer manufacturing conditions. A homogeneous and small-sized pore distribution could be obtained in the geopolymer matrix by providing extra mixing time during the bloating process

Keywords: Lightweight geopolymers, IGCC slag, Si Sludge, Bloating, Collapse rate