Lightweight geopolymers were fabricated by using IGCC (integrated gasification combined cycle) slag and Si sludge which are classified as general wastes (recyclable resources). Three curing methods were tried to investigate thechanges in compressive strength and density according to the curing method and immersion time. Immersion period was tried up to 21 days to observe long-term performance in water. Compressive strength of the specimens cured in ovendecreased abruptly with an increase in immersion time. Compressive strength of the specimen cured in autoclave was low after 3 and 7 day immersion; however, increased rapidly after 21 day immersion. On the contrary, compressive strength of the specimen cured in autoclave and oven was high but substantially decreased after 21 day immersion. Conclusively, it was speculated that oven curing is effective for the compressive strength development at early age; however, autoclave curing is more desirable for the long-term performance in water.