Effects of fly ash source and curing procedure on strength development of geopolymers

Fly ash geopolymer is regarded as a 'green' alternative for cement, which can be synthesized at a lower temperature using industrial waste as the major raw materials. Lightweight fly ash geopolymer offers many environmental and commercial advantages. This study compared the mechanical properties of geopolymers by NaOH solution activation of fly ash from different sources. Results showed that the high calcium and iron content in fly ash was the primary reason accounting for the high compressive strength of the corresponding geopolymer sample. High temperature curing at 90°C caused high shrinkage of geopolymer paste while mild temperature curing at 40°C resulted in prolonged strength development and lower final strength. To obtain a volume stable foamed geopolymer, a two-step curing procedure was developed with mild temperature 40°C curing at first following by 90°C curing.