The transition of fly ash-based geopolymer gels into ordered structures and the effect on the compressive strength

•The transition of amorphous gels into ordered structures can take place at 25 °C.•The transition can be accelerated at 80 °C high humidity conditions.•The transition takes place on amorphous gels randomly, rather than in pores.•It is worthy to consider the transition at design and manufacturing stages.

This paper reports the investigation of microstructural properties associated with the transition of amorphous gels into more ordered structures in fly ash-based geopolymers. Three typical geopolymer mixes are synthesized with Class F fly ash and slag. Accelerated ageing scheme is performed at the 80 °C hydrothermal conditions, lasting for 90 days. It is observed that under the hydrothermal conditions, the two geopolymer pastes containing solely fly ash activated with sodium hydroxide and sodium silicate exhibit exhibit a common tendency to partial crystallization after 28 days. Crystalline zeolitic phases (including analcime) are detected by XRD and FTIR techniques. The SEM observation shows that the transition from the amorphous gels into more ordered structures takes place in the binder matrix randomly, rather than in pores or from particular structures. The blending of slag can reduce the transition (crystallization) trend, which is attributed to the formation of calcium containing gels. The decrease of compressive strength of geopolymer at late ageing stage can be correlated to the phase transition. Proper molecular designs of geopolymers to kinetically control the transition must be considered for the applications of this family of materials.