Ultra-high early strength fly ash-based geopolymer paste cured by microwave radiation

This paper investigates the strength development of fly ash (FA)-based geopolymer paste cured by thermal and microwave radiations, together with microstructural characterisations. The results indicate that both the two methods can significantly increase the compressive strength compared to room temperature curing. Higher microwave power can promote the strength development without changing the reaction products and the final strength, owing to the accelerated formation of N-A-S-H in the reaction product under the raised temperature, characterised by the increased Si–O Q2-3 bonds and decreased Si/Al ratio. Over-time microwave radiation weakens the strength due to the formation of micro-cracks. Microwave presents higher curing efficiency than heating, producing a compressive strength of 37 MPa within 15 min. Moreover, microwave radiation can accelerate the setting progress of the fresh paste without compromising its final strength after curing. The present work can guide the preparation of FA-based geopolymers with high early strength in practice.