Impact of micro characteristics on the formation of high-strength Class F fly ash-based geopolymers cured at ambient conditions

Abstract

• The Class F fly ash-based geopolymer pastes with 28 d’s strength over 75 MPa is produced under ambient conditions. • The mechanical properties are strongly related to the synergistic action of amorphous phase content, reactive phase’s Si/Al molar ratio, and porosity. • Water content plays a critical role among the options of adjusting mix proportions in improving the micro characteristics. The high-strength Class F fly ash-based geopolymer pastes cured at ambient conditions with 28 d compressive strength over 75 MPa is rarely reported. Herein based on such reference, the geopolymer pastes synthesized by cognate fly ash with lower activity are produced to investigate the combined effects of geopolymer’s micro characteristics, such as amorphous phase content, Si/Al molar ratio of reactive phase, and porosity, on the enhancement of mechanical properties. The fly ash-based geopolymers are prepared with different Si/Al molar ratios, Na/Al molar ratios, and water/fly ash mass ratios. The results at the micro-level indicate that the development of mechanical properties of fly ash-based geopolymers is mainly affected by the reactive phase’s Si/Al molar ratio, followed by the porosity and then amorphous phase content. In addition, experiments show that decreasing the water/fly ash mass ratio leads to the positive promotion in all the micro characteristics with small reactive phase’s Si/Al molar ratio, low porosity, and high amorphous phase content. Therefore, the dramatic increase in the compressive strength and Young’s modulus of geopolymer specimens can be obtained. In summary, it can be concluded that, given the workability of precursor preparation, the application of a water/fly ash ratio lower than 22.5 % can increase the strength close to 75 MPa, which is beneficial to the potential application of fly ash-based geopolymers in practical engineering. The other related characteristics of high-strength fly ash-based geopolymers will be investigated in future studies to further reveal its latent practicability.