Fly ash-based geopolymers: The relationship between composition, pore structure and efflorescence

• Efflorescence of geopolymers depends on activator, temperature and slag addition.
• NaOH, high temperature and partial slag are favour for slow efflorescence.
• Soluble Si and slag addition have limited effect on overall efflorescence potential.
• Crystallisation appears to be effective of reducing efflorescence potential.

This study reports the observation of efflorescence in fly ash-based geopolymers. The efflorescence rate strongly depends on the activation conditions; at the same alkali content under ambient temperature curing, NaOH-activated geopolymers show less and slower efflorescence than sodium silicate-activated specimens. Geopolymers synthesised at high temperature exhibit much lower efflorescence than those synthesised at low temperature, except for the sodium silicate-activated samples. The substitution of 20% fly ash by slag reduces the efflorescence rate. A relationship between alkali leaching from monosized fractured particles and ‘efflorescence potential’ is proposed. Soluble silicate and slag addition are beneficial in reducing efflorescence rate, but have very limited influence on the overall efflorescence potential, as they appear to have a delaying rather than mitigating effect. The partial crystallisation of geopolymers, by curing at high temperature, appears to be the most effective method of reducing the efflorescence potential.