Zeta potential, gel formation and compressive strength of low calcium fly ash geopolymers

• Workability is a key factor in the properties of low calcium fly ash geopolymer.
• Particle size smaller than 20 μm is best indicator of the suitability of fly ash.
• Specific surface area of fly ash leads dissolution, coagulation and gel formation.
• Negative zeta potential for the fly ash is indicative of a more reactive material.
• A smaller negative zeta potential of geopolymer indicates more gel formation.

A major challenge in the specification of geopolymer mix designs is the variability in the fly ash used and the impact of that variability on the performance of the geopolymer produced. The factors affecting the performance of geopolymers made from a total of five chemically and physically distinct fly ashes are reported. The key factor identified as influencing the strength was the workability, with a flow in the range between 110 ± 5% and 140 ± 5% required for optimal performance. In this flow range, the strength of geopolymer is governed by the specific surface area of precursor fly ash coupled with the quantity of the 10 μm and 20 μm particles. In addition a negative zeta potential of the fly ash was identified as assisting gel formation with the smaller the negative zeta potential of the geopolymer product the more gel formation and high compressive strength observed.