Effect of additives on the setting time and compressive strength of activated high-calcium fly ash-based geopolymers

This study aims to study the feasibility of using additives to prolong the setting times of high-calcium fly ash-based geopolymer. Moreover, the effect of sodium silicate and sodium hydroxide ratios (S/N) on the compressive strength was observed by varying ratio at 1.0–2.5 by weight (wt%). For solid to alkaline activator (F/L) ratio investigation, the mixing proportions were varied at 0.5–3.3 wt%. The proper S/N and F/L ratios in this study were 1.0 and 2.0, respectively. The ratio was chosen to study the effect of additives on geopolymers. Thirteen additives were mixed in geopolymer pastes in two different mixing methods, wet and dry mixing. The results showed that both initial and final setting times of the pastes with sugars addition increased in the ranges of 133.6%− 135.3% and 125.3%− 148.7%, respectively, by wet mixing. Additionally, the percentage of sugars addition at 1–5 wt% and the dissolving time at 15–1440 min were also investigated. The results depicted that the initial and final setting times were prolonged in the range of 78%− 204% and 55%− 280% with 3 wt% of glucose addition. Moreover, the investigation of heat release, drying shrinkage, phase analysis and microstructural analysis were also reported. However, the compressive strength of geopolymer specimens with sugar addition at 90 curing days were slightly degraded. Hence, the results from this study could be guidance for practical application in which strength is not severely demanded.