Effect of chemical admixtures on properties of alkali-activated Class C fly ash

Geopolymers are considered an alternative to portland cement in the construction industry since they can be formulated to possess comparable performance. Geopolymers are synthesized by alkaline activation of aluminosilicate materials. In this study, three sources of high calcium, Class C fly ash were used as the aluminosilicate sources for geopolymer synthesis. One major problem of high-calcium fly ash geopolymers is the workability. The goal of this work was to find a way to improve the workability of geopolymer pastes. The workability of the geopolymer pastes was assessed by the mini slump test and rheological testing. The addition of two chemical agents (sodium gluconate and a commercial hydration stabilizer sold under the trade name “Recover”) as chemical admixtures was shown to increase the workable time of the geopolymer pastes, while additions of borax, naphthalene sulfonate, and sodium sulfate could not improve the workability. Optimum dosages to improve the workability for sodium gluconate and Recover additions were 0.35% and 1.50% by mass of the fly ash, respectively, when the paste was mixed using a mixer as prescribed in ASTM C305. However, additions of sodium gluconate and Recover at the optimum dosages resulted in reductions in compressive strength of the geopolymer pastes compared to the pastes without chemical admixtures. Fumed silica was also added as a chemical admixture to NaOH solution with Ms = 1; fumed silica was added to obtain a molar ratio SiO2/Na2O of 1. The pastes prepared with fumed silica addition sustained large mini slump areas for over 60 minutes for all fly ashes. However, the fumed silica addition increased the compressive strengths for WP and BC fly ashes, but resulted in significant drops in the strengths for MR fly ash. Workability was impacted by the mixing process used to prepare the paste. High shear mixing at 1000 rpm extended workable times with similar initial mini slump areas of the pastes. This study showed that it is possible to control the workable times of alkali-activated high calcium, Class C fly ash, but this can come at a cost of reduced compressive strength.