Improving the workability and workable time of sodium hydroxide-activated ground granulated blast furnace slag binder-based concrete

In this study, an inorganic retarder and a synthesized dispersant (based on PCE) were used to improve the retention and workability of alkali-activated ground granulated blast furnace slag (GGBFS), with NaOH as the sole activator. The objective of the study was to formulate pumpable concrete mixtures with workable time of more than 90 min. The prolonged retention in slump was attained by the addition of the retarder. The effect of the dispersants, synthesized with different monomer to macromonomer ratios, on the workability of the paste was investigated by analyzing the fundamental rheological parameters. The addition of dispersant reduced the initial storage modulus and improved the workability of the alkali-activated paste mixtures. The interaction between the dispersant and NaOH-activated GGBFS systems was investigated by means of adsorption studies and zeta potential measurements. The dispersing ability and the amount adsorbed on GGBFS increased with an increase in the anionic charge of the dispersant. Zeta potential measurements suggested that the dispersion mechanism is primarily due to steric hindrance. Concrete mixtures of compressive strength in the range of ordinary concrete with pumpable workability for 90 and 120 min were achieved with the addition of both retarder and dispersant. The study concludes that a retarder is necessary to prolong the workable times, whereas a dispersant with a higher anionic charge is required to improve the workability of sodium hydroxide-activated GGBFS mixtures.