Effect of Anhydrite on the Hydration of Calcium Oxide-Activated Ground Granulated Blast Furnace Slag Binders

To explore the potential use of anhydrite in calcium oxide-activated ground granulated blast furnace slag binders, this paper investigated the influence of anhydrite on the compressive strength, the pH of the pore solution and the non-evaporable water content for up to 28 days. The hydration products were studied by X-ray diffraction, thermogravimetric analysis, nuclear magnetic resonance spectroscopy and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy. Mechanical tests showed that the optimal anhydrite content was 5 wt.% of the total binder regardless of the curing time. The increase in compressive strength was mainly due to the formation of ettringite, which had a beneficial reinforcing effect in the matrix. Although a higher anhydrite content (over 5 wt.%) resulted in a remarkable increase in the non-evaporable water content, it did not make any contribution to the compressive strength due to the decrease in the calcium silicate hydrate gels content. Additionally, the presence of anhydrite inhibited the formation of hydrotalcite-like phases because most aluminum ions were incorporated into ettringite. It was also found that the addition of anhydrite reduced the mean chain length and aluminum proportion of the calcium aluminum silicate hydrate gels.