Effect of reverse-osmosis brine and sodium aluminate on the hydration properties and strength of alkali-activated slag cement

The effects of sodium aluminate (NA) and reverse-osmosis brine (RW) on alkali-activated slag cement (AASC) using sodium hydroxide and sodium silicate as activators were investigated. NA was mixed at 0%, 2.5%, and 5.0% by binder weight, and the mix waters tested were tap water (TW) and RW. In the experiment NA showed the effect of improving the compressive strength, with the highest strength value obtained at 2.5%. However, the strength improvement effect of NA was more clearly demonstrated in RW samples than in TW samples. Changes in the C4AH13 (Tetracalcium aluminate hydrate; Ca4Al2(OH)14.6 H2O), C2ASH8 (stratlingite; Ca2Al2SiO7.8 H2O), and C3AH6 (katoite; Ca3Al2(OH)12) phases were observed according to the increase in NA through hydration reaction analysis. Friedel’s salt (Ca4Al2O6Cl2.10 H2O) was additionally observed in the RW sample. In the TW samples, more of the C3AH6 phase was formed than in the RW samples, which affected the strength reduction. In the RW samples, increases in the chloride ions and calcite phase in the pore solution due to carbonation of Friedel’s salt were confirmed. This phenomenon of the RW samples became more pronounced as the NA concentration increased. The technique of utilizing RW in AASC showed the possibility of being used as a novel stabilization/solidification method for reverse osmosis brine.