Concentrated salt water can partially replace sodium hydroxide in alkali activation of blast furnace slag

Abstract

Abstract

Alkali-activated materials are low-CO2 alternative binders, but the alkali-activator component contributes significantly to the CO2 emissions, costs, and safety risks. This study explores the potential of partially replacing NaOH with highly concentrated salt waters as an activator for blast furnace slag and related chemical mechanisms. Four water types were compared: simulated brine (∼105‰ salinity), NaCl and Na2SO4 solutions, and deionized water. Results show that NaOH molarity can be reduced to 0.1 mol/L (or 0.16 weight% NaOH of slag weight—a significantly lower value than typically required) using simulated brine, while achieving a 28-day compressive strength of 28.0 MPa—more than double compared to deionized water. Slag dissolution increased substantially upon using salt waters with 0.1 mol/L NaOH in comparison to deionized water. Thermodynamic modeling and slag dissolution studies indicated that salt anions (e.g., SO42− and Cl−) formed ion-pair complexes with Ca and Mg, and thus drove the enhanced slag dissolution. The similar likely occurs also when using Na2SO4 as an alkali activator for which the mechanism has been unclear in the existing literature. From the practical point of view, the highly concentrated salt waters could largely replace NaOH in certain applications of alkali-activated slags when no steel reinforcements are required.