Mild alkali activation or latent hydration of blast furnace slag with brine: Durability studies of mortars

Alkali-activated materials have gained increasing interest as potentially sustainable alternatives to Portland cement-based binders. However, the alkali-activator component of the mix designs may compromise their environmental or cost efficiency. One approach could be to replace the alkali-activator partially or completely with high-salinity brine. This study investigates the durability of blast furnace slag mortars prepared with an unconventionally low 0.1 M NaOH concentration (i.e., 0.16% Na 2 O of slag mass) or without NaOH (i.e., via the “latent hydration” of slag) using brine (105‰ salinity) instead of fresh water. An evaluation was made regarding some of the key durability aspects: shrinkage, leaching, freeze-thaw resistance, and acid resistance. Shrinkage tests revealed that NaOH-free mortars tended to shrink during aging, whereas mixes with 0.1 M NaOH showed better dimensional stability. In terms of leaching of chloride and sulfate (the main anions in brine), all samples were in the ‘inert waste’ category based on the European directive 2003/33/EC. After 144 freeze-thaw cycles, all mixes exhibited minimal mass change (< 0.8 %) and ultrasound pulse velocities were in the ‘good’ quality range. Under 5 % H₂SO₄ exposure for 28 days, compressive strength fell by approximately 40–60 % in all samples. Despite this deterioration, all mixes retained > 7 MPa strength, exceeding typical requirements of mine backfill, for example. These findings demonstrate that using high-salinity brine with little or no NaOH can achieve satisfactory mortar-level durability, highlighting a potential route to lower-alkali-demand cementitious binders.