Abstract Durability of alkali‐activated slags has been assessed by measuring the permeability using the beam‐bending technique. This nondestructive technique obtains permeability values by measuring the hydrodynamic load relaxation curve of a saturated slender beam of solid paste placed in three‐point bending. The impact of activator selection on the permeability of alkali‐activated slags has been assessed together with the effects of water‐to‐precursor ratio, revealing that free silica in the activating solution is the dominant factor controlling permeability. The permeability results have been correlated with nitrogen sorption and mercury intrusion porosimetry to elucidate the percolated pore size controlling permeability. Comparison with existing beam‐bending results for ordinary portland cement ( OPC ) paste shows that alkali‐activated slags can be synthesized with permeability having the same order of magnitude as for OPC , indicating that certain alkali‐activated slags are as resistant as OPC to ingress of solutions associated with deterioration processes.