Drying shrinkage performance of alkali-activated ground granulated blast-furnace slag governed by alkali-modulated viscoelastic relaxation

The drying shrinkage of alkali-activated ground granulated blast-furnace slag (AAS) exhibits distinctive and more pronounced time-dependent behavior than ordinary Portland cement (OPC) pastes, yet how alkali impacts this behavior remains insufficiently understood. This study investigates the effect of alkali content on drying shrinkage of cementitious materials by comparing the performance of plain OPC (without external alkali addition), alkali-enriched OPC, and AAS pastes over a wide relative humidity (RH) range (12–85%). The results show that alkali-enriched systems demonstrate markedly different shrinkage responses even under similar cumulative moisture losses, whereas plain OPC paste maintains a comparatively consistent shrinkage–moisture-loss relationship. Viscoelastic analysis reveals that alkalis modify the time-dependent mechanical response of the cementitious solids, leading to larger delayed compliance and generally longer effective relaxation timescales. These changes enhance viscoelastic relaxation and stress-history effects, shifting drying shrinkage behavior from a moisture-loss-controlled regime toward a stress-driven, history-dependent response in alkali-enriched systems. The results further show that, under various drying conditions, the influence of alkalis is manifested through coupled changes in internal moisture state, pore structure, and pore-fluid thermodynamics, which together govern internal capillary-stress evolution. Consequently, drying-induced deformation increasingly depends on the evolution of capillary-stress history rather than on cumulative moisture loss alone. These findings demonstrate that the primary distinction between alkali-less and alkali-enriched systems lies in alkali-modulated viscoelastic relaxation.