Coordinating early-age strength, shrinkage resistance, and interfacial bonding in alkali-activated slag repair mortar using NaCl and light-burned MgO

Alkali-activated slag (AAS) repair mortars provide high early strength, but shrinkage can impair bonding. This study investigated combined NaCl and light-burned MgO additions using a 4×4 dosage matrix. With 4 wt% NaCl and 5 wt% MgO, 1-day compressive and flexural strengths reached 21.68 and 6.58 MPa, increasing by 92.2% and 72.3% over unmodified AAS. At 28 days, drying and post-demolding autogenous shrinkage decreased by 60.1% and 41.3%, respectively. Tensile bond strength at 48 h increased from 0.54 to 2.31 MPa. NaCl favored Friedel’s salt formation, while MgO contributed to Mg(OH) 2 formation. Measured porosity decreased from 21.29% to 12.19%. These microstructural observations were consistent with improved mechanical performance and shrinkage resistance. On a 1 t mortar-mixture basis, production-stage emissions within the binder/modifier assessment boundary were 65.5% lower than those of the Portland cement reference. These findings support the potential of this mixture for non-reinforced concrete repair applications in corrosion-insensitive service environments.