Time-Dependent Transition in the Re-Tempering Behavior of Cement Mortar

Construction delays frequently reduce the workability of cement mortar, and re-tempering by adding extra water is commonly practiced to restore its fluidity. Although re-tempering improves fresh-state workability, it may simultaneously impair the hardened properties of mortar. This study investigated the combined effects of waiting time (0, 15, 30, and 45 min) and additional water content (0, 2, and 4% of the initial mixing water) on the fresh and hardened properties of cement mortar under controlled laboratory conditions. Mortar flow, compressive strength, and 28-day water absorption were evaluated to characterize the re-tempering behavior. Re-tempering effectively restored workability; however, its recovery efficiency decreased with increasing waiting time. In contrast, compressive strength decreased, and water absorption increased progressively as both waiting time and additional water content increased. To quantitatively evaluate the balance between workability recovery and strength deterioration, a dimensionless Workability–Strength Trade-off Ratio (WSTR) was introduced as a comparative indicator based on normalized flow recovery and normalized 28-day strength loss. WSTR consistently decreased with increasing waiting time, with a marked reduction after approximately 30 min, indicating that the engineering benefit of re-tempering diminished as hydration progressed. These findings demonstrate that the dominant effect of re-tempering gradually shifts from workability recovery to deterioration of hardened properties, providing a quantitative framework for evaluating re-tempering under construction delay conditions. The findings should be interpreted within the investigated experimental conditions, and further studies involving different mortar mixtures and material systems are required to validate the proposed interpretation.