Effects of Recycled Fine Aggregate Prewetting and External Water Dosage on Mortar Hydration Indicators and Interfacial Characteristics

Prewetting recycled fine aggregate (RFA) changes both the amount and initial distribution of water, complicating the balance between workability and hardened performance. Two complementary mortar series were investigated using nominal prewetting levels of 55–100% and external water-to-cement ratios (w/c) of 0.381–0.446. Prewetting dosages were referenced to the 24 h saturated surface-dry absorption capacity. Flowability and strength measurements were combined with ignition-loss-based apparent hydration indices, backscattered electron (BSE) analysis, and qualitative staining observations. At an external w/c of 0.420, flowability increased from 142.86 to 202.87 mm between S55 and S100. The corresponding 28 d apparent hydration index increased from 75.68% to 82.68%. Among the investigated RFA mixtures at an external w/c of 0.420, S65 showed the highest mean 28 d compressive strength (40.60 MPa), whereas S100 reached 26.42 MPa. From S75 to S95, the near-RFA low-gray area fraction increased from 2.85% to 3.86%, accompanied by lower strength. Thus, improved flowability and higher apparent hydration indices were not consistently accompanied by greater strength. The combined trends may reflect competing effects of residual RFA absorption demand and water availability. For the tested RFA, prewetting dosage should therefore be selected together with external water addition to balance workability and strength.