Strength and Water-Ingress Resistance of Recycled Fine Aggregate Mortar Modified by Silane and Waste Ceramic Micropowder

Residual mortar on recycled fine aggregate contains pores and microcracks that weaken mortar strength and resistance to water ingress. This study treated recycled fine aggregate with 5 wt.% KH570 silane and used waste ceramic micropowder, at 4–12% of the total recycled fine aggregate mass, to replace an equal mass of the fraction not exceeding 0.6 mm. At a constant water-to-cement ratio and comparable flowability, compressive strength, flexural strength, 28 d water absorption, and maximum water pressure without visible leakage were evaluated, together with SEM, XRD, and MIP analyses. Mortar strength first increased and then decreased with increasing ceramic micropowder content, whereas water absorption showed the opposite trend. Among the tested replacement levels, SCR8 achieved the most favorable mean performance, with 28 d compressive and flexural strengths of 42.4 and 6.0 MPa, respectively, 38.6% and 25.0% higher than those of the unmodified recycled mortar. Its water absorption decreased by 36.4% to 7.48%, and it sustained 0.6 MPa without visible leakage. Microstructural results indicated fewer local defects, improved interfacial continuity, lower cumulative mercury intrusion, and a shift toward smaller pore throats. These improvements could be associated with combined regulation of aggregate wettability and pore-throat structure.