Concrete durability in aggressive environments is primarily governed by permeability and chloride-ion resistance, particularly when recycled aggregates are incorporated. This study investigated the effectiveness of a hydrophobic pore blocking additive (HPBA) to enhance the mechanical and permeability properties of low-carbon concrete using blended binders (70% cement, 20% fly ash, and 10% ground granulated blast furnace slag) and 50% recycled coarse aggregates. Workability was evaluated using the slump test, mechanical performance was assessed using compressive strength, and durability performance was evaluated using water penetration, sorptivity, and rapid chloride penetration test (RCPT). A 26% increase in 28-day compressive strength was observed relative to the control mix when 2% HPBA was used, although the workability of the mix was reduced by 22%. For the concretes incorporating 1% and 2% HPBA, the permeability-related properties, such as water penetration depth, total charge passed in RCPT, and initial absorption, decreased by 26–38%, 9–22%, and 45–59%, respectively. Overall, the results demonstrate that the use of a HPBA could effectively mitigate permeability-related performance losses associated with the incorporation of recycled aggregates, while maintaining strength and supporting circular-economy-driven concrete design.