The long-term performance of Portland cement concrete (PCC) in hot, humid tropical climates remains a critical yet underexplored area in African construction materials research. Juba, South Sudan, presents an extreme case: mean annual temperatures of 28–38°C, relative humidity cycling between 30% in the dry season and 88% in the wet season, intense ultraviolet radiation, and the near-total absence of quality-controlled locally produced construction materials. This study investigates the mechanical and durability properties of four PCC mix designs — plain OPC, OPC with 20% fly ash (FA) replacement, OPC with 40% ground-granulated blast-furnace slag (GGBS) replacement, and a ternary OPC-FA-GGBS blend — under accelerated and natural Juba exposure conditions over a 90-day test programme. Mechanical performance was assessed through compressive strength (CS) tests at 7, 14, 28, 56, and 90 days; durability was evaluated through rapid chloride penetration tests (RCPT), carbonation depth measurements, water absorption, and sorptivity coefficients. The ternary blend achieved 28-day CS of 38.2 MPa (target class C30/37) with a 22% reduction in chloride penetration relative to plain OPC. Carbonation modelling using a modified square-root-of-time law indicates that unprotected OPC concrete in Juba would reach reinforcement cover depth (40 mm) in 12–18 years — well within the design service life. A Juba Concrete Durability Classification (JCDC) system with five exposure classes is proposed to guide mix design selection and minimum cover requirements for structural concrete in South Sudan. The study provides the first comprehensive experimental dataset on PCC durability under Juba conditions, filling a critical gap in regional construction standards.