Chloride Resilient Recycled Aggregate Concrete: Optimising Marine Durability through Age Enhanced SCM Integration

The demand for sustainable construction materials is rising, and Recycled Aggregate Concrete (RAC) is one of the most discussed alternatives to conventional concrete. RAC, however, performs poorly in marine environments because chloride ions from seawater enter through the recycled aggregate and reach the reinforcement faster than in concrete made with natural aggregate, causing corrosion and shortening service life. This study uses a curated dataset of 1030 concrete records from the UCI Machine Learning Repository, extended with 287 marine-exposed records drawn from peer-reviewed studies, to investigate how Supplementary Cementitious Materials (SCMs) and age-dependent hydration influence the chloride resistance of RAC. Statistical analysis is combined with three machine learning algorithms (Random Forest, Support Vector Machine, and Artificial Neural Network) and extended through polynomial regression, Sobol global sensitivity analysis, and Monte Carlo simulation for probabilistic service-life prediction. A simplified Life Cycle Assessment is performed following ISO 14040 and ISO 14044. Replacing 30-50% of natural coarse aggregate with recycled aggregate, combined with 25-35% SCM by mass of binder, produces concrete with compressive strength above 45 MPa, chloride conductivity below 0.5 mS/cm, and a Monte Carlo mean service life beyond 100 years. The optimised mixes meet the durability requirements of ACI 357.3R-14, EN 206 (XS3), and JSCE Standard Specifications. The proposed hybrid mix containing 15% fly ash and 20% slag gives the best overall performance with a 41% reduction in CO2 emissions relative to conventional marine concrete, providing a basis for using RAC in marine infrastructure projects in Africa and other coastal regions.