A Leaching-Index-Driven Framework for Durability-Oriented Design of Mineral Binders: Validation on Acid-Induced Degradation of an NHL–Pozzolan System and Prospective Extensions to Circular Materials

Most studies on circular mineral-based materials report short-term mechanical properties without providing predictive frameworks that link chemical degradation to long-term mechanical performance. This study develops and validates a leaching-index-driven chemo-mechanical framework for predicting the degradation of a natural hydraulic lime (NHL)–pozzolan mortar exposed to sulfuric acid. A normalized ionic-release index was used to drive all parameters of a trilinear continuum damage mechanics (CDM) model, enabling the prediction of complete stress–strain responses from leachate chemistry alone. The framework was calibrated using an extensive experimental dataset comprising accelerated acidic exposure at pH 1.5, 2.0, and 3.0 for durations up to 6000 h. Iron release was identified as the most suitable degradation indicator based on its monotonic evolution and strong correlation with mechanical deterioration. Power-law relationships linking the normalized leaching index to elastic modulus, peak strength, transition strain, and post-peak energy were established and validated against independent exposure groups, yielding prediction errors generally below 20%. Validation was performed against three independent blind exposure groups withheld from calibration, yielding mean deviations of approximately 18% in the elastic modulus and 13% in the peak strength, so that the quantitative validation rests on this limited three-group set, whereas the extension to circular mineral binders is presented only on a prospective, non-validated basis. A kinetic sub-model was further introduced to relate exposure conditions to the leaching index, enabling a complete predictive chain from environmental exposure to mechanical response and service-life estimation. Sensitivity analysis showed that post-peak energy dissipation degrades approximately 1.3–1.5 times faster than stiffness and strength, indicating that ductility-related parameters govern long-term reliability. The methodological architecture is further discussed, on a prospective and non-validated basis, in relation to circular mineral binders relevant to Baltic industrial and municipal by-product streams, including municipal-waste bottom ash, slag, fly ash, and recycled glass. Although experimental validation is limited to the NHL–pozzolan system, the proposed framework provides a physically grounded and data-efficient pathway for durability assessment and future durability-oriented design of circular mineral binders in the Baltic region.