Durability Performance of Concrete Incorporating Calcined Montmorillonite-Rich Marl from Mine Interburden

The long-term performance of concrete structures depends critically on their durability, governed by transport mechanisms within the matrix. This study aims to determine whether carbonate-rich calcined marl derived from Moroccan phosphate waste can enhance transport-related durability properties of concrete while maintaining long-term mechanical performance at high substitution levels. Concrete mixtures were designed with varying water–binder ratios (0.45 and 0.55) and substitution levels (up to 40% by weight) to evaluate workability, mechanical performance, and durability. Incorporating calcined marl improved electrical resistivity, reduced water absorption, and enhanced sulfate resistance due to its pozzolanic activity and pore refinement effect. Although early-age strength decreased slightly because of dilution, long-term strength development was remarkable, reaching 81% above the control mix at 365 days. The main limitation observed was reduced carbonation resistance, which requires further investigation for practical applications.