How siderite (FeCO3) enhances the sulfate resistance of cementitious systems

Siderite has recently emerged as a promising reactive SCM in Portland cement systems, with potential to enhance concrete durability. This study systematically investigates the resistance of siderite-containing mortars and pastes to external sulfate attack by evaluating both their expansion behavior and chemical stability in Na 2 SO 4 solutions. Microstructural, mineralogical, and chemical changes were thoroughly characterized. When used either as a fine aggregate or as a partial binder replacement (10–50 wt%), siderite significantly improved the sulfate resistance of OPC systems. Remarkably, OPC–FeCO 3 blends exhibited lower expansion than even highly sulfate-resistant cements such as C 3 A-free CEM I SR-0 and CEM III/B. This enhanced resistance is attributed to the partial consumption of portlandite, the stabilization of primary ettringite during hydration, and the formation of less expansive Fe-rich ettringite during sulfate exposure. Our findings highlight how highly relevant durability issues of OPC can be improved with readily available natural Fe carbonates.