Reactivity of Siderite (FeCO₃) in Cementitious Systems and Its Potential Use as a Future SCM

As the supply of some traditional supplementary cementitious materials (SCMs) decreases, the necessity to study new materials to reduce the CO₂ emissions of the concrete industry arises. In that context, this study investigates the new and mostly unexplored potential SCM siderite (FeCO₃), an iron carbonate extracted from the Austrian Erzberg mine. For this contribution, different batches of siderite were mixed into siderite-cement binders and tested for heat of hydration, activity index, and miniature sintering potential. According to heat of hydration, siderite retards the hydration of cement, expectedly from the reaction of Fe²⁺ with cement. Siderite-cement mortars show a decrease in 2-day strength in the range of 37–90%, in comparison to pure cement mortars. Factors such as fineness and storage time of the siderite powder, as well as quarry differences, influence the reactivity and, consequently, the magnitude of the strength reduction. Late strength, however, can remain higher than 80%, according to past studies. Miniature sintering potential of accelerated siderite-containing binders depicts an improvement in the durability of concrete, with a 21% reduction of Ca²⁺ leaching. This is attributed to the consumption of portlandite by siderite to form Fe-AFm phases, preventing the Ca(OH)₂ from dissolving. This investigation shows that siderite could be used as an SCM in future applications, as it can improve the durability properties of concrete while maintaining high late strengths. More evaluations need to be conducted to properly understand the hydration mechanisms. Herewith proper recommendations and optimization of cement mixtures could be estimated and implemented in the future.