Calcium aluminate rich secondary stainless steel slag as a supplementary cementitious material

•Characterization of slag derived from stainless steelmaking refining processes.•Secondary stainless steel slag contain hydraulic phases, predominantly calcium aluminates.•Study of cement composites in which cement is replaced by secondary stainless steel slag.•Investigation of strength development, hydration products, and porosity.•Secondary stainless steel slag is a promising supplementary cementitious material. Detailed characterization of calcium aluminate rich secondary steel slag derived from two different refining processes of stainless steel production has been performed in order to gain generic knowledge about this by-product, and its behaviour as a supplementary cementitious material. Additionally, slag blended cement composites were investigated and compared to a limestone filler blended cement composites and to a reference cement composites. The results showed that the investigated slag contained several hydraulic phases, mainly in the form of calcium aluminates. In the case of the slag cement composites, a larger proportion of hydration products was observed than in the case of the limestone cement composites, as well as a higher rate of strength increase.