Alternative hydraulic concretes based on calcium sulfate-granulated blast furnace slag composites

Two series of hydraulic concretes were prepared with alternative and sustainable composite binders of mineral calcium sulfate hemihydrate or waste anhydrite fluorgypsum, both supplemented with granulated blast-furnace slag, and fly ash or silica fume. The mixtures with 300 and 400 kg m−3 of binder cured for up to 360 days showed strengths of up to 30 MPa and hydraulic character. The microstructural configuration, characterized by SEM, EDS and XRD, indicated that the precipitated framework of gypsum crystals was stable under water after being engulfed by C–S–H like products formed by the reactions of the slag, pozzolanas and Portland cement. Ettringite was identified and assumed in moderate amounts in some samples, indicating the sulphatic activation for the slag and fly ash. The environmental impact assessed by a simple LCA indicated that the studied concretes release GHG in average 64.6 or 81.3 kgCO2eq·m−3 depending on the binder content, which is up to 5.3 times lower relative to conventional Portland cement mixtures. Depending on the distances travelled, freight was an important factor of GHG, while grinding and sieving were very low. [Display omitted] •Sustainable concretes made with gypsum-blast furnace slag-pozzolanas were studied.•Good mechanical strength was developed while maintaining insolubility under water.•Dihydrate, C–S–H and ettringite were the main hydration products.•Mixes generate up to 5.3 times less greenhouse gas emissions than Portland cement.•This study constitutes the basis for extensive research on gypsum-based concretes.