The influence of gypsum ratio on the mechanical performance of slag cement accelerated by calcium sulfoaluminate cement

This paper investigates the use of calcium sulfoaluminate cement to improve the strength of high-grade slag cement composed of granulated blast-furnace slag and anhydrite. Two quaternary binders, composed of granulated blast-furnace slag, Portland cement, calcium sulfoaluminate clinker and calcium sulphate are compared with the reference. The only parameter studied is the gypsum ratio in the calcium sulfoaluminate cement. The results of compressive strength testing on standard mortar (in accordance with the European standard NF EN 196-1) indicate that the early and medium-term mechanical performances increase as the amount of gypsum increases. To explain this, pore size distribution in the mortar was studied and XRD, DTA–TGA and SEM techniques were applied to pure paste. The investigation showed that the hydration reactions of the individual constituents are dependent on the gypsum ratio and that its variation can lead to the formation of different hydrates.

Research highlights
► Acceleration is due to ettringite from hydration of yeelimite and gypsum.
► Activation of yeelimite by portlandite depends on gypsum ratio in CSA.
► Medium term strength is due to sulfo-calciq slag activation.
► Increased quantity of hydrates or change of morphology of hydrates.