Hydration mechanisms of super sulphated slag cement

The hydration and the strength evolution of supersulphated cements (SSC) produced by the activation of two different ground granulated blast furnace slags with anhydrite and small amounts of an alkaline activator have been investigated. The main differences between the two mixtures are found to be the strength development, the dissolution rate of the slags, the amount and volume of the individual hydration products formed and the growth mechanisms of the ettringite. The chemical composition of the slag had a large influence on the amount of the hydrates formed and thus on the volume of the hydrated slag. Advancement of the amount of hydrates of a slag with low reactivity by adding aluminium sulphate and calcium hydroxide did increase the amount of ettringite. Nevertheless, the early compressive strength was not increased, but late strength shows a slow increase. It was concluded that the early compressive strength of an SSC using low reactive slag cannot be overcome by adding stoichiometric amounts of constituents which are used for the formation of a specific hydration product. The best way to increase early compressive strength is to increase the intrinsic dissolution rate.