Hydration behaviour of sulphate-activated slag cements

Abstract

The sulphate activation of glassy ground granulated blast-furnace slag (GGBS), such as supersulphated cement (SSC), was investigated. Environmental scanning electron microscope (ESEM) and X-ray diffractometry analyses showed ettringite and C–S–H as the main hydration products of the hardened SSC paste. Changes of the composition of the pore solution were determined during hydration. Thermodynamic calculations of saturation indices for possible reaction products supported the phase observations. A satisfactory strength development of SSC mortar was observed despite low reaction rates, maximum 26 wt % of the slag, after 56 days hydration. A greater fineness enhanced the degree of reaction of the GGBS and the compressive strength of SSC mortar. The high tendency towards carbonation is a negative property of SSC. Due to a fast progressive carbonation rate a coarsening of the microstructure occurred in conjunction with a loss of flexural strength. Excessive dosage of alkaline activator led to a significant loss of strength of SSC mortar. ESEM micrographs showed that ettringite crystals grew directly on the surface of the slag grains and caused a sterical separation of the hydrated slag particles. This coarsening of the hydrated structure was the reason for the loss of strength. According to the results of this study, SSC can be designed to obtain a sustainable, ecologically minded, binding material with low CO2 emission during production.