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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.

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Tricalcium silicate (Ca 3 SiO 5 ) with a very small particle size of approximately 50 nm has been prepared and hydrated for a very short time (5 min) by two different modes in a paste experiment, using a water/solidratio of 1.20, and by hydration as a suspension employing a water/solid-ratio of 4000. A phase containing uncondensed silicate monomers close to hydrogen atoms (either hydroxyl groups or water molecules) was formed in both experiments. This phase is distinct from anhydrous tricalcium silicate and from the calciumsilicate-hydrate (C-S-H) phase, commonly identified as the hydration product of tricalcium silicate. In the paste experiment, approximately 79% of silicon atoms were present in the hydrated phase containing silicate monomers as determined from 29 Si{ 1 H} CP/MAS NMR. This result is used to show that the hydrated silicate monomers are part of a separate phase and that they cannot be attributed to a hydroxylated surface of tricalcium silicate after contact with water. The phase containing hydrated silicate monomers is metastable with respect to the C-S-H phase since it transforms into the latter in a half saturated calcium hydroxide solution. These data is used to emphasize that the hydration of tricalcium silicate proceeds in two consecutive steps. In the first reaction, an intermediate phase containing hydrated silicate monomers is formed which is subsequently transformed into C-S-H as the final hydration product in the second step. The introduction of an intermediate phase in calculations of the early hydration of tricalcium silicate can explain the presence of the induction period. It is shown that heterogeneous nucleation on appropriate crystal surfaces is able to reduce the length of the induction period and thus to accelerate the reaction of tricalcium silicate with water.

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