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10New results show that the microstructure development of cement-silica fume blends is very different 11 from plain cement. portlandite (CH) tends to precipitate as platelets and even around clinker grains as 12 "CH rims" and is consumed by pozzolanic reaction with silica fume. The Ca/Si ratio in the inner 13 product (IP) C-S-H decreases as CH is consumed to reach Ca/Si ≈ 1.40-1.50 at the point when CH has 14 disappeared, and then drops down to 1.00 in absence of CH. At later ages, the IP C-S-H is often 15 composed of two distinct regions. The outermost (formed first) consists of originally high Ca/Si C-S-H, 16 which Ca/Si slowly decreases. The second (formed later) forms only once CH is no longer present and 17 has a lower Ca/Si. Between 10 and 38°C, the main effect of increasing the temperature is to accelerate 18 the reaction of cement and increase the reactivity of silica fume.

Microanalysis of characteristic X-rays in the SEM is a powerful method to assess the chemical composition of phases in cement pastes, in particular the calcium silicate hydrate containing aluminium (C-AS -H). Nevertheless, many variables may influence the results obtained, due mainly to the intimate mixing of C-AS -H with other hydrate phases and the susceptibility of this phase to damage by the electron beam. In this study the effect of various acquisition parameters was examined, along with methods to determine an "average" C-AS -H composition. The results acquired in the SEM were compared with the analysis of the same samples in the TEM, where phases can be analysed without intermixing. A simple method was used to obtain compositions from SEM based analysis that are very close to those which can be obtained in the TEM.

The Sn concentration gradients across the A15 phase ha ve been studied by Energy Dispersive X-ray Spectroscopy (EDS) measurements. High spatial resolution EDS measurements in the Transmission Electron Microscope reveal a comparatively strong Sn concentration gradient from the periphery towards the centre of individual (Nb Ta) 3 Sn grains.

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