Progress in building materials analysis (Part 2)

15 C₃S after 18 h hydration time: a) Helix detector image of the needle-like C-S-H phases in their native state; b) the same C-S-H phases imaged with too high charge density (damage to the C-S-H crystal structure – typical formation of bubbles)

16 Cryo-SEM: a) high content of K-sulfate OPC – 90 min hydr. time: ettringite and syngenite are detectable; b) low content of K-sulfate OPC
– 40 min hydr. time: only ettringite and foil-like C-A-H phases are detectable – no syngenite

17 UHPC matrix with reacted silica fume particles (see arrows): a) Helix detector image at 0.4 mbar water vapor pressure; b) vCD – BSE-detector image: beam deceleration mode landing energy of the electrons 1.5 keV

18 UHPC matrix with spherical shaped silica fume particles – uncoated sample imaged in LV-SEM mode at 2 kV: a direct comparison of a progressing pozzolanic reaction [a) Silica fume particle with signs of dissolution, b) respectively in its original state]

19 OIM analysis of a strained quartz: subgrains smaller than 10 µm are selected (see image small); these are color-coded in the map. 20 % of the entire scan area consists of these subgrains

Reaction products of blast furnace slag (left) and silica fume particles

Summary: By means of modern electron microscopy and preparation technologies it could be proved without doubt that the temporary syngenite formation during the early hydration of OPC with a high content of potassium sulphates is not an ESEM artefact. It can be derived from the microscopic investigations that the C-S-H phases occurring during the hydration process of OPC or C₃S in the outer hydration rim always show the same needle-like habit and they have a crystalline nature. These C-S-H phases are very sensitive to radiation damage. Furthermore some applications of the integrated EDS-EBSD system for phase identification and orientation mapping for the subgrain crystal size determination are presented.

4 Results and discussion

In the following some examples are described relating to the manifold microscopic and analytical capabilities of the FEI Nova NanoSEM in combination with high-pressure cryo-preparation techniques. These methods are especially useful to answer fundamental questions on cement chemistry.

Are the C-S-H phases crystalline which are
formed during the OPC hydration?

The C-S-H phases formed during the hydration process at ordinary temperatures and for realistic water/cement ratios of C₂S, C₃S and OPC always appear in the same needle-like morphology when ESEM-FEG and NanoSEM...

Progress in building materials analysis (Part 2)

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ZKG 02/2010