The crypto-crystalline structure of C-S-H gel in cement pastes inferences from X-ray diffraction and dielectric capacitivity data
✍ Scribed by Åke Grudemo
- Publisher
- Elsevier Science
- Year
- 1987
- Tongue
- English
- Weight
- 498 KB
- Volume
- 17
- Category
- Article
- ISSN
- 0008-8846
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✦ Synopsis
X-ray diffraction data indicate that the structure of the C-S-H gel in cement paste might be analogous to that of crystalline CH, but with a somewhat distorted unit cell, formed simply by replacing some of the Ca ions by SJ ions in octahedral positions, or by octahedral voids. Due to the change in relative sizes of the ions involved, the Si ions seem to become mobile, oscillating or migrating between adjacent lattice positions, and strongly polarized, if an e]ectric field is applied. Measurements of the capacitivity ("dielectric constant") of cement paste have given values amounting to more than 500, or several times the value found for pure ice, or liquid water, which is about 80, and caused exclusively by the polarization of protons in the hydrogen bonds. The remarkably-high capacitivity of dry cement paste can be compared with these of other substances, for which very high dielectric constants have been observed, such as ceramic materials of the perovskite type. The property of ferro-electriclty is obviously associated with oscillatory movements of certain cations within oxygen polyhedra, or even between polvhedra. The mechanism of polarization in cement gels might be of a similar type. ]t is further observed that, in the early reaction in cement pastes, a thermodynamically-unstable mixture of hydrate phases is formed. There are certain indications that the distribution of cations tends to equilibrate at a slow rate, possibly resulting in considerable reductions of strength in certain paste materials, chiefly those of high initial densities.
Similarities between the structures of crystalline CH and colloidal C-S-H 9el
In a recent report (1), the cement hydration processes and mechanisms of reaction are reviewed, and different alternatives for the crystal structures developing in these processes are considered. In particular, judging from measurements of peak area intensities in X-ray diffraction patterns of long-time hydrated cement pastes of widely varying w/c ratios, new principles are proposed for the crystal formation in C-S-H gels, with structures essentially different from those suggested in earlier works, i e those resembling tobermorite or jennite.