Influence of Cellulose Ethers on the Kinetics of Early Portland Cement Hydration

[ir_71]

Influence of Cellulose Ethers on the Kinetics of Early Portland Cement Hydration

Author: Ingo Müller | Size: 8.96 MB | Format: PDF | Quality: Unspecified | Publisher: Universität Karlsruhe | Year: 2006 | pages: 120 | ISBN: 9783866440777

The present study is focused on the effects of cellulose ethers (CEs) on the kinetics of early
Portland cement hydration. The retarding effects of cellulose ethers on the development of
single cement phases could be traced to the adsorption behaviour of the polymers. A method
was developed to quantify the amount of cellulose ether adsorbed on the surfaces of cement
phases. The specific surface area of some hydrated phases was analysed by determination of
nitrogen gas adsorption isotherms. The amount of adsorbed cellulose ether was determined by
analysing the residual free water of a cement paste by size exclusion chromatography. It could
be demonstrated that the adsorption behaviour of the polysaccharide depends on the mineral
phase and on the degree of substitution (DS), a key parameter of cellulose ether
characterization.
Beside other methods, the hydration processes were monitored in-situ by the means of ultra
sonic measurements, synchrotron X-ray and neutron powder diffraction and environmental
scanning electron microscopy using a field emission gun. The results of all independent
methods showed a good correlation. The following phase reactions could be observed within
the first 24 hours of Portland cement hydration:
1. tri-calcium aluminate + bassanite + water → ettringite (1st generation)
2. bassanite + water → gypsum
3. anhydrite + water → gypsum
4. tri-calcium aluminate + gypsum + water → ettringite (2nd generation)
5. tri-calcium silicate + water → calcium silicate hydrates + portlandite
It turned out that cellulose ethers had a strong direct effect on the hydration of the silicate
components of cement. Tri-calcium silicate and its hydration products showed the highest
values of polymer adsorption. The adsorption led to a surface intoxication and therefore to
delayed hydration reaction. The effects of cellulose ethers on the aluminates were less
specific. No CE adsorbed on the surface of ettringite, a hydration product of tri-calcium
aluminate and calcium sulphates. Nevertheless the ettringite crystallization after 4 hours was
strongly retarded by cellulose ethers depending on their DS. This retardation was an indirect
phenomenon caused by delayed development of ion-concentration in the pore water. This
delay again was a consequence of the retardation of the silicate hydration caused by cellulose
ethers. In general it can be said that the lower the degree of substitution of the cellulose
ethers, the larger the amount of adsorbed polymers and the stronger the retarding effect.

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