Kurzfassung (Abstract)

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.