Auswirkungen nachträglich hydratisierender Portlandzement-Partikel in Zementstein und Mörtel bei sehr niedrigen w/z-Werten

The aim of this research project was to answer the question under which conditions unhydrated Portland cement particles do react in the hardened cement paste with additional water permeated into the specimen and if this late hydration is a kind of an expansion reaction altering the microstructure by means of precepitation of voluminous hydration products thus reducing the durability and strength of the paste.
Is was shown that Portland cement pastes prepared with low water/cement-ratios, thus having a high content of unhydrated clinker grains, show countinuous hydration until 360 d when stored at a relative humidity higher than 95 % or under water, accompanied with all wellknown and desired consequences with regard to microstructure densification and gain in compressive strength. But after 540 d there were signs for a deteriorated microstructure which was not systematically connected with a loss in compressive strength. By means of UVmicroscopy only one specimen with a deteriorated microstructure in terms of starlight-shaped cracks originating from partially hydrated clinker particles could be identified. This was a CEM I 32.5 R with a LSF of 0.99 and addition of 10 M.-% of clinker 125-250 µm after curing at 50 °C under water isothermally.
This leads to the conclusion that one of the most important porperties of Portland cement pastes, its compressive strength, is not deteriorated systematically during the observed period of time and under the applied curing conditions due to late hydration. Only under unusual conditions - like elevated temperatures connected with a coarse cement - post hydrating clinker particles could be determined by means of UV-microscopy being responsible for adeteriorated microstructure.