Morphological, mineralogical and chemical features of steam-cured concretes containing densified silica fume and various alkali levels

Densified silica fume was incorporated in concrete mixes, containing various alkali contents, at the rate of 10% silica fume replacing 20% cement by mass. The cast specimens were steam-cured at 75°C and were examined by scanning electron microscopy, X-ray diffraction and thermal analysis soon after steam curing (1 day), and after periods of28 days and 1 year of external exposure. Mortars with and without silica fume were made with the same cement as that used in the concretes, and their pore solutions were expressed at different ages for the determination of their chemical compositions. The results show that the silica fume is very effective in removing alkali from the pore solution. However, densified silica fume agglomerates of 40-100 μm size persisted in the concrete, and in the presence of high alkali levels acted like reactive aggregate. Nevertheless, the silica fume was very effective in reducing the concrete expansion caused by a reactive granite aggregate, although it added slightly to the expansion of concretes containing a non-reactive basalt aggregate. The silica fume also had an influence on the type of sulphoaluminate phase formed in the concrete, and favoured the formation of ettringite. Therefore, silica fume may be effective in controlling both the alkali—aggregate reaction and secondary-ettringite formation in steamcured concrete.