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This paper reports the effects of nanosilica (nS), microsilica (silica fume, SF) and their simultaneous use (nSzSF) on ''both'' the microstructure of cement pastes and the mechanical properties of mortars. After water curing at 21uC for 7, 28 and 90 days, samples with water/binder w/b ratio of 0?35 were characterised by thermogravimetric analysis, X-ray diffraction, scanning electron microscopy and compressive strength test. Single or mixed mineral additions did not generate ''any'' distinct hydration phases compared to the reference material without additives. A decrease in the calcium hydroxide contents in later curing ages indicated a pozzolanic effect of nS and SF. The chemical action promoted by nS together with the physical effect due to the small particle size distribution given by SF result in higher compressive strength and better hardened properties, suggesting the synergistic action of nSzSF compared with single additions.

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