This paper presents the results of the effect of fine inert and hydraulic fillers on the early hydration of mortars incorporating these powders. The activity of the mineral additives is assessed by measuring the heat of hydration of the different blended mortars which consist of 90percent Portland cement and 10percent admixture with a 0.45 water/binder ratio. During the early stages the fine particles accelerate the hydration rate by providing an increased number of nuclei sites for hydrates growth. These results concern the inert filler (alumina) and the hydraulic and/or pozzolanic microffillers (ultrafine calcite, silica fume and quartz powder). At very early stages the finest particles increase the compressive strength of blended mortars compared with control mortar. Moreover silica powders, especially silica fume, develop a pozzolanic activity which later increases the compressive strength.
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