Hydration of Portland cement paste mixed with densified silica fume: From the point of view of fineness

To be easily transported and stored, silica fume is commonly densified before using in concrete. The size of densified silica fume (DSF) agglomeration is up to several millimeters. In general, the agglomeration is hard to be dispersed into individual silica fume sphere. The properties of concrete are closely associated with the fineness of DSF. In order to explore the effects of DSF fineness on the hydration of Portland cement, four techniques, such as isothermal calorimetry, X-ray diffraction (XRD), thermogravimetry (TG) and scanning electron microscope (SEM) are carried out. In the end, the effect of the fineness of DSF on the mechanical property of cement mortar is discussed. Four series of DSF with different particle size distribution are investigated. Three dosages (0%, 10% and 20%) are considered. The results show that the screened DSF is difficult to be dispersed in the cement paste regardless of fineness. The fineness of DSF has obvious effect on the hydration of cement. With the increase of particle size, the delay action of DSF agglomeration on the hydration of cement becomes noticeable. The rate of cement hydration is decreased and the acceleration period is prolonged. The large DSF agglomeration shows less pozzolanic activity. In this binary system, calcium hydroxide cannot be consumed completely by DSF under high replacement level at 28 days of curing. Finer DSF definitely contributes to the improvement of the compressive strength of cement mortar, but only at a curing age of 28 days.