Orientation of calcium hydroxide at the portland cement paste-aggregate interface in mortars in the presence of silica fume: A contribution

A scanning electron microscope (SEM) with an energy dispersive X-ray analyzing facility has been used to study the orientation of calcium hydroxide, CH at the interface between cement paste and inert sand aggregates. The investigation was carried out on fractured surfaces of portland cement mortars with and without silica fume using secondary electron imaging in the SEM. Evidence from the SEM examination shows that silica fume tends to affect the pattern of crystallization and degree of orientation of CH crystals at the aggregate surface during the first few days of cement hydration. By 28 days of hydration, however, the calcium hydroxide crystals had disappeared from the aggregate interface in the silica fume-blended mortars. The interference with the orientation of the CH crystals by the microsilica is believed to be due to the tendency of the CH crystals to nucleate and grow on the fine particles of the silica fume when the latter occur at or on the aggregate surface.