Microsilica is currently being used in a wide range of refractory castables and its quality influences performance, particularly flow and set of castables. In the present work, two different types of microsilica “Elkem ASA Materials” (grade 983U) and polish “Silimic” have been investigated. It was established that the differences between the characteristics of paste and suspensions of microsilica (pH, electric conductivity, viscosity) depend on the quality of microsilica. The influence of type and amount of microsilica on the pH, electric conductivity and viscosity behavior of alumina cement Gorkal-40 and microsilica pastes has been investigated in details. Deflocculants Castament FS-20 and sodium tripolyphosphatum were used to control the characteristics of the investigated materials. The influence of their amount on the characteristics of various suspensions and pastes was estimated. It is shown that rheological characteristic of middle cement castable, which contains in its composition with microsilica of low quality, may be increased considerably if the right amount of the right deflocculant is added
