This paper presents a comparison between Si silica fume (a by-product of silicon metal or ferrosilicon alloys) used in civil engineering applications and CaSi silica fume (a by-product of calcium-silicon alloys). Si silica fume has been recognized as being a by-product of more importance to the concrete industry. A lot of studies have looked at its properties and at how it can contribute to the improvements in fresh and hardened concrete. On the contrary, few papers have been published on CaSi silica fume. Chemical, mineralogical and physical characteristics are analyzed here. Rheological and mechanical behaviors of cement pastes and mortars composed of Si and CaSi silica fume are also studied in order to observe the effect of CaSi silica fume as cement replacement. The results show that these two types of silica fume do not affect in the same way the fresh and hardened properties of cementitious materials. Replacing a part of cement by CaSi silica fume does not improve the compressive strength as Si silica fume does but a good rheological behavior is maintained. To complete this work, the alkaline activation of 100%CaSi silica fume mortars is investigated. These preliminary results show that this by-product is worth to be studied and lead us to believe that it could be used in construction materials. (C) 2015 Elsevier Ltd. All rights reserved.