This paper investigates the influence of different types of silica fume on the crystallization process of medium density calcium silicate based products. The products are formed by a new technology that consists of two steps. In the first step, a mixture containing calcium silicate hydrates (C-S-H) is formed by reaction of lime with special silicas at temperatures below 100°C. This mixture is then molded into boards by a filter-pressing technique. In the second step, the boards are treated in hydrothermal conditions enabling the conversion of the C-S-H into important contents of xonotlite (Ca6Si6O17(OH)2); this is the most stable calcium silicate hydrate phase at high temperatures. In order to make C-S-H in pressure less conditions, the use of reactive forms of silica is required. In this work we used silica fume as reactive silica. To understand the influence of the silica fume on the formation of xonotlite, several properties were studied, such as particle size, purity and specific surface area (BET). It was found that the particle size distribution and degree of agglomeration for the silica fume were the most important properties. A proper dispersion technique must be applied in order to break the silica fume agglomerates, forming particles small enough to react with dissolved lime and to form C-S-H phases that are able to be converted into xonotlite under hydrothermal conditions. Finally, it was also found that the formation of xonotlite is favored by the use of high purity silica fume.

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