Most silica fumes contain a small amount of silicon metal. Silicon reacts with alkali or alkaline-earth hydroxides when water is present to yield hydrogen. The reaction mechanism is similar to the one used to produce cellular concrete or expanding grout with aluminum powder.

There is concern that hydrogen release can cause problems when silica fume is used in grouting prestressing tendons because atomic hydrogen embrittles steel, thereby shortening the life of the prestressed concrete structure.

Moreover, if the partial pressure of hydrogen in air attains a certain level, there is a risk of explosion in the presence of a source of heat or spark. Portland cement-based materials with silica fume that contains some silicon do not present an induction period that could delay hydrogen release. Therefore, users of silica fume should be aware of this potential danger.

Because most users of portland cement are unfamiliar with techniques for measuring the silicon content of silica fume, a simple and rapid measurement method is shown here to evaluate the amount of hydrogen that can be released from a given silica fume. This value can be used to calculate the amount of silicon contained in the silica fume.