Many millions of cubic meters of microsilica concrete have been placed over the years. Still, concern has been raised over possible risks related to the formation of hydrogen due to free silicon metal (silicon) contained in microsilica (silica fume, condensed silica fume).
The paper addresses this question from four different viewpoints: • Chemical reaction mechanism and theoretical amounts of hydrogen that can be generated. • Definition of explosion limits on hydrogen content in the air. • Test method for silicon-metal content in microsilica in one laboratory and comparison between laboratories. • Hydrogen development in practical situations, that is, in tests with mortar in an isolated chamber and in simulation tests of hollow-core elements.
From the present study it is concluded that only in extreme cases, with very high silicon-metal contents and with heat curing, can sufficient hydrogen be developed to cause an explosive mixture of gases. Further, the limit of 0.4% metallic silicon, as determined by the recommended method, will remove even these concerns.
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