This specification covers high-reactivity pozzolans for use as a mineral admixture in portland cement concrete and mortar to fill small voids and/or where pozzolanic action is desired. High-reactivity pozzolans are microsilica products with a particle size typically one to two orders of magnitude smaller than portland cement. These materials are usually supplied in undensified or densified dry form. If the material is supplied in a densified form, the tests should be performed on the as-collected undensified material before being processed into densified form. This standard is not intended for specifying silica fume. It is recommended that M 307 or ASTM C 1240 be used for that purpose.