“Shirasu” is a whitish sand found in pyroclastic flow deposits that includes a crystal mineral and an amorphous silicate. It has the properties of both a fine aggregate and a mineral admixture owing to its pozzolanic reaction. To utilize Shirasu as industrial resource, separation of the amorphous silicate and the crystal mineral is required. It is shown in this study that dry gravity classification is effective for dividing Shirasu into the crystal mineral and pumice, and can be used to obtain the amorphous silicate. This paper reports fundamental experiments on a high-strength concrete with a pulverized fine Shirasu powder used as a mineral admixture. In particular, fine Shirasu powder comprising the amorphous silicate pulverized to a particle size of 1μm exhibited high performance as mineral admixture equivalent to that of silica fume.