Q.: For a job in New Mexico we must site-cast some double-tees. The coarse aggregate in the area contains 61 percent free silica but the Underwriters Laboratories permits no more than 20 percent free silica for fire endurance. Is there any way to treat the aggregate to reduce the silica content? The only alternative is to truck a suitable aggregate in from about 150 miles away.
A.: The only method of beneficiating the local aggregate that we know about is heavy-media separation. However, the constituents of the aggregates in the New Mexico area are reported to be so close together in specific gravity that heavy-media separation is inefficient and expensive. The best solution appears to be to truck in a limestone or other suitable aggregate.