It is rated at 94#/cf because that is the "bulk" density due to the fineness. The actual particles have a specific gravity of 3.15.
When you are doing a concrete mix design everything is based on the actual weight of the material to maintain a measureable standard.
It is very similar to the situation with aggregate, where the actual specific gravity of the particles is 2.7 +or- while the bulk density many 95 to 110 pcf. Aggregate also has the property of bulking depending on the moisture content and whether it is surface dry or saturated. When the concrete is batched, everything should be converted back to dry weights to control the amount of actual materials, proportions and the w/c of the completed mix.
One exception to using the volume of materials for good proportioning is the preparation of mortar, where all proportions are based on the bulk volume of the materials (cement, lime, sand). This is because of the vast history or performance of mortar and the site mixing process. Trying to use weights would be impossible and would no provide the desired workability, which is the most important property of mortar (ASTM C270 Appendix).
