They carried any variety of bulk materials. Due to differing densities of bulk materials, the cars had specific loading instructions to keep within weight limits. Denser materials were loaded X feet and inches from the top according the instructions.
More information here:
https://riverraisinmodels.com/libraryData/prrh30h32.pdfFrom the above source:
"While these cars are generally thought of as cement cars, they hauled many different commodities, some more dense and some less dense than cement. The general lading for these cars, in order of increasing density, included pulverized coal, soda ash (anhydrous sodium carbonate, used for industrial cleaning and chemical or soap production), lime (calcium oxide, used in making cement and mortar), cement, sand, sodium nitrate (saltpeter, used in fertilizers, explosives, and various chemical processes), feldspar (crystalline mineral mainly of aluminum silicates, used in metal alloys), and dolomite (mineral consisting of magnesium and calcium carbonates, used in the chemical and steel industries). "