Q.: At our precasting plant we put sulfur caps on test cylinders the day after they're cast but before they're placed in the moist room. Is there anything wrong with doing this?
A.: ASTM's standard practice for capping cylindrical concrete specimens (ASTM C 617-87) doesn't directly address this practice. Section 5.2 says hardened specimens which have been moist cured may be capped with high-strength gypsum plaster or sulfur mortar. It doesn't say, however, that specimens which haven't been moist cured can't be capped with sulfur mortar.
If cylinders capped with high-strength gypsum plaster are stored in a moist room, the caps must be protected from dripping water and moist room storage must not exceed 4 hours. That's because water may weaken the caps. Cap weakening isn't a concern with sulfur caps because they aren't soluble in water.
You do have to be sure that cylinders don't dry too much after they're stripped from the mold and capped. Guard against this by wrapping the stripped cylinders in wet burlap before and after capping until they're placed in the moist room.