Mud
Mud turns up in 3 fields: refractories, HVAC, concrete. Here is what it means to the people in each one.
Mud in refractories
Shop word for the clay used to plug a blast furnace taphole, and bricklayers' word for mortar.
In the cast house, mud is taphole clay, a resin- or tar-bonded mix of alumina, silicon carbide, coke and clay that the mud gun pushes into the taphole to close it after a cast and that bakes into a plug. Its quality decides taphole length and stability. On a relining crew, mud is simply mortar, as in mixing a box of mud. Context always tells which one is meant.
"Mud gun pushed 120 kilos, hole's closed, tell the drill crew we're on the other side next cast."
Mud in HVAC
Duct mastic, the thick paste brushed over duct joints and seams to seal them airtight.
Water-based mastic is brushed or troweled onto joints, with fiberglass mesh tape bridging larger gaps, and cures into a flexible, permanent seal that far outlasts cloth duct tape. As a verb, mud the joints means seal them with mastic. Inspectors and energy raters look for it at every takeoff, boot and plenum connection.
"Mud every takeoff and the boots before the insulation goes on, the leakage test is Thursday."
Mud in concrete
Jobsite slang for fresh concrete, used by finishers, pumpers and ready-mix drivers alike.
The default slang name for wet concrete on American sites, as in mud truck or 'how much mud is left in the drum'. It covers the plastic state only; once the material sets, crews go back to calling it concrete or the slab. Plasterers and drywall finishers use the same word for their own compounds, so the trade doing the talking decides the meaning.
"First truck's on the street, get the mud moving to the back corner before the pump starts chasing it."