Space Tug by Murray Leinster

Time passed. Hours, then days. Things began to happen. Trucks appeared, loaded down with sacks of white powder. The powder was very messily mixed with water and smeared lavishly over the now waterproofed wooden mockup of a space ship. It came off again in sections of white plaster, which were numbered and set to dry in warm chambers that were constructed with almost magical speed. More trucks arrived, bearing such diverse objects as loads of steel turnings, a regenerative helium-cooling plant from a gaswell -- it could cool metal down to the point where it crumbled to impalpable powder at a blow -- and assorted fuel tanks, dynamos, and electronic machinery.

Ten days after Mike's first proposal of concreted steel as a material for space ship construction, the parts of the first casting of the mockup were assembled. They were a mold for the hull of a space ship. There were more plaster sections for a second mold ready to be dried out now, but meanwhile vehicles like concrete mixers mixed turnings and filings and powder in vast quantities and poured the dry mass here and there in the first completed mold. Then men began to wrap the gigantic object with iron wire. Presently that iron wire glowed slightly, and the whole huge mold grew hotter and hotter and hotter. And after a time it was allowed to cool.