How do you get cement to set under water?

Adding to the other answers — cement setting is a hydration reaction. Water chemically bonds with the cement slurry to form first a gel and then a solid. Extra water actually speeds it up (and makes it weaker).

In oilfield cementing, we try to pump so much cement that the water is mostly pushed out of the zone where strength is needed. When water contamination is expected, the cement blend can be changed (eg more silica) to make it tolerant of the excess water.

How do you make cement smooth like the floors at Home Depot?

The local Home Depot stores I do business with (3 within 40 miles) have hard troweled concrete floors with a sealer. The hard trowel process must be done as part of the finishing process. After a floor slab is placed and screeded off, the finishers will use a bull float to float the wet concrete. This raises cement grout (the “float” part of bull float) and pushes the aggregates down slightly. After the concrete begins to set, a troweling machine (powertrowel) or hand trowels are used to finish the concrete. Concrete slabs that receive flooring don’t have to be troweled perfectly smooth, but exposed, finished concrete floors do.

Troweling a slab is done in stages as the concrete hardens. The first trip on the slab with the troweling machine ‘’cuts it in’’ and flattens out the bull float marks. Next, the concrete is troweled again, to fill tiny holes and to smooth the surface. After the concrete has become hard enough to walk on it with almost no tracks, it is hard troweled, with the blades of the machine set as high as possible (increasing the pressure per square inch of contact). Often a little water is sprinkled on the concrete to act as a lubricant to the troweling blades. The concrete is troweled until all marks disappear and it is very smooth. We sometimes ‘’ burn the slab in’’, troweling it so hard the concrete becomes glossy black, but usually this isn’t really needed.

After the concrete has fully cured (28 days or more), the concrete is sealed with a hardener/sealer. This leaves the concrete slightly glossy (but not slippery), and the hardener fills the microscopic pores in the floors so stains do set and become permanent.

For polished concrete, a fine, finer, then finest diamond polishing stone is used to grind the surface to make it very smooth, almost glassy. This is a technique for finishing where a high gloss is desired, and can leave an almost flawless surface. The stones used for this process are something like 800 grit, 1200 grit, and 1600 grit, which are really very fine grit stones.