Cracks or damage to concrete ceilings indicate a problem. Something in the support structure or in the concrete itself is failing. The foundation may shift as the ground beneath moves. Soil-settling can shift the walls and crack concrete ceilings. You can't fix it with ordinary cement because cement shrinks when it dries. New materials have been developed that are very strong, durable and expand when they dry. With a little prep work, you can create a lasting, permanent repair.
Clean the ceiling thoroughly with soap and water, then apply a cleaning solvent to remove grease and oils from the repair surface. Dry the surface thoroughly.
Work around the lip of the crack or hole with hammer and chisel, undercutting the edge of the crack and widening it behind the surface edges, creating a flat-topped inverted "V" like a dovetail joint in a dresser drawer corner. On a ceiling, the wider part will actually be above the surface of the crack, so the "V" will, in fact, be right side up in relation to gravity. Remember, you are working upside down on a ceiling. Don't cut the crack in a "V" shape with the narrow end away from the surface. The expanding repair compound will actually push itself up out of the crack. The dovetail shape will cause the repair material to push itself more securely into the crack.
Mix epoxy-based concrete repair compound according to manufacturer's instructions. Use the type that bonds more slowly. This can be problematic in a ceiling repair, so you'll have to provide some backing to the repair. The dovetail shape of the crack will help hold the repair compound in the crack, but inject plenty into the fissure so you get a good seal throughout the depth of the crack.
Smooth the top of the repair compound with the putty knife, pressing the material well into the crack. Add more if it squeezes further in. Don't skimp on the repair. If some compound squeezes all the way through the crack into the attic or onto the roof, it's all well and good. It will help better support the repair that way.
Cover the repair compound while it cures. Lay strips of cardboard over the repair and tape it into place with duct tape. You can even duct-tape directly over the compound. Once it's set, the tape will peel right off. Allow to set and cure.
Sand over the top of the repair to blend it into the ceiling. If the repair is inside a finished space, recoat or paint the ceiling where the repair was done. The repair compound will have pushed up a little, so you'll need a coarse-grade sandpaper designed for metal or concrete to smooth the surface for painting or sealing. Paint or seal. To get an invisible repair, you'll probably have to redo the whole ceiling at least in the section where you've made the repair.
Use the putty knife to make the repair as flat as possible while the repair compound is still malleable. Remember the repair compound expands rather than shrinks.
Be sure to set up fans or wear a respirator when painting the ceiling or working with solvents. Keep the area well-ventilated. Wear gloves when handling epoxy-based repair compound. Clean up spills immediately.