Inspect the roof over the cement ceiling. If you have a leaky roof that is allowing the cement ceiling to fill with water, you need to fix the leaky roof first or your cement ceiling problems will reappear.
Start the repair of the cement ceiling by cleaning away any dirt dust, flakes and loose concrete from the cement ceiling with a wire brush.
Mix a batch of stiff cement. Follow the instructions on the bag for the particular type of cement you are using. It is especially critical that you have the right mix when adding cement patches to a ceiling. If the mix is too dry, it will flake and fall off; if it is too wet, it will drip before it has a chance to set.
Dampen the surface of the ceiling with a spray bottle. The surface should be slightly wet to the touch but not so wet that it's dripping. This dampness helps the new cement to adhere to the old ceiling.
Trowel the cement into all gaps and cracks on the ceiling. Work with small amounts of cement at one time, and press them firmly against the existing surface to create a strong bond. For any gaps that are wider or deeper than 1/2 inch, cover them only partway with cement, allow it to cure for a day, then add another layer. If you try to fill large areas with cement all at one time, the cement falls out from its own weight.
Smooth over the repaired area with the back of the trowel to hide the repairs and blend them in with the existing ceiling.