If there is serious crumbling on the surface then no paint will stick very well. If the crumbling is minor, then start by gently scrubbing the surface with a wire brush. If you apply too much pressure, a lot will fall out. Just eliminate what is truly loose. The next step depends on what kind of paint you plan to apply. To prepare the surface for acrylic paint, first etch the walls and floor with muriatic acid. The acid will also clean off the surface some of the fine dust that was not removed with the wire brush. If you plan to apply latex, do the same thing, but use phosphoric acid instead of muriatic acid. Remember, if you choose either an acrylic or a latex paint, you will have to first apply a primer. Another possibility is to use a cementitious paint (above). There are several different brands on the market. If you choose, you can even add color to cementitious paint.