Help. Powder dry, concrete floor. Paint scrapes up won’t stick.

I need big time help as soon as I can get it. Im trying to paint my cement floors and failing miserably.  I am removing 30 yr old carpet. Yes 30 yrs old. The cement slab is crumbly and like a powder paint isn’t sticking I have researched, asked authorities, experimented with all kinds of bonding agents, sealers, polyurethane products. Water based product. Oil products. With this said. I’m at a complete loss. I am defeated in this task because the paint on my floor will not stick. I have to strip the floor which I just painted  the cement is like powder. crumbles up underneath the paint. I desperately need to finish my house. How can fix this?