crumbling cement slab by windows

I need to repair areas of our cement slab near the windows. The previous owners allowed water to leak through the windows (probably for YEARS!) and the moisture saturated the slab and has made it crumble into dust and chunks. The gaps range from .5" - 2" and none of the radiant heat pipes are exposed.

We chipped out all the damaged cement and now need to fill these gaps with new cement. (we need to do this so we can put down new flooring.) Has anyone else had to do this?

Hopefully I can find a pre-mixed heavy duty mortar that will be just as good as cement, (since cement is such a pain to mix), but I would appreciate any advice. thanks!