Need suggestion for filling destroyed slab ductwork!

[jtprice]

Hello! Couple months back I bought my 1962 split level home a few years back and every thaw I would hear water collect in my duct work below my 1st floor slab. I recently bought a borescope to inspect the damage and it was worse than I ever imagined. Completely destroyed! I since capped off the supply trunk that feeds 4 registers that were affected and rerouted the ductwork which was not fun... (I only had 7" from the top of my supply trunk and 14 inches between joists to make it work, not ignoring the k&t wiring that ran right through the joists  ).

Anyways, I am to the point where I am ready to fill in the abandoned duct work and am looking for some direction on how to approach filling them in. Slab is in good shape, no foundation cracks. I know I want to fill in the section where the ground water is infiltrating (pictured attached) to make sure it doesn't create problems down the line for me.

Is there a mix I should be looking for? Any tips for application? And should be noted I would be doing this at the register boots as well.

Any recommendations are appreciated.