Plugging 12"X12" Hole in Cement Slab HELP!

I had a leaking problem at the clamping flange/drain assembly in a shower drain. I decided to cut out the slab around the drain head. Approximately 12" x 12" box down to the dirt, exposing the drain head and trap. I scored and snapped off the original drain head, about 40 years old! I want to replace the drain assembly , cement in and then put in a shower pan. Problem I am running into is how to fill it so it is structurally solid. I decided on a product called CementAll. It is a fast curing cement. The dirt under the slab has settled over the years and so I had to build up the area under the hole with dirt and put a layer of silver sand on top. On a couple of the corners the sand just slips through. It was a tight area and I decided not to build a form as it would have been difficult with pipes in the way, etc. I drilled 1/8" pilot holes and hammered in masonary nails to act as keys and will fill with cement. Is there anything that will help the cement to adhere better to the sides of the opening and prevent it from slipping through?
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Paint the sides with Daraweld (a concrete "glue"). Also, if you still have holes in the corners, you can either fill them with Great Stuff (expanding foam sealant) or just mix a little more concrete and keep pushing it down the hole. However, depending on how things have settled, you might push 4 or 5 yards down there...
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Michael Roback wrote:

Does everything in your life culminate with HELP!!? You work in your loafers don't you?
Fill the void back up with dirt to approx. 3-4" below the slab. Tamp it a little and finish it off with simple mortar.. Use plain old Elmers glue or Weld-Crete, on the edges of the slab since you think it's going to cause a sinkhole and eventually engulf your whole house.
--
Liam


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