Please tell me is it correct, that instead of backfill with dirts, how about fill it with dry concrete mix?

Question:Sincere appreciation to the answers from those gentlemen. I was thinking instead of backfill with more dirts, how about fill them with mixed concrete? And my reason are as following:

A) The hallow spaces under the surface around the piers are numerous and full of watery mud, they need to be drilled an 1.5 inch hole of 3 feet deep. Dry concrete is easier to find than the dry dirt especialy in some township.

B) Around the cured piers, some of the holes of 3 feet deep are full of water or watery mud, their hallow spaces are 5 or 10 times bigger than the straight hole. Because when I did pour in the concrete mix, the hole kept eating continuously like entering a big water tank in the base. In the mean time, I kept using a small sticker to keep the hole path clear. I did this untill finally the filling became solid and dry, and pact them with a tamper.

C) Dry concrete powder mix will absorb water inside / under the hallow spaces, and pretty soon it will dryup becoming solid concre

Answers:
It can be used but does have several drawbacks. First concrete is a lot more expensive than fill dirt. You will still need to compact the concrete just like dirt because it will get wet and then dry at the exterior which will prevent more water from getting to the interior to cure it. I know this is true because a long time ago I tried placing dry concrete mix around some posts in post holes. Years later when I dug them up the center was still dry concrete mix and only the exterior had cured.