Back when I was a roving cellular telecom engineer I supervised the construction of over 5000 cell towers all over the USA and in other parts of the world. It was typical to allow the tower caisson(s) to cure for at least 21 days before test plugs taken from the same concrete pour were compressed to the point of failure to check the strength of the concrete before we would stack the tower. They may be using a different type of concrete here, though.
It ain’t concrete, it’s just a cementicious slurry without aggregate rock. Very pump-able, and fast setting. However, the set up time is affected by temperature and pressure and other down-hole conditions that are hard to gauge from the surface.
One of my wells was ruined by unhardened “green” cement and an over-anxious crew. They tipped the bit in-hole to drill out the residual cement plug. The heat generated by the bit cured the cement around the bit and drill pipe, locking it up.
They dicked around for three weeks trying to un-stick the bit, and finally parted the drill pipe, leaving $1 Million worth of gear lost in the hole forever.
Moral of story: sh!t happens in the oil patch, rarely intentionally, but always costly.
Why not use steel caisons? They don’t need to very large. They cane be welded together.