Quikcrete
Quikcrete is normally found at home improvement stores but can also be used on tanks! This post will go along with Letter I- Insulation. Quikcrete is typically used over insulation to reduce the overall weight of this type of background.
Here is a build thread from user ArielElf on Dendroboard. If you are looking at creating a concrete background you need to check out this thread. This is a very inspirational build and it turns out awesome!
Insulation cut & in place
first coat of concrete
2nd coat of concrete
3rd coat of concrete. You can see that it is now able to be sculpted.
The tank then had to sit for awhile while being sprayed with vinegar to neutralize the ph. After many sprays of vinegar it will need to be flushed with water and verify the PH until it gets to around 7.0. After the neutralization the painting can begin!
Concrete was now painted to create some greater depth in the rocks.
Preliminary planting
Tank after being grown-in for awhile
close-up of waterfall
This tank has inspired me to want to create a tank like this as my next tank. Have you ever made a concrete background? How did it hold-up?
The Frog Lady 
Hi! Popping in from AtoZ. This is so interesting to me. Takes me back to a previous life many years ago. I’d have loved to have known this and you then. I dread to think what I designed at the time. Must go back one day to face the music..
Thanks for stopping by! It is crazy to see how fast things change. Just thinking back to my first tank makes me cringe!