What cementing techniques should I apply?

I’ve started to build a small sized monolithic dome.
It needs to withstand -20 -ish Celsius, rain, draught and heat.
I’ve applied a sheet of tar onto the cemented layer at the foundation and have covered it with another thin layer of cement. (I intended this to be a bit of Isolation from the bare soil underneath).
I am using only materials immediately available in the surrounding environment, much of which are leftovers from previous building activities. It will be about 150 centimetres high and circa 2 wide.
Up to now I’ve only been using tiles (fragmented and whole), rocks and cement mixture.
Are there any suggestions you might offer ? Advice?
cheers all.