the different mixtures turned out to be really unstable so i setteled for the 1:2 mix
and finally did the first layer of cement on the background
so now i let it cure for a couple of days and apply the next layer. that one will be a bit thicker and should allow me to do a bit more sculpting but i have to see what kind of rocky surface i want to recreate... need to do some googleing.
if you have some inspiration please feel free to post some links.