Mortor = Cement + Sand
you mix 2 parts sand to 1 part cement, for a background i would add 3 parts sand to i part cement, a good idea is to use play sand and wash it prior to use, this way there is no chance of getting contaminated sand that can leach unwanted minerals or poisons far after the cement has cured...
Some people use this type of mix to make there base rock in salt water setups, usein crush corals and such, they use it like putty, a very dry mixture...
Do you know about cureing the cement to prevent the ph from spikeing to 12+