We use concrete mixes frequently for use in tank backrounds and to build artificial reef structures...but it is an involved process.
Use a premium grade portland cement. If you are going to be doing a lot of detail work, mix it 50/50 with plastic cement.
Once your work is done, set it aside to dry for 7-10 days. If you can get it outside after the first 36 hours, you will get a better cure.
Now comes the fun part. If it is a free-standing piece (seperate from the tank) you can soak it in a plastic container, if not you have to soak the entire tank. Fill whichever receptacle with tapwater and add 2 cups of muriatic acid (used for pools) per twenty gallons, being careful not to pour it directly onto the cement. Mix the solution and let the cement soak for three days. Drain, and repeat. Drain and rinse off thoroughly. Let stand to dry for 2-3 days. Don't freak out that the cement has turned brown...it is a normal reaction .
Fill the tank with r/o water and introduce some goldfish or the like to test and make sure you leached the concrete enough...if they live, go ahead and start your salt tank, or if it is a seperate piece, place it into your tank...
Sounds waaay more complex than it really is...let me know how it goes!