I use low alkali portland cement from home depot. I've used crushed oyster shells mixed in with it, although you can use lots of other things mixed in the cement. Some use crushed coral or aragonite sand. I can't remember exactly the ratio, but I believe I use about 1 part cement to 5 parts shell or sand mixture.

After the "rock" dries, you must soak it for several weeks in (fresh)water so the rock doesn't cause the pH in your tank to skyrocket (because cement has a high pH). Some call this "curing". I'd change the fresh water every week. You may want to test the pH after several weeks using a fw test kit. pH should not exceed 8.4.

Andrew