Shower Pan Install
Not sure if cement type is as important as to type floor it lays over. Is it a cement slab, or a wooden floor? If a cement, there is less chance of movement and the type you use is less important. If over a wood floor, which is prone to movement, use a fiber-filled cement product or, after laying and roughing in the new cement, press a nylon or steel mesh into the mix to prevent cracks that will open and seperate. Cracks are not critical, but cracks that open and seperate are. That's what you want to prevent.
To minimize cracks, maintain a 2" minimum depth (even at the drain throat) and deeper the further you get from the drain to establish slope for the water to be gravity driven to the drain.
Slope can be established by the new cement alone, or by a wood underlayment, then surfaced over with a consistant 2" depth of cement (use a nail poking as you go to ensure depth is consistant). If no wooden slope, the cement must be deeper as you get further from the drain.
For a wood slope, let's use an example of a 4' x 4' shower. Lay a 2 x 2 around the entire perimeter. Cut 3/4" CDX the size of the shower floor, then cut it from corner to corner with your circular saw (the intersection crosses over the drain location. If the drain is not centered, adjust the cross cuts to intersect over the drain.). Cut the sharp tips off an inch or so from around the drain opening, shim appropriately so the plywood doesn't sag (liquid nail the shims in place), fasten ply to suit (liquid nails to cement, screws to wood, etc., smooth the joints then lay your rubber mat over the top of it with enough excess to climb up under the wall sheathing and now you're ready for the cement. CLASACT was right on when he said to make sure its clumpy. Otherwise it will begin to sag and "fall" toward the drain. Make sure to keep it 2" thick & you'll be good to go!
Good Luck,
Bob