Cement versus Plaster - Weathering vs. Speed

I have been playing with a wide range of readily available cements, mortars and grouts the past few weeks trying to imitate the detail and strength that I have been getting with gypsum based plaster products in my castings. Cement is extremely difficult compared to the plaster products due to the amount of time that is required for the cement products to set up. Secondly all of the cement products seem to shrink as they set, which means they fall out of their frames. I have tried to make this an advantage by casting a bunch of brick panels in order to adhere them to the back of some epoxy or plaster front panels so that I can have a two-sided finished looking piece that can be used in a wall. The idea is good, but waiting for the cement pieces to finish setting and dry enough to paint or seal has been troublesome. I guess that if I get 1 for every 2 pieces I make due to handling and breakage. The cement does not seem to be inherently as strong as the plaster and can't take the abuse that I am used to administering to the plaster pieces. I am hopeful that as I wait longer, now a minimum of one week or more, that the cement panels will gain strength and then I can consider coatings to seal and finish the panels before inserting them like tiles into some frames.