All portland cement mortars (screed beds, thin-set mortars, grouts, etc.) will not reach their full strength and serviceability if they dry out before curing. Generally, the only way to retain water in an installation is by preventing its loss through the use of polyethylene film or Kraft paper. Also, water can be added to the mortar after it has initially set. And it may need to be added for several days in a row. Drier, hotter conditions only reinforce this need. Wherever weak, crumbling mortars have appeared the usual problem is lack of a proper cure. Considering the cost of labor and tile in today's market, damp curing is a low cost insurance policy indeed. The graph below depicts the strength advantage of damp cured mortars.