Cement strength by moistening
I'm not sure what you mean by "cementing" a wall. However, cement (typically Portland cement) is the paste that binds together the aggregate (gravel and sand) in concrete. The better the bond between the cement and the aggregate, the higher the resultant compressive strength of the concrete.

When cement is mixed with water in concrete it binds to the aggregate through a chemical reaction known as "hydration." 

http://en.wikipedia.org/wiki/Hydration_reaction

A hydration reaction consumes water as a reactant, which is why you must keep concrete wet for several days after it is placed. If the concrete dries out then it has no water to continue the hydration reaction. The end result then is that you're left with cement that is not fully adhered to the aggregate (less compressive strength).

Careful though - you only need to keep the concrete wet after it has set up and is curing. Adding too much water to the mix (giving it a high water to cement ratio, w/c) actually drastically reduces the compressive strength of the concrete.

Curing compounds are especially helpful for properly curing concrete. (speaking from a heavy civil general contractor's point of view) We use this product all all the time:

http://www.wrmeadows.com/1100-clear-resin-water-based-concrete-curing-compound/