When cement mortar dries, it hardens, resulting in a stiff aggregate structure. However, because mortar is easier and less expensive to fix than building blocks, it operates as a weaker component than the building blocks and serves as a sacrificial element in the construction. This means that although mortar will dry and harden, it does not have the same strength as when it was wet. Mortar sets up within an hour to a few hours after it's been poured. The setting time varies depending on the temperature of the mixture. Cement paste that is cold from storage will set faster than warm paste. Mortar that is kept mixed for too long before it's used will be more fluid and may not set at all. If this happens, add more cement powder to thin out the mix. Mortar is flexible when it's fresh and can be shaped without breaking. As it cures, the mortar becomes harder and more brittle. This makes it useful only for open joints where it will not be subjected to pressure. Setting mortars can be removed from the surface they're applied to by using a soft brush or spackling knife. The dried material can then be resealed with more mortar or another coat of paint if necessary.