The mortar of a recently completed masonry wall has a very blotchy appearance. There are many areas where the mortar is significantly darker than others. Why is this occurring?
There are several reasons why mortar discoloration may occur. After rains, mortar sometimes absorbs moisture in different amounts, depending on variations in its porosity; such discolorations typically will disappear once the walls have dried out. If discoloration remains after the walls have dried out, it may relate to the mortar's moisture content, the tooling method, the type of jointer used, the retempering of colored mortars or the wall cleaning method. Tooling mortar joints when they are wet in some areas and somewhat dry in others can vary the wall's appearance significantly. Typically, joints tooled when dry will be darker than those tooled when wet. Mortar joints should be tooled when thumbprint hard. Sometimes the type of jointer used affects the appearance of mortar joints. Steel jointers may give a darker appearance to the wall. Some people use plastic or plastic-coated jointers to avoid this problem. When colored mortars are used, retempering often impacts mortar color. For this reason, most references state that colored mortar should not be retempered. Finally, washing walls with solutions containing hydrochloric acid often changes the appearance of mortars. The acid does two things: It oxidizes mortar compounds at the surface, and it removes some of the cement paste from the surface of the joint, exposing the sand. Thus, acid cleaning can change the color and texture of the mortar joint surface. The effect of acid cleaning on appearance depends on the mortar used and the cleaning technique. The procedure may darken the appearance of light-colored mortars or lighten the appearance of dark-colored mortars.