In most walls, some mortar projects into the cells of concrete masonry units before grouting takes place. If large enough, these projections cause the grout to hang up. How far can the mortar project into the cells before it is a problem?
The Masonry Standards Joint Committee Specification for Masonry Structures in paragraph 3.3B.1.c limits mortar protrusions to less than 1/2 inch. A protrusion of 1/2 inch or greater must be removed before grouting. Any loose aggregate, mortar droppings, debris, or deleterious material must be removed from cells or cavities before they are grouted. In determining whether to use fine or coarse grout (as defined by ASTM C 476 "Standard Specification for Grout for Masonry"), the minimum grout dimension is measured as the clear dimension between any masonry protrusions. These requirements are included to ensure that grout will flow smoothly around the reinforcing bars.