Why must I use fluid grout to grout a collar joint? If I do a good job of packing mortar into the collar joint, won't this work?
In theory, it would. In practice, however, it is too difficult to pack mortar into the collar joint without leaving voids. Because grout flows readily, voids are much less likely to occur in the collar joint. Voids in the collar joint can permit water to bridge from one wythe to another and cause interior leakage.