Masonry cement became popular in the 1930s, at about the same time when brick manufacturers began producing harder bricks. Masonry cement mix contains sand, ground limestone and up to 65 percent of portland cement, which is hard and waterproof. Modern brick workers prefer using portland cement because it is cheaper and widely available. Masonry cement also sets quickly. This material works well if your brick wall is made up of new, hard bricks. Using masonry cement for old bricks, which are porous and soft, may lead to problems.
Before masonry cement came along, masons used lime-based mortars, which contain only sand and limestone putty. This mixture has stood the test of time, as sand and lime hold the bricks of the Egyptian pyramids together. Unlike the hard masonry cement, lime-based cement is flexible. It allows soft bricks to expand and contract with environmental changes. If your brick wall was erected before World War II, you may have to use lime-based mortar to get a strong bond between the brick and the new mortar.
Before filling the cracks with mortar, you have to first clean the cracked joints. Use a grout saw for joints 1/4-inch wide or smaller, or use a chisel and a hammer for wider joints. Remove the loose debris with a stiff brush and wet the bricks until they drip. After the bricks dry, use a trowel to force the mortar into the spaces between the bricks. Once the mortar dries, brush off any excess mortar.
Some types of cracks signal that you have a serious structural problem and will need professional help. If the cracks form a stair-step pattern along the joints in the brick wall, then you need to call a structural engineer. Another sign of serious trouble is a long horizontal crack along the mortar joints of the wall. These signs may mean that the wall is about to fail. Simply filling the cracks will not solve the problem because they will reappear.