When old mortar begins to crack or crumble replacing the mortar itself -- instead of the entire wall -- is the typical remedy, and this process is known as stone pointing or repointing. You'll need to mix two batches of mortar to replace the single old mortar batch. The first batch serves as a filler mix, filling most of the joint between the mortar and adhering with existing undamaged mortar, which can differ greatly in content from modern mortar mixes. The second mix is a more modern combination, a finishing mortar. This mix provides the flexibility needed to move along with the bricks, ensuring that it won't pull away from the bricks after drying.
Pour the mortar materials for the initial replacement layer into a large bucket. Combine seven parts sharp sand with one part lime and one part Portland cement as your dry mix.
Add water to the mix. Use an electric drill containing a paddle bit attachment to work the water evenly through the dry mix. Add the water slowly to the bucket until you have a mortar mix that's about the same consistency of peanut butter.
Scoop the mortar from the bucket to a hawk, then use a pointing trowel to push the mortar from the hawk into the joints. Fill the joints between the bricks or stones for the first 5 cm (2 inches) in depth. Compact the mortar into the joints tightly, compressing it to fill the space as completely as possible.
Mix the finish layer of mortar, to fill the last 1.25 cm (1/2 inch) layer of mortar in the joint, using a board instead of a bucket. Pour six parts sharp sand, one part lime and three-quarter part Portland cement into the middle of a large board. Add water to the mix and fold with a shovel until you have a mortar that's the same peanut butter like consistency as the first.
Scoop the finish mortar from the board to the hawk. Push the mortar from the hawk into the joints with the pointing trowel, filling the rest of the opening between the stones to an even level.
Wear work gloves, safety goggles and a respirator to avoid contact with mortar materials while mixing and applying the mixture.