Mortar joints can be seriously eroded over a period of years due to the changing seasons and wetting and freezing cycles. This is especially true in the older historical buildings where the brick and mortar joints are softer. The mortar joints can be renewed and replaced by repointing, which is the process of removing the damaged mortar and replacing it with fresh mortar. It is not a job for an amateur, and does require a high degree of skill and knowledge about matching the original mortar.
Proper time to repoint
Repointing should be started when the existing mortar joints are eroded or exposed to a depth of 1/2″ or more, or when the mortar has shrunk away from the majority of edges of the brick. Many of the older types of lime-based mortar, while soft, are still structurally sound, as this was the nature of lime-based mortars. It is not generally necessary to completely repoint all of the mortar joints in an old building, but rather to repoint only the bad places, taking care to match the original color,composition,and psi strength of the original mortar. The goal is to restore the brick work to its original appearance. The texture of the aggregate is also important in achieving that original look.
My recommendation for mixing a typical historical repointing mortar is to use 1 part of portland cement to 2 parts of mason`s hydrated lime to 8 or 9 parts of sand or aggregate. This will result in a mortar with a strength of approximately 350 psi, which is considered soft. The addition of the extra lime to the mix causes the mortar to adhere to the edges of the brick and also provides for the maximum expansion and contraction of the joint without cracking. Any hairline cracks will usually reseal themselves when rainwater comes in contact with the joints. This process in known as autogenous healing. Historical Buildings in Calgary
The first step in repointing is to remove any of the old deteriorated mortar joints using a special plugging, electric grinder, or joint chisel, being careful not to chip the edges any more then possible. It is recommended to cut the joints out to a depth of 3/4″ to 1″ for the best results. There are mechanical grinders made that will do this on a large scale, but they often damage the surrounding edges of the bricks.
After the joints have been cut out to the required depth, they should be brushed clean of all particles or old mortar and then wetted with a brush and water before repointing. The repointing mortar should be mixed at a slightly drier consistency than that used when laying brick in a wall, and should stick to the trowel and slicker pointing tool without falling off.