Historic brick is significantly softer than modern brick. Use of any material other than historically accurate lime mortar can cause damage to the brick because, for example, portland cement based mortar is much harder than lime mortar. As the brick expands and shrinks with changing seasons and weather, portland cement mortar joints do not causing damage to the softer brick. In this case the brick must be replaced with similar historic brick and injected using historically appropriate materials then repointed with the appropriate lime mortar so the brick and mortar can expand and contract harmoniously. The final step in joint repair would be to match the color and strike of the original work.