The right mortar for repairing masonry cracks and repointing is different for every house. This is due to inconsistencies in the mixing of mortar, different types of mortar being used, and/or different aging and staining of the mortar. Even if you match the mortar composition and compressive strength of the mortar, the repair can look bad and stand out if the mortar color and gradation of the sand are not matched.
Most of the professional masonry repair specialists assume that they can match mortar by simply matching the color of mortar with pigments or stains, and some try to match the sand. While this may work most of the time for them, it only works because the customer does not know that it was wrong and neither party are understanding the problems this method can cause. This issue happens most often in right-to-work states that require no license for masonry repair and city codes do not enforce a standard of repair that relates to this issue.
With that being said, the correct and only way to identify your mortar and find a suitable repair mortar is through mortar analysis.
There are different types of mortar analysis available. The most commonly used is acid digestion which is limited for accuracy. In fact, all official individual testing methods have wide ranges of error. It is only through a combination on different mortar testing procedures that accurate results can be made. By collecting more information about the mortar, it’s compressive strength, acid digestion, chemical reactions, weight loss, and comparing these results along with a calculation of the specific gravity of the binder materials and sand, the mortar composition can be more accurately identified.
Only after identifying the mortar composition can the color begin to be matched correctly. This is because many base tones of the mortar color are a direct result of the mortar composition. For instance a white sand with grey and black particles in a white mortar will give a bright white mortar with a slight grey tone in the light, but a blonde sand with red particles will appear buff to pink in a white mortar.
After the base formula is determined, the color can be matched by making minor adjustments of different colors of the base materials as long as the ratio and formula remain constant. Then the final adjustments can be made with pigments or stains as needed.
With a proper repair mortar, masonry repairs will bond better and last longer without causing damage to the wall.