I am a specification writer in an architectural firm. Our specifications typically permit retempering of mortars. I have heard, however, that retempering can change the color of the mortar and reduce its compressive strength. Should I be permitting retempering?
In most cases, retempering should be not only permitted, but expected. Unless water lost to evaporation is replaced, the mortar will become stiff and will not bond properly to the masonry units. Retempering will slightly reduce the compressive strength of mortar. The benefits gained in bond strength, however, far outweigh any reduction. Retempering of colored mortars, however, should not be permitted. Retempering can change the appearance of colored mortars significantly and therefore lead to areas of masonry that do not match. When using pigmented mortars, mix smaller batches that can be used up quickly.