I tuckpointed the joints on some limestone coping stones about 2 months. ( I cut the old cement out with a grinder and then repointed.) I used a mix of about 1 part masonry cement to 2 1/4 parts fine sand. I am not usually meticulous about the cement to sand mix, but tend to go with about 2 to 2 1/2 parts cement to sand, erroring on the side of more cement to the usual ratio of 3 parts sand.

I had a helper who did most of the actual pointing. He was not very experienced pointing, but had done it a few times for me before. At the time, it seemed like he did them pretty good and made them pretty smooth and tight. It was a hot day when we did it. The mix was very dry (very little water) per usual pointing mix.

I went back yesterday and saw all the cement in the joints had shrunk, leaving a crack between the cement and the stone on one or both sides of the joint. Some of the cement had spalled off. Some was in the process of spalling, and I was able to remove some chunks with my hand.

Any ideas why these joints might have failed?

 

Failure to dampen the brick beforehand or the mix dried too fast.

 

I don't usually dampen the brick or in this case the limestone, because the pointing cement if it gets on the face and the face is wet, will stain it.

 

Because Limestone is such a porous material it will absorb the moisture of the Mortar at a rapid rated causing extreme shrinkage. Since you were adding just a small amount of Mortar this will just add to the amount of shrinkage. Maybe a additive like a bounding agent or a retarder may prevent shrinkage. You would definitely want to be sure the material is pressed into the joint and much as possible. What method did you use to finish the joints? With extremely fast setting material a second pass of the tool later on can help some. Even a pass with a sponge and clean water will help.

Another thing to look for would be a Mixture to Rich(to much Cement) this can have a adverse affect also.