What kind of mortar should I use to point a fieldstone foundation?
The old mortar is crumbling, and sandy mortar is falling out onto the basement floor. I plan to remove the crumbling mortar (see related question on how to remove), then point the foundation with new mortar. But what kind of mortar should I use?
I have had a hard time finding definite answers about fieldstone foundations on the Internet, but I'll include some of what I've found here. First, David Valley has this to say about fieldstone foundations:
Most fieldstone foundations have, or had at some time, a thin mortar coating on the face of the stone. The purpose of this coating was to assist in holding the stones in place. This thin mortar coating will inevitably flake off from any moisture migration, revealing the surface of the stones. As this coating continues to erode, the mortar between the stones will begin to crumble and the soft, sandy mortar begins to slowly fall out onto the basement floor. It looks like a small pile of sand at the base of the foundation. When this occurs, tuck pointing is needed to refill the voids where the old mortar has fallen out. It's very important that you scrape away or chip away the crumbling mortar (in between the stones) in order to establish a small cavity or key way which can hold the new mortar that is being applied. Never apply new mortar over any old crumbling mortar, as this is a temporary fix and it will only last a couple years. All crumbling mortar must be removed in order for the newly applied surface to bond properly. While upgrading your field-stone foundation, only work on a section at a time. Do not remove the old mortar throughout the entire basement all at once. Complete all removal, tuck pointing and parging one section at a time.
To avoid annual tuck pointing, you should finish the foundation with a complete top coating of mortar. Be sure it is lime based. This top coat does not have to look like a stone artisan's creation: It merely has to serve the purpose of keeping the newly installed mortar in place. It's sort of like applying a frosting to a cake.
This page seems to suggest that I should be using not straight type-S hydrated lime, but type-S mortar, which consists of one part Portland cement, ¼ to ½ part lime, and an amount of sand equal to 2¼ to 3 times the total volume of cement and lime.
Do I need to analyze the existing mortar to make sure that the new mortar has the same lime content, as suggested by some?
Should I expect the new mortar to start crumbling and releasing dust soon, or should it stay solid and dust-free for many years?