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I have a set of stone steps out front, built into a rubble-stone retaining wall on both sides. The steps are mortared into place with concrete. Over time a crack (~1/4" wide) has developed between the stones and the mortar. Obviously I don't want ice getting in there and making it worse.
What should I use to fill this? More concrete? Masonry cement? Silicone caulk?
First of all, I hope the mortar wasn't concrete. Concrete and mortar, while related, are two different things. Mortar is meant to be softer and give (rather than the brick or stone giving and thereby cracking).
The proper solution is to re-tuck-point the joint. That means chip/grind out the old mortar and put new mortar back in.
Alas, mortared steps are a maintenance nightmare and you'll likely be doing this forever at least bi-annually. I eventually caved and tore out our aging brick stairs (meaning half the brick was spalling and flaking, and weeds were in every joint) and replaced them with colored concrete. Not quite as rustic, but a whole lot safer (the old bricks were slick) and maintenance free.
The answer turns out to be polymeric mortar. It has enough stretch and give to make up the difference in expansion between the stone and the concrete. Since this isn't taking any load, just preventing freeze/thaw cycles from breaking the stone apart, the lower strength of the mortar is of less concern.
We'll see how it holds up after a few winters, of course...
