My brick fireplace hearth is supported by large stones sticking out perpendicularly to my foundation wall in the basement. It forms a kind of shelf on top of which the hearth was laid. The stones themselves were attached together and to the foundation wall using mortar. This mortar was very crumbly and falling apart. I removed all the old mortar therefore the hearth is not supported by the stones anymore. I'm looking at replacing the old mortar and providing structural support to the hearth again. I was advised by a mason to inject new mortar above and between the stones using a mortar bag from below. He advised me to use type S mortar for this.
I already have a 60lb bag of Quikrete FastSet Repair Mortar. Is it wise to use this product in place of regular mortar, or should I really use mortar? The FastSet seems to be a structural solution so I don't see why it couldn't be used in this situation, but it's only advertised for concrete repairs, so I don't know.