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.

2 Answers 2


Quikrete FASTSET Repair Mortar is a great product. The biggest difference between it and a typical type S mortar is that it is "fastset" (duh) meaning it will be hard in about 20 min. Not cured, but hardened. And that it is intended for commercial use as it will eventually reach 6000 psi after it's full 28 day cure. This product does not need any bonding agent as it is already added. http://www.quikrete.com/ProductLines/FastSetRepairMortar.asp

Any regular type S mortar will harden in about a day or two depending on the enviorment. And will eventually reach 1800psi http://www.quikrete.com/ProductLines/MasonMix.asp

You may think "oooo stronger is better" but not so with mortar. You always want the mortar to be weaker then the surrounding material (stone, brick, cinder block) That is why it comes in so many strengths. From strongest to weakest type, M S N O K.

Sorry to be so long winded.

Basically, go with the type S, and use the FastSet Repair mortar for repairs around the house.

  • @concreteman Didn't know that they actually add bonding agent to the mix already... I guess what I have been doing all along has been unnecessary. Hasn't hurt anything though ;) Jun 14, 2012 at 11:16
  • Thanks concreteman for the comments, I think we're on the same page but I wanted confirmation. I will use type S with the bag in between the stones and then use the fastset to finish off the work (building up the stones to re-form the slab that was there before). Should I avoid using the fastset for that too since it will be in contact with the stone?
    – md1337
    Jun 14, 2012 at 17:20

Type S mortar has a high bonding property which is perfect for bonding to other concrete and masonry after the fact. It is also thin enough that you can get it deep in the cracks where the mortar used to be.

I can't say for sure that FastSet repair mortar will do the trick too, but if you do go this route, make sure that you get concrete bonding adhesive like below.


You replace a small amount of the water with this adhesive and also use a brush to apply liberally onto the surface that you wish to apply it. This will probably work well but in the end I would probably just go with the advice of the mason.

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