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Nivk 04-19-2011 03:39 PM

Old Stone Foundation Repair
I'm looking for some advice on how to best repair the old stone foundation of my house. The place was built in 1870 and the mortar between the stones has turned to dust in places. A previous owner tried to "repair' this problem using insulating spray foam for some reason :furious:, though I kinda doubt that's helping to support the house...

Anyway, i've been vacuuming out the dust and repointing with new mortar. So far I've only completed a portion of one wall. I was recently told by an acquaintance that I should be adding lime to the mortar, but so far I haven't done so. At this point I'm using quickcrete mortar mix No.1102 type N (which says on the bag it is appropriate for tuck pointing stone). I chose this because it should not be as unforgiving as a type S mortar which could damage the natural stone and because of what I read here,, which said it should be ok for interior walls even though it is cement based:

"Inside, it's a different story: Pre-packaged mortar mix can be used for the repointing and also the recoating. Simply follow the mixing instructions on the bag of mortar,troweling the mix between the stones and finishing with a complete coating."

Is is necessary to add lime to the mix? Can I add it to the mix I am using or do I need to mix something from scratch? If so how much lime should I add and where do I get it? Also what is the reason for adding lime anyway?

The image below shows the problem (the gaps in the mortar go much deeper than can been seen in this photo), and my attempt at repairing it (top left). Thanks in advance for any advice.

Daniel Holzman 04-19-2011 04:59 PM

The original mixture for mortar was worked out by Vituvius of Rome, and consisted of one part lime, one part sand, and one part water. This mixture worked very well for about two thousand years, and still works today. However, around 1900 masons began using portland cement based mortar, which is stronger, but is more brittle than lime based mortar. Many masons who specialize in old houses will tell you that modern portland cement based mortar is too strong and inflexible, and damages brick or stone.

If you are planning to use portland cement based mortar, there is no reason to add lime. If you are using lime based mortar, there is no reason to add portland cement. In either case, you need to follow the mixture directions on the bag. And the mortar adds little strength to the stone foundation, it is really there just to keep the stones together. This is quite different from concrete, where a substantial part of the strength is derived from the cement.

Tscarborough 04-19-2011 07:46 PM

Actually, if you do a search here for "lime mortar" and "foundation", you will find complete instructions for what you need to do. What you are currently doing is just about as bad as the spray foam, if not worse.

Nivk 04-19-2011 08:30 PM

Well fortunately I haven't gotten too far, and there is already plenty of cement patches from even earlier repairs. I had been having a hard time finding a straight recipe for lime mortar mix online, but I just came across this from the page on historic preservation:

For Limestone (ASTM C270 Type "N"):

1 part portland cement
1 parts lime
4-6 parts aggregate
Enough water to form a workable consistency

I will switch to this using sand as an aggregate, once I find a source for hydrated lime around here.

CplDevilDog 04-20-2011 07:10 AM

Just returned from Ireland and got to look at a few stone walls done in lime mortar. Some as old 800 AD. Amazing.

So the Portland Cement is hard and brittle, while the stone is soft and flexible? Leading to cracking of the joint quickly, correct? Assuming Lime Mortar is hard to come by, would Type S or Type K Portland Mortar be more appropriate to this application?

I believe they have lower Portland concentrations and therefore, a higher tensile strength.

Tscarborough 04-20-2011 08:31 AM

I would limit the portland to less than 20% by volume of cementious materials (lime and portland), and 10% would be even better. You only need the portland to allow for an initial set, as the lime mortar requires some time. The mix above is NOT a lime mortar, it is Type N portland cement mortar, and is not suitable for this application.

jomama45 04-20-2011 09:23 AM

As usual, Tscar is right on the money on this scenario. Lime mortar allows moisture/vapor to pass through the wall as it enters from the exterior. Patching the inside with a less permeable mortar will only trap that moisture in the wall, further eroding the remaining lime mortar in the wall.

Nivk 04-20-2011 11:35 AM

Ok, so this is starting to make sense to me. At first I thought it wasn't a big deal for stones since they are harder than brick and wont be damaged by the mortar, but it sounds like its less an issue of face damage and more of an issue of moisture permeability.

From what I had read in the gov't restoration guides the softer (high lime) mortars are critical for brickwork that is exposed to the elements, but for a stone wall that is on the interior side of a basement it seems like the harder mortars would be less of a problem. But this does not address the moisture issue.

Anyhow, what you are describing sounds more like a type K mix, or 1 part cement to 3 parts lime to 10 parts sand. But even softer since the type K is still 25% cement. So would it be ok to adjust this recipe to: 1 part cement to 6 parts lime to 10 parts sand, so it is less than 20% cement, or do I need to adjust the aggregate content as well?

Also, how do you mix these mortars? Can I just combine volumes of portland cement, hydrated lime, and sand with water, or do I really have to soak the lime and make a putty first?

Tscarborough 04-21-2011 10:40 AM

It is acceptable to mix 1 part lime and 3-5 parts sand (by volume, always by volume).

For a blended mortar, you can use Type S lime straight from the bag, for lime mortar, the lime needs to be hydrated. Gently cover with water and keep covered with water for at least 2 days, then pour off (and save) excess water and the lime is ready to be used. As long as you keep it covered with water it will remain in a usable condition. It is extremely caustic when wet, so wear protection.

Vincer 04-21-2011 12:19 PM

Home Depot and Lowes here (in the San Francisco Bay Area) carries 50lb bags of lime in the cement mix section.

Nivk 04-21-2011 12:34 PM

Alright... I'll look for some lime and give it a shot this weekend. Unfortunately they don't even know what it is at the local Lowe's/HD. I'll call around to some masonry supply places and see what I can find. It must be sold somewhere in central PA. There's a factory that produces the stuff right down the road.

Tscarborough 04-21-2011 02:39 PM

Make sure it is Type S lime, not ag lime or the lime known technically as $hit-house lime.

Vincer 04-21-2011 05:42 PM


Originally Posted by Nivk (Post 633887)
Alright... I'll look for some lime and give it a shot this weekend. Unfortunately they don't even know what it is at the local Lowe's/HD. I'll call around to some masonry supply places and see what I can find. It must be sold somewhere in central PA. There's a factory that produces the stuff right down the road.

Home depot, type s lime

Nivk 04-25-2011 12:41 PM

I found the lime at the local true value, and mixed up a test batch of mortar as suggested above with portland cement composing 20% by volume of cementious materials. It was like a type K mortar with less cement. (0.75 cement : 3 lime : 10 sand). It looks like it did the trick, but I have 2 concerns about it:

1) it took quite a bit more water to get to a workable consistency - as compared to the quickcrete type N mortar

2) it is taking quite a bit longer to dry and is noticeably less hard than the quickcrete stuff.

I assume softness is to be expected because it is a softer mortar, and since more water went into the mix it makes sense that it is taking longer to dry, but I was just hoping to confirm that this is normal and I didn't screw something up. Thanks.

fieldstone 06-17-2011 12:17 PM

Nivk, how did you make out with your lime mortar? I've got a lot of pointing on my to do list...

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