Dave Kett

Good luck!

Ok,  I looked at your pic closely and have a couple more comments.  First, get a "flat' not curved jointer for your tool.  You will want to have the joints be flatish or planar rather than curved - to match the existing joints.  Second, (this may be a bit tricky but the good news is that mortar is water soluble so you if you don't like the finished product, you can remove and redo it before it sets up hard.  So, no stress) if you want the repair to match the old work well, you will want to very gently brush it with a wire brush a day later when it is hard but the surface can still be abraded).  The idea is to get the surface to be more sandy looking than super smooth. This is to match the weathered look of the old mortar.   You can also get the same effect by spraying water on it with a hand operated sprayer (like a windex sprayer (with water)) spraying hard.  This will dissolve some of the cement/lime in the mortar, leaving more sand apparent on the surface.  If you look at the pic you can see see that this is what has happened over time.  One other thing.  Once your repair is done and hard, you can come back with a little brush and buttermilk and brush it on the new joint if you want to encourage new moss growth quickly.  This also works on ferrocement walls, for those of you who know about ferrocement.

A previous poster, B. wrote:

Hey R,

"mortar is a mix of aggregate (sand) and a binder (cement or lime) and water.
Plaster is a specific type of mortar.
So they're all very similar.

Modern adhesive cement has additives (glue?) to make it stick much more than regular mortar. Adhesive cement might come in a bucket ready to use, in a bag to just add water or as an additive (powder or liquid) that you add when making your cement mortar.

Your plan is to basically glue the stones back in place where they were? In that case you need an adhesive cement mortar, with some flexibility and very small aggregate (fine sand or no sand). "Adhesive cement for repairs" might help you as a word combination when talking to the store clerks.

But let me tell you that I don't know how well that's going to work. You would probably have a better chance of success by chiseling away some of the mortar in the places the stones fell out to make some room for a good coating of mortar (1/3" at least)."

This is all correct.  I want to emphasize that you really will want to chisel away or grind away (grinders are cheap but dangerous if you are not careful; you would need a masonry disk, not the disk it comes with) mortar still attached to the stone.  Wear safety goggles.  As for adhesive, you can use portland cement, premix mortar, or tile "mastic."  Assuming the original mortar was just regular mortar, that might make the most sense.  You will use it to 'glue' the rocks back into place, and then come back once it's hard and fixed to fill in the joint gaps.  You may need to use some nails or pieces of wood, or pens or pencils as spacers to hold the rocks so as to maintain bottom joint space.  You can gently pull these out about 12 hours later.  

Now, this is in an ideal situation where the rocks refit well and you mix the mortar to an optimal enough consistency to hold it in place while setting.  If this does not work and the rocks fall out, you can add lime to the mortar mix (new batch ; also you should only be mixing about a quart at a time for this).  This makes it "stickier."  

OR, you can just cheat and use some "fast grabbing" construction adhesive instead.  Liquid nails should work but there are weaker brands that actually grab better.  I recommend that approach.  All you are trying to do at this point is get the rocks replaced and affixed.  (You will grout later and that grout will also hold them into place.)  You would need to clean off the rock faces and make sure they are dry.  Put the fixative only in the wall opening/spaces and use a decent amount, but not so much that it is going to squeeze out of the joints.  IF you make a mess, just let it harden and chip/cut it out a day or so later.  If you try cleaning it while wet, you make smear it all over the other rocks, and then will need acetone or whatever the clean up solvent is...  Big pain in the rear.  Anyway once it hardens up and is holding the rocks into place, come back and grout the joints with mortar.  BTW you will need a jointing tool (Look up "OX Pro Brick Jointer" for EXAMPLE.   Select one that is about the same width as your existing joints.  YOu don't need this brand.   Just an example.  Might want to go to you tube to look at a video on brick and stone jointing)

Basically, rock and brick work are one of those things that are fundamentally simple but can be elevated to an art form.  Your project is actually pretty basic, so address it with confidence.  Write back if you need clarification.   FWIW, I have been doing this and concrete off and on for 40 years, and consider myself a competent amateur.  In my case this means I can and have done footers, walls, steps, slabs, etc. for myself and for pay, and successfully.  BUT make no mistake, the people who do this day in and day out for *years* are the true masters and artists.  Anytime you can watch one at work for a little while (ask for permission) do so.  You will always learn something.  It's like drywall... anyone can do it, but doing it fast and well is another art form.

Came across this recently and thought I'd provide it to the community:

https://www.oftwominds.com/SR.html

p.s. if you ware going to mix copper naph solution with an oil, be cure to use the non aqueous type.  The carrier mineral oil will allow it to mix with the other oil.  If you use the aqueous type it won't mix and will separate in the sprayer reservoir.  

My experience on using vegetable oil and linseed oil as a preservative:

I have used boiled linseed oil on beehives since 1990, mostly on the east coast (Philadelphia and environs).  It worked well and the bees didn't mind.  I used it only on the exterior of the hivebodies and supers.  The bees coat the interior wit propolis, which is/acts as varnish.  Seeing it's effectiveness, I also used it on other outdoor wood products and it seemed to work.  Note that 'unboiled' linseed oil is untreated.  I think they put boron in the boiled stuff.  I started using unboiled once  I discovered there was a difference.  It takes a lot longer to dry and is more viscous.  Bottom line is that I used both kinds there with success.

Here in Oregon, I have used a 50/50 mixture of unboiled linseed oil with a mineral spirit copper naphthenate (as opposed to aqueous version) in a power sprayer to coat my 50 year old cedar shakes (on the walls - I have metal roofs).  The mixture is absorbed in to the shakes to a degree, and slowly seals up.  Three years since this application, the shakes still feel smoothish and on warm days very subtly tacky, so I suppose it is still curing or sealing.  On warm days (we don't get a lot of days over 80 where I live, on the coast) you can smell the linseed oil a bit.  The treated side sheds water very nicely.  I haven't noticed any rot or deterioration and it seems to be keeping any further deterioration of the shakes.

As for used vegetable oil,  I was part of a biodiesel coop back in Philly.  My job was to secure the relationships with restaurants and collect it.  We made excellent biodiesel for our trucks.  One tip, if you are serious about doing this:  Build a super sucker...  I.e. get a big propane tank and attach a vacuum pump to it and run it while on your way to the collection site.  When you get there you shut the vacuum line valve and immerse your 1 inch or two inch hose from the tank into the barrel or container of oil.  Then open the valve from the tank that is attached to that hose.  This will suck the oil up into the tank.  The advantages are that this allows use of a large hose, reducing the chance of dead mice or avocado pits or what not clogging things up since there is no pump on this line.  Also, and very important.  IF you get a breach in that line you won;t have a ton of oil spraying everywhere.  I am sure there is a thread on all this somewhere.  We used a 200 gallon tank welded to a steel pallet and a forklift to fit it into my 1 ton truck for collection.  You could use a smaller propane tank if your collection needs and available resources were more modest.  A bit of an investment, but it beat having hundreds of 5 gallon plastic jugs to dispose of.  

Here in Oregon, I initially was aggressively collecting from a few of the restaurants near me, but stopped at 200 gallons.  I don't have much of a use for it except as a fire starting aid.  Am planning on building a burner for it once I get my greenhouse up and running though.  Again, there are plenty of plans and resources on the web about this.  As for a preservtive, I imagine it would work reasonably well on tool handles and shovel blades...  Will have to set up some tests on diferent wood and report back in a couple years.

Theas is right about the dye being difficult to remove.  I never worried about it because I would always have to do wall or surface repair after the leaks were fixed anyway.   It all depends on hpw permeable the surface is and how quickly ou can get to cleaning it up.