Best mortar mix for strength pointing

eamon11

hello Folks,

I am extending, what was originally a 200 year old dry Stone wall 2 foot thick and 6 feet high. Wall had a clay mix which was scraped out and well pointed approx 20 years ago, as it was falling, and clay was dry and virtually non existent..
The pointing I have done is crumbling. I have used 1 to 3 mix, yet its still very sandy after a few days and crumbles easily. What was done 20 years ago is very hard and solid as a rock . It's slightly lightly in colour so I suppose lime was used. This pointing Is really what keeps the wall together as nothing else there.
What's the ideal mix for strength and durability.
Thanks a lot.

TiNcAn

I am no expert but you should typically use a ratio between 1:1.5 to 1:3. The size and quality of the sand is quite important so you want to be using ~4 mm. The mortar normally takes 30 days to reach close to its maximum strength so it will be weaker in the first few days. But if it is very crumbley then there might be something wrong here. It could be that the mortar is drying out too quickly and in this case you need to wet the background more before you begin and also dampen the mortar in the first few days if the weather is hot and protect from sunlight.

eamon11

thanks a lot Tincan, much appreciated.

TomOnBoard

Are you putting lime in the mix along with the cement? Also as previous post said, wet the wall well before pointing and keep it damp for as l8ng as possible after pointing. If you can hang a canvas/hessian sack over the wall for a few days and wet that down every so often it may help it to dry more slowly.

eamon11

was not putting lime in. What might be a good ratio with the lime?

TomOnBoard

eamon11 wrote: »

was not putting lime in. What might be a good ratio with the lime?

The lime will greatly improve adhesion and will give you a more sticky/putty-like consistency. It's a long time since I did outside stone walls, but I'd use a 1:1:6(c:l:s) mix for maximum workability and a 2:1:9 where more structural strength was needed. Back in the day, walls were regularly built with no cement at all, using a lime mortar 1:3 or 1:4. Good mixing of the lime/ sand mix with water is very important.

I've seen lads make very well mixed lime/ sand mortar the evening before and adding cement before using it. Something about maximising the lime's properties.

For your job, I'd make a few recipes and check how well your recipe works to deal with the crumbling issue, assuming you can spread the job out. I'd experiment, starting with 2:1:9 at the bottom of the wall, coming up a few feet, and possibly go to 1:1:6 closer to the top. Worth a try.

If its a retaining wall, put in some drainage to relieve water pressure behind the wall.