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What mortar mix for smooth granite cobble retaining wall? Greenhouse base?

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  • #1

    What mortar mix for smooth granite cobble retaining wall? Greenhouse base?

    I have a few masonry repairs to do around the garden and not sure what mortar mixes to use. Anyone who could advise please?
    There is a granite cobblestone retaining wall / raised bed 1/2m high around the perimeter of my greenhouse which has really poor mortar joints. Cobbles come away when subject to any lateral loading like me stepping over to get to the greenhouse.
    What would be the strongest and most durable mortar mix for this application Soil is slightly acid but concerned at leaching of lime.

    Also the (Halls Popular 86) greenhouse was built directly onto breezeblock foundations rather than on the optional base. It wa levelled on timber shims which have of course rotted and swollen. The mortar bedding has gone in other places and the gaps provide a passage for gastropods to get into the greenhouse. I'll give this greenhouse a few more years and replace with a Rhino on proper foundations.
    What would be the best mortar mix for bedding and levelling the greenhouse onto. It's going to be subject to moisture all year round, snow and frosts, direct sunlight, a bit of wind loading and perhaps some standing water. I guess a mortar with some flexibility but without too much lime, which might leach into the beds.

    I've taken evening classes in bricklaying so can do the actual trowelwork but unfortunately there wasn't a session on mortar materials.
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  • #2
    Hi, lime is a difficult building material to use if it hasnt had much attention when preparing, standard cement mix is 5:1 brickwork using a soft sand (usually called builders red) use any type of ballast to make concrete up to 4:1 mix, lime can be mixed down to 5:2 but always use a aggregate sand and it needs to be mixed properly, lime given the right conditions should set within a few days and cure after a few weeks (for lateral strength) I wouldn't worry about leeching if it's been mixed correctly. There plenty of info online for the best lime mixes - I'd just use a 3:1 sharp/natural hydraulic lime ...... But for the cost and the fact that's it's being replaced in a few years a cheaper option is 4:1 cement with a smidge of plasticiser and not mixed for ever (keep air bubbles down) should last a season or too before falling apart. Just don't mix lime & cement in a mortar as the cement inhibits the carbonation process and will cause premature failure.
    Bit long winded but hope it helps !!!

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    • #3
      Originally posted by babyjohnny View Post
      Hi, lime is a difficult building material to use if it hasnt had much attention when preparing, standard cement mix is 5:1 brickwork using a soft sand (usually called builders red) use any type of ballast to make concrete up to 4:1 mix, lime can be mixed down to 5:2 but always use a aggregate sand and it needs to be mixed properly, lime given the right conditions should set within a few days and cure after a few weeks (for lateral strength) I wouldn't worry about leeching if it's been mixed correctly. There plenty of info online for the best lime mixes - I'd just use a 3:1 sharp/natural hydraulic lime ...... But for the cost and the fact that's it's being replaced in a few years a cheaper option is 4:1 cement with a smidge of plasticiser and not mixed for ever (keep air bubbles down) should last a season or too before falling apart. Just don't mix lime & cement in a mortar as the cement inhibits the carbonation process and will cause premature failure.
      Bit long winded but hope it helps !!!
      Thanks very much babyjohnny
      I'm certain I replied to this, maybe it went in the black hole of autologoff or my PC went into hibernation?

      I had a few trips over to B&Q in the bus the other day to get bags of plaster, sands, cement, lime and admixtures for this and other trowel jobs in the house

      A garden wall my late dad built blew over last year in the gales. There were hardly two bricks the same, and there's extensive frost damage (spalling etc) everything is spalled Maybe in a few years time when I can afford it, I'll have the wall replaced by someone that knows what they are doing (I don't) with something a bit more uniform, but I'm not sure what yet (fence, wall, stone cage?)

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      • #4
        With the faces of bricks blowing indicates a soft brick which absorbs and retains water, if you build with a cement based mortar then the cement prevents the bricks from breathing from the joints and when the frosts come the bricks don't have a chance, lime however allows water to permeate the joints allowing the excess moisture to evaporate, cement only replaced lime in buildings when the developed better methods of firing bricks ie, they got harder and stronger, so in future if you get the wall rebuilt using older bricks then use lime, if you drive about then you can see people go have had reclaimed brick wall built using cement and within 5 years half of the bricks would have blown :-/

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        • #5
          Originally posted by babyjohnny View Post
          With the faces of bricks blowing indicates a soft brick which absorbs and retains water, if you build with a cement based mortar then the cement prevents the bricks from breathing from the joints and when the frosts come the bricks don't have a chance, lime however allows water to permeate the joints allowing the excess moisture to evaporate, cement only replaced lime in buildings when the developed better methods of firing bricks ie, they got harder and stronger, so in future if you get the wall rebuilt using older bricks then use lime, if you drive about then you can see people go have had reclaimed brick wall built using cement and within 5 years half of the bricks would have blown :-/
          Great, thanks, aye the bricks were all reclaimed from flytip by my late dad, and carried home one at a time in his bag. There are engineering, patio, clay, reconstituted (fyfestone), concrete.. allsorts..
          At my evening class we worked only with lime-based mortars, (ready mixed) because the college said they were able to grind it up again and reuse it. It nice and jammy and was workable for weeks!

          While on the subject I an replastering in the kitchen behind the sink and around the now Polyyisoed window frames so the cills and house are more temp stable for my pot plants and me). Gypsum has gone to mush due to splashes and poor tlie seal. Not sure the substrate or whether to plaster the moisture-prone areas with plastering sand , portland and waterproofer, some sort of lime-based plaster or something else?

          How to I determine if mortar joints are is portland or lime-based? From the age of the property ? 1920s From the texture? Add water and pulverise?
          Attached Files

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          • #6
            From the picture it looks like lime mortar just the way the bricks sit, if you pick some of the mortar out and I suspect you could find some kind of aggregate in there, probably a fine grit that's very pale in colour, if it's a 1920's house then it very likely to be lime - all the way up to the 1940's lime was more commonly used. Be careful when attempting to remove plaster as you might find the key has gone and you then have to strip an entire wall. To re-plaster I wouldnt worry too much about what you use (I'm not a plasterer lol) as with modern central heating keeps the walls fairly damp free, just ensure that you damp wipe any dust off the wall first and give it a good coat of pva to help new plaster to key. I would use an adhesive plaster for scratch then a skim coat then seal and paint over with a silk paint, You might find a plasterer on here with better advice though ...

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