Mortar mix help

Discussion in 'Landscaping and Outdoors' started by johnboy82, Sep 14, 2021.

  1. johnboy82

    johnboy82 New Member

    Good morning, i am posting in the hope that someone can help me with an ongoing issue i have. I began laying my own patio (first time) and have been preparing a 4:1 ratio of cement to sharp sand. My issue is that the next day, the slabs have not taken properly to the mortar bed. i have been using the blue circle general purpose cement with my sand mix. I am wondering if i am putting enough cement into the mix, out of a 25kg bag, how much should be in the mix? i have used a shovel full of cement going by the ratio but i saw someone posted they used half a bag for each time they prepare a mix. Is this correct because i do feel that i am not using enough cement with the results i am getting. I have made sure the water content is correct so other than that i dont know where i have went wrong. Any help would be much appreciated.

    thanks
     
  2. Jord86

    Jord86 Screwfix Select

    Are you mixing in a cement mixer or wheelbarrow? 4 sand 1 cement is more than fine, dampen the backs of the slabs that you are about to lay first or prime them with SBR to control any suction and help them bond.
     
  3. johnboy82

    johnboy82 New Member

    Yes Jord i am mixing in a wheelbarrow, i feel my cement input is not enough as thats only thing i feel could be wrong. I have not used SBR, what is that exactly and where do i get it from? also, does it go in the mortar mix? i just worry now that im going to have to lift them, the foundations of the ground will now be disturbed and need to start from very beginning again.

    in terms of a 25kg bag of blue circle general purpose cement, how much would i put in? im using shovel full but doesnt seem enough, some people saying half bag per mortar mix.
     
  4. Jord86

    Jord86 Screwfix Select

    It's four to one, ie four shovels of sand to one shovel of cement, four buckets of sand to one bucket of cement, four spoons of...... you get the idea. It's more than enough cement, less would be ok. Mixing in a wheelbarrow, you have to make sure the mortar is mixed and turned over thoroughly. SBR is available from any builders merchants, it's a primer amongst many things and helps bond materials and surfaces together, you paint the backs of the slabs with it before laying.
     
  5. johnboy82

    johnboy82 New Member

    Yeah i thought as much in terms of the ratio, just thought maybe i wasnt using enough cement though, would half a bag of cement into 4 parts sand be too much, i also dont think i am making it thin enough. really stressing me out now.
     
  6. pppmacca43

    pppmacca43 Super Member

    4:1 is fine, as jord has explained you are using the right mix. Half a bag of cement is too much for four shovelfuls of sand
     
  7. johnboy82

    johnboy82 New Member

    i dont know what is going wrong then if the slabs are not taking to the mortar mix, should they actually stick to it or should they be able to be lifted up? ive seen mixed comments on this as i had impression the whole point of mortar mix was to fix them down to avoid movement when walked on
     
  8. pppmacca43

    pppmacca43 Super Member

    Is it a dry mix or wet mix, I’m not an expert but the more details the more people can help. I’ve laid my own slabs on both dry and wet mix and they took ok. As jord said wetting the underside of the slabs or priming them with a suitable primer might help.
     
  9. johnboy82

    johnboy82 New Member

    its a wet mix, i made the consistency so i could make a ball of it up and it retained shape without breaking or oozing water. ive researched that on many sites and all said similar. My only fear now is what to do if i need to lift them and start again, should it be ok to try tamp down the existing bed of mortar and lay back on top of it?
     
  10. pppmacca43

    pppmacca43 Super Member

    I suspect u will need to take up unfortunately but wait for someone with more knowledge than me to confirm. I would guess that priming the backs like jord said will help with suction and stop it drying out to quickly and help them bed. Once they are down obviously don’t walk on or disturb them either
     
  11. johnboy82

    johnboy82 New Member

    to be honest ive had to lift up a good few of them due to them rocking. i may have to painfully pay someone to fix the issue. bit gutted as i was hoping i couldve done it, pretty frustrating it hasnt worked out.
     
  12. DIYDave.

    DIYDave. Screwfix Select

    Lot of mixed advice on the ‘simple’ task of laying slabs and even if they should bond to mortar bed or not

    Some say doesn’t matter as pointing will hold them firm and mortar bed is for levelling them

    Personally, don’t agree with that but like you, I’m only diy so not laid hundreds of patios !!

    Can’t see how it’s a good idea that only the edges of slabs are bonded once pointed as this doesn’t give much surface area - compared to the underside of slab

    You don’t say what material the slabs are made from ?

    If porcelain, 100% every time they must be primed with SBR slurry as porcelain isn’t so porous so the mortar can’t get a grip

    Stone / cement slabs can be dusty, sandy and porous, so they either suck the water out the mortar before it cured or it simple doesn’t stick due to dust

    The few I have laid, I’ve primed the backs, regardless of material with SBR Slurry

    Can either buy this ready made, Priming Slurry, has an extended ‘pot life’ as has retarders added so convenient to use over days / hours

    Or mix your own-
    SBR at 1-4 water
    Whisk in cement powder to form a thick, brushable slurry

    Stir whilst using so cement doesn’t sink to bottom, keep out of sunlight and keep it as cool as possible, don’t use the next day so mix smallish batches

    Paint on the backs of slabs (or use a heavy pile roller) and bed slab onto wet mortar bed, level, etc

    Set up a ‘slurry station’ for painting away from laid slabs as you want to avoid splashing newly laid slabs

    Work with a supply of clean water and rags and wipe up any slurry from face of slabs as you go

    The loose slabs you have, can again try the slurry method to stick them down, but paint both mortar bed and slab, lay them back down then add some slabs on top to improve contact - leave for couple days - avoid rain

    Also read up online and take a look at ‘The Paving Expert’

    Excellent site that covers everything to do with groundworks, paving, etc. They’ve had a site revamp so now easy to navigate and find what you need

    Anyway, give the SBR Slurry a try and let us know the results - good luck
     
  13. stevie22

    stevie22 Screwfix Select

    Rocking and not bonding are different things.

    None of the slabs round my place rock but none are bonded. Most are original (mid 70s) but many have been lifted and replaced for various reasons.

    We did an awful lot of paving in the City of London back in the day, again no bonding.

    You need a solid base then bed the slabs properly and not disturb them for a couple of days. Take the time to do a good job on the pointing.
     
  14. johnboy82

    johnboy82 New Member

    So dave I need to buy the SBR and mix it with water? Ive seen the best one is the ever build type if that is right? Is it 1 part sbr to 4 parts water and what would be best to measure it with, a jug or something?
     
  15. pppmacca43

    pppmacca43 Super Member

    Mix it as per the manufacturers instructions
     
  16. DIYDave.

    DIYDave. Screwfix Select

    If your mixing your own, then yes, Everbuild products seem well regarded in reviews, including SBR

    As above, follow M I printed on the label for each specific task as like many products, these have several uses and the ratios may well differ, depending on use

    From memory, it’s 1 part SBR to 4 parts water, mix together, then add in cement powder to form a brushable, thickish slurry and stir well to break up the powder as it clumps together

    Keep a stick in the mix so you can stir regularly, either brush on or use a paint roller (I may have mentioned this above)

    Can also buy ready made ‘paving primer/slurry’ , some powder form or others as a thick paste that you add water to

    Goggle ‘Paving Primer’ for more info (or mix your own)

    As to best implement for measuring - well …… depends on what you have to hand and the quantity your looking at mixing

    Simple ‘recipe’ @ 1 SBR to 4 water (but do check this on the label)

    So as long as you use the same receptacle for both the SBR and the Water, it doesn’t matter what you use as it’s just a simple ratio - 1 teaspoon SBR + 4 teaspoons water

    1 jug SBR + 4 jugs water …… etc

    As for the cement, just eyeball it to reach the desired consistency - I don’t think the above needs to be super critically accurate to the gram or millimetre ….. you ain’t baking a cake here mate :)

    One last thing - whatever you mix the slurry in, make sure your happy to either dump it afterwards or keep for similiar tasks as you won’t be cleaning it out. A simple bucket / empty 10lt paint pot is all you need for this
     

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