Bad mortar ?

[Olderisbetter]

First time this has ever happened but the other day i put some slate down ready to brick off using my 3 to 1 mix with additive and just gone to look at it and the slate came away like it was just resting in place, the mortar mix was very weak and no good bond, bad cement?

 

[mike os]

probably dried out rather than cured, also slate is notoriously bad for bonding to mortar

 

[eLuSiVeMiTe]

I usually paint slurry over the underside of slate slabs before laying. It seems to help with the bond.

 

[gordon stephenson]

Did you weight down the slate while drying?

 

[Olderisbetter]

Yes i put 2 layers of imperial engineering bricks on top

 

[TechnicAl]

add some PVA to the mix.......I know I did the same....they will hold now with No Nails

 

[brightspark]

id use sbr rather than pva it breaks down when its damp. but as said it doesn't bond to slate very well . pasting the slate 1st with a slurry of sbr cement and sand will give a good key for a mortar bed

 

[TechnicAl]

id use sbr rather than pva it breaks down when its damp. but as said it doesn't bond to slate very well . pasting the slate 1st with a slurry of sbr cement and sand will give a good key for a mortar bed

SBR ? never heard of that

 

[brightspark]

here https://cpc.farnell.com/everbuild/s...MI-6HT9K_36gIVt4BQBh0KugebEAAYASAAEgInePD_BwE

 

[Dcal]

Was everything really dry?
Looks like the motor dried out before it could cure by hydration.

Like bright spark I would use SBR and make sure all the surfaces are damp.
If it's a difficult surface (very closed surface) as @eLuSiVeMiTe says a slurry with SPR and cement will work very well but just watch the pot life, it's pretty short.

 

[Seadog]

Concrete takes a month to fully harden. It needs to be kept moist as it cures or it loses a significant amount of strength. I left the first part of my workshop base for two weeks before uncovering it, and I hosed it down daily.

 

[Bladevane]

When a couple of our Indian sandstone bricks became dislodged the builder told me to use this. Everbuild Lumberjack Wood Adhesive (I know, wait a minute). It goes off in 5 minutes, sticks like the proverbial to a blanket and expands. It is amazing stuff. I used it ot glue my neighbours brick wall back together after a car clipped the edge. Also brilliant on wood. It's not cheap, but you don't need much.

 

[Matchless]

When a couple of our Indian sandstone bricks became dislodged the builder told me to use this. Everbuild Lumberjack Wood Adhesive (I know, wait a minute). It goes off in 5 minutes, sticks like the proverbial to a blanket and expands. It is amazing stuff. I used it ot glue my neighbours brick wall back together after a car clipped the edge. Also brilliant on wood. It's not cheap, but you don't need much.

sounds the same as Gorilla glue, use it a lot, mend the patio concrete balustrade after I hit it with the mower, have glued broken roof tiles together, daughters door mirror glass back to the motor, plus woodwork when bodging my shed,

 

[Olderisbetter]

Was everything really dry?
Looks like the motor dried out before it could cure by hydration.

Like bright spark I would use SBR and make sure all the surfaces are damp.
If it's a difficult surface (very closed surface) as @eLuSiVeMiTe says a slurry with SPR and cement will work very well but just watch the pot life, it's pretty short.

It was a moist day when i did this and i kept it covered with plastic so it should have cured not dried out.

 

[sg66]

As above with the slurry mix.

 

[Olderisbetter]

Well i cleaned it all up ready to try again, the mortar came away in slabs really easy so that was a bonus

 

[Dcal]

That mortar looks rubbish.

Was the sand very fine?
What was your mix? (looks more like 6 to 1 than 3 to 1 which is a very strong mix)
Did you add lots of plasticiser?
Did you use no / little plasticiser but lots of water?
Was the cement old?
What type of cement was it? (Ordinary portland or eco cement with lots of fly ash?)

Very fine sand kills the cement because you need more cement to coat the surfaced of the sand particles.
It's the reason the Arab States have to import sand for construction.

 

[Olderisbetter]

That mortar looks rubbish.

Was the sand very fine?
What was your mix? (looks more like 6 to 1 than 3 to 1 which is a very strong mix)
Did you add lots of plasticiser?
Did you use no / little plasticiser but lots of water?
Was the cement old?
What type of cement was it? (Ordinary portland or eco cement with lots of fly ash?)

Very fine sand kills the cement because you need more cement to coat the surfaced of the sand particles.
It's the reason the Arab States have to import sand for construction.

The sand was the same stuff i have been block laying with and that all went fine, the mix is 3 to 1 and i use sika plasticiser and i mix it in a tub to get the ratio right, THe cement was my first suspect it is hanson OPC and dated just a few weeks old, I am going to make a few small batches up and do tests but as you say the mortar looks rubbish and it felt light, the slabs of mortar that cam off snapped up with a tap so really no strength.

 

[brightspark]

wasn't sharp sand u were using was it or was it yellow builders sand

 

[Olderisbetter]

Its just ordinary sand for brick laying..