Just curious really, was wondering if anyone could explain why some types of slabs need SBR primer to aid adhesion but other types dont?
What's the difference in the stone causing adhesion problems?
Why do some slabs need sbr primed but others dont?
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MikeG
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To do with how much the slab will absorb content from the bedding mix. The more porous the material the better the ‘grip’.
So non porous, like porcelain or slate, need the sbr primer to act as a bridge to stick to both the slab and the bedding mix to give it some grip which wouldn’t occur otherwise.
Another reason is to act as a barrier to help reduce the movement of potential stain from the bedding mix to the visible top surface of the slab, which conversely is most likely on the most porous stones.
Maybe some more reasons but can’t think of any off the top of my head...
So non porous, like porcelain or slate, need the sbr primer to act as a bridge to stick to both the slab and the bedding mix to give it some grip which wouldn’t occur otherwise.
Another reason is to act as a barrier to help reduce the movement of potential stain from the bedding mix to the visible top surface of the slab, which conversely is most likely on the most porous stones.
Maybe some more reasons but can’t think of any off the top of my head...
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Tony McC
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MikeG is right - it's all related to porosity.
Think of the cementitious 'paste' that forms part of any mortar or grout, the really fine mix of cement powder and water. When the substrate (in this case, the paving) is somewhat porous, that 'paste' can ooze its way into the miniscule holes, voids and interstices that exist at the surface and into the body of the paving units.
That paste will eventually harden and when it does, it's like a lock in a keyhole - an irregular mass (although it is, as I said, minute) that cannot be pulled out. Or think of it like the monkey jar trick, where the monkey puts its hand into a narrow-necked jar to retrieve some peanuts but then can't get its greedy peanut-filled clenched fist out of the jar as it's now too big.
And this happens all over the contact plane between bedding mortar and paving unit, with millions of these tiny "clenched fists" grabbing tight hold of the paving above.
With impermeable materials, such as porcelain and slate, there are very few or no holes/voids/interstices, and therefore the mortar can't 'grab' the paving, leaving it relatively loose and liable to move once the mortar has set.
Accordingly, with these awkward materials, we use an intermediary, one with incredible adhesive power at a microscopic scale, a would-be superglue, in effect. This is the SBR or primer slurry (which tends to be SBR with very finely-ground cement).
This bonds to the paving by finding its way into and around the tiniest of surface snags, bumps and hollows, irregularities that are way too small for the comparatively coarse cement paste, gets a hold on the paving, and then, in a similar way, does the same with the bedding mortar, and so renders the impermeable paving well-and-truly gripped by the laying course.
There are, as MikeG says, other benefits, particularly in relation to sub-surface stain prevention, rising moisture and salt migration, which are more down to the rubber-like 'membrane' that the SBR creates as it cures, but the main reason for using SBR or a slurry primer remains bed adhesion. The rest is just a bonus!
Edited By Tony McC on 1578774555
Think of the cementitious 'paste' that forms part of any mortar or grout, the really fine mix of cement powder and water. When the substrate (in this case, the paving) is somewhat porous, that 'paste' can ooze its way into the miniscule holes, voids and interstices that exist at the surface and into the body of the paving units.
That paste will eventually harden and when it does, it's like a lock in a keyhole - an irregular mass (although it is, as I said, minute) that cannot be pulled out. Or think of it like the monkey jar trick, where the monkey puts its hand into a narrow-necked jar to retrieve some peanuts but then can't get its greedy peanut-filled clenched fist out of the jar as it's now too big.
And this happens all over the contact plane between bedding mortar and paving unit, with millions of these tiny "clenched fists" grabbing tight hold of the paving above.
With impermeable materials, such as porcelain and slate, there are very few or no holes/voids/interstices, and therefore the mortar can't 'grab' the paving, leaving it relatively loose and liable to move once the mortar has set.
Accordingly, with these awkward materials, we use an intermediary, one with incredible adhesive power at a microscopic scale, a would-be superglue, in effect. This is the SBR or primer slurry (which tends to be SBR with very finely-ground cement).
This bonds to the paving by finding its way into and around the tiniest of surface snags, bumps and hollows, irregularities that are way too small for the comparatively coarse cement paste, gets a hold on the paving, and then, in a similar way, does the same with the bedding mortar, and so renders the impermeable paving well-and-truly gripped by the laying course.
There are, as MikeG says, other benefits, particularly in relation to sub-surface stain prevention, rising moisture and salt migration, which are more down to the rubber-like 'membrane' that the SBR creates as it cures, but the main reason for using SBR or a slurry primer remains bed adhesion. The rest is just a bonus!
Edited By Tony McC on 1578774555
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KAMIKAZEE DIY
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Thanks Tony for such a detailed explanation. As a long time reader of site i respect your knowledge of paving. Based on my experiences indian sandstone requires sbr primer, so to a novice like me if I apply your stone porosity logic you would think india n sandstone wouldnt stain easily, yet I've had some great examples of staining? Could you share your knowledge as to why a paving slab that needs primed for adhesion can stain so easily?
Cheers
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Tony McC
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Indian sandstone, or at least most of it, is relatively porous so generally does not need a primer, although using one does no harm and brings the added benefits previously mentioned, particularly resistance to salt migration and sub-surface moisture.
It's vulnerability to surface staining is, yetr again, down to porosity. Just as the cement paste can easily 'grab' the underside of a paving unit dies to all those interstices, a staining substance (red wine, burger fat, general crap) can just as easily gain ingress to the top face, and once it's in, it can be hard to shift.
Geenrally speaking, the paving materials that must have a primer (slate/porcelain/some quartzites/etc) offset that inconvenience by being much more resistant to stains - they take with one hand but give back with the other!
This also explains why these materials do not need a sealant.....no matter what the sealant manufacturers would tell you!
It's vulnerability to surface staining is, yetr again, down to porosity. Just as the cement paste can easily 'grab' the underside of a paving unit dies to all those interstices, a staining substance (red wine, burger fat, general crap) can just as easily gain ingress to the top face, and once it's in, it can be hard to shift.
Geenrally speaking, the paving materials that must have a primer (slate/porcelain/some quartzites/etc) offset that inconvenience by being much more resistant to stains - they take with one hand but give back with the other!
This also explains why these materials do not need a sealant.....no matter what the sealant manufacturers would tell you!
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