On 03/12/2015 21:36, Andrew Gabriel wrote:
In article ,
stuart noble writes:
On 03/12/2015 18:56, John Rumm wrote:
PVA,
being water-soluble, is a sort-term fix that will eventually be washed
away."

Disagree. What PVA does (and the only thing it does) is prevent water
being sucked out of the mortar before it has had time to cure. Doesn't
matter what happens to it after that. Interesting that the end result is
the same whether you mix the PVA with the mortar or coat the surface first.

PVA/EVA/SBR all increase the tensile strength of cement based
mortars by cross linking with the set cement crystal structure.

PVA is unsuitable for use where moisture will be present, as water
dissolves PVA. EVA (usually sold as Exterior PVA although it's not
really PVA at all) is more waterproof, but *only* when used as a
mortar additive, because the cross-bonds with set cement are
insoluble. (Any excess EVA in the mix is still water soluble,
which can still cause weakness if it gets wet and there was too
much EVA or it wasn't well mixed in.) SBR goes one step further,
in that's it's not water soluble at all.

The other use for the bonding agents is where you want to bond
cement to something it doesn't stick to too well (including cement
which is already well set). Then a slurry made from cement and one
of the bonding agents at the join can work very well.


IME unadulterated mortar sticks well to any reasonably solid surface,
including a builder's bucket if the plastic isn't flexed. It only ever
fails when the underlying surface is porous. Never tried, but I'm sure
you could render a jam jar given the inflexibility and zero porosity.

SBR is interesting stuff but I get the impression there is no chemical
reaction with the cement, in fact you have to keep stirring it to
achieve any kind on integration.