PVA in cement

[Guy King]

I saw someone putting a dollop of PVA into some cement he was mixing.

What's that do then?

[Chris Bacon]

It makes the mortar tougher. It can also increase adhesion. It's also almost always not needed, often not desirable.

[Guy King]

The message from Chris Bacon contains these words:

What would make it undesirable?

[The Natural Philosopher]

What it does is make the grains of sand slide over each other a little, this 'plasticising' the mix, and also , if its a fairly weak (low cement) mix fills up the gaps between the grains and stops water soaking in quite as much.

I.e. reduces porosity and 'waterproofs' the mix.

[The3rd Earl Of Derby]

In reality it serves no purpose whatsover.

Most victorian properties have been up a hundred years,there was no PVA then. :-)

/awaits the antagonist.

[Grunff]

IME nothing - PVA is a great additive, it plasticises, and improves adhesion significantly. If you add enough of it, you end up with a relatively non-porous finish.

[Chris Bacon]

Extra cost.

Reduced porosity.

Increased bond strength

Increased hardness of mortar.

The last three can be good as well as bad. It depends on the circumstances. The last I heard, which was about 10 years ago, there were some concerns about the effect of exposure to damp on mortars containing PVA admixtures. I don't know what the outcome was.

[Steve Walker]

I agree - I lob it in every mix.

[Weatherlawyer]

It would become a PITA to remove from the facework even with brick acid. It is used in self leveler. If you are using it in a grout it will stop it spalling or failing through drying too quickly.

You can use it in small mixes that you make just for patching. It's a waste of time and money in large jobs.

[Andrew Gabriel]

Actually, you can only use a relatively small amount of PVA in mortar which is going to be exposed to moisture, or it will cause the PVA/mortar to eventually fail. You only need a small amount anyway -- something like a teaspoon per shovel load of cement to get maximum effect. If you need any more than this in circumstances where the mortar is going to be exposed to moisture e.g. to make up some sort of bonding mix, then you must use EVA (which is sold as waterproof PVA, although it's not strictly PVA). SBR is another alternative suitable for moist conditions (Lafarge's helpline suggested it to me on one occasion), but not having tried it myself, I don't know which of PVA's various properties it imparts to the mortar.

[Stuart Noble]

IME SBR doesn't mix with mortar in the way that pva does. Prefers its own company and has to be constantly re-mixed to stop it separating out. Both work better as a seal applied to the surface rathet than as an admix