PVA Additives... good or bad?

[chalkyt]

Hi All

Time for a debate and/or enlightenment. A few days ago I replied to a post about leveling a bathroom floor section and suggested using an additive like Cemstick or Bondcrete as a primer and cement "enhancer". Another post cautioned "don't use PVA in wet areas". Having checked some of the additive manufacturer's web sites, most of the products seem to be PVA (or similar) based with only one saying "not suitable for permanently wet areas". The various tile company primers and grout additives etc also seem to be PVA based.

So, anyone got a view on PVA good or bad in cement and reasons for this view. I have "always" primed the substrate and sloshed a bit in when doing any patching etc and am now wondering if this is not such a good thing even though so far I haven't noticed any problems (apart from a slight darkening on the mix when it dries).

All views welcome!

[Batpig]

Dear Chalky,

Good question, and well worth asking. As far as priming is concerned, the said PVA doesn't actually need to be waterproof, because once the cement has set, the Primer's job is done. This is because it's sole purpose is to stop water in the fresh cement product that is being applied from being instantly sucked out and into the existing substrate by capillary action. In the absence of such priming, the effect of the enusuing moisture loss is:
(1) The fresh cement will usually not "take", or just plain fall off,
and,
(2) The strength of the fresh cement product will be (theoretically...) affected due to the resulting drop in its moisture level.

Worst-Case scenario I can think of as an example was trying to cement-render an unprimed vertical brick wall where the properly-baked faces of the bricks had all been chipped off during removal of the previous tiles by jackhammer. Because the revealed innards of the bricks were more porous to water than the faces had been, such rendering was an almost impossible feat to achieve without priming the masonry beforehand.

But having said this, once the fresh cement has set, the Primer's job is at an end. There's usually enough mechanical interlock between the old substrate and the new cement product to do a satisfactory job of adhesion thereafter (although the Primer down there at the interface will continue to help the new cement coating to cure more properly...) Try therefore to make sure when priming that the ratio of PVA in water is no more than 1 part of PVA in 4 to 6 parts of water, so that it can easily soak in. It's better to have it a bit thinner so that it can sink in to the substrate and leave a rough surface behind, then it is to make it too thick to sink in, because then you're trying to stick cement to PVA rather than substrate, and if the stuff isn't waterproof, well I guess it will eventually break your heart after enough dampness has got to it.

As far as using it as an Additive is concerned, this is a different kettle of fish. But I've always assumed that Bondcrete is a water-resistant type of PVA anyway (similar to Selley's Aquadhere "Exterior"; it's water-soluble when wet, but water-resistant when dry...) I personally reckon the Bondcrete would be up to it as an Additive, but I've never really felt the crushing need or desire to use it as one.

For what it's worth, last time I checked, 4L of Bondcrete was not much dearer - if at all - than 4L of Selley's Aquadhere "Exterior". I wouldn't touch the ready-mixed primers like "Ultraprime" - you're just paying for water...:-

Best Wishes,
Batpig.

[heavytrevy]

Pva (vinyl acrylic) when used in a high humidity environment will cause it to reactivate. This becomes a huge problem for tilers .There has been many documented cases of tiles detaching and falling from walls .
P.V.A. Versus Primers (http://www.tilersforums.co.uk/tile-adhesive-grout-substrate-preparation/2511-p-v-versus-primers.html)

[heavytrevy]

From the thread i posted earlier,
Cement, Gypsum, Anhydrite, Ettringite Crystals, and PVA; A Discourse


Introduction

There have been many debates on these subjects, and my intention is to, in a reasonably easily understood manner, explain the "why" of it all. I'm not an expert on this, but I have read up on it enough to form an opinion, and I wish to pass on my findings.

This post is based on a pair I did very recently, in a thread pertaining to these matters.


A Brief Summary

If you do not want to read the whole post, here follows a summary of what I'm going to explain.


PVA is not suitable in any tiling related application.
Cementious materials and anydrite (or gypsum) materials are not compatible, and must be completely separated by, for example, a primer.

There. Now onto the main part of this discourse.


PVA and Cementious Materials

I hadn't even heard of using PVA for anything tiling related before I came on here. Thus, I read up on it, and here follows my findings:

PVA stands for polyvinyl acetate, and it is a rubbery synthetic polymer. It is commonly emulsified in water and used as glue. Many know it simply as "wood glue", or "carpenter's glue".

Cementious materials, such as many tile adhesives and grouts, or other materials which contain cement, such as concrete, arealkaline. Simplified, that means they have a high pH.

Alkali slowly attacks polyvinyl acetate, forming acetic acid, which has a low pH. Cement doesn't dry per se; it cures throughhydration, which means it binds the water you mix it with chemically. This causes the pH of the substance to rise dramatically. Introducing an acid negates that process to some extent, preventing the cement or conrete from binding all the water it needs to harden properly.

It is hydrolysis which gives cement and concrete products strength, and holds them together. Without this process, it would merely be the powder you started with.

The acetic acid which is formed when cement and PVA comes into contact, either through mixing them, or "priming" with PVA, will continually free the water bound in the cement, and that will weaken the bond and/or integrity of the material. The effect is accelerated if the material is subjected to moisture, which is more or less always the case.

PVA isn't water resistant. It becomes slightly live when exposed to moisture, and this in combination with the exposure to alkali, accelerates the forming of acetic acid. PVA which is marketed as "water resistant" or "exterior grade", has additives which makes them water resistant, but they're not alkali-resistant.


Anhydrite, gypsum, and cement

Anhydrite products are mainly composed of calcium sulfate, and gypsum products are mainly composed of calcium sulfate dihydrate. When anhydrite is exposed to water, it forms gypsum. In other words, it hydrates. Essentially, it cures, but not to the same extent as cement.

Gypsum always has a proportion on anhydrite crystals left in it.

Cement has a proportion of calcium aluminate. Calcium aluminate reacts with calcium sulfate, which is the main component of anhydrite products, and which is present in gypsum. The reaction forms hexacalcium aluminate trisulfate hydration; in other words, ettringite crystals. These expand, and force away anything which is fixed onto where they form.

As I've previously explained, cement cures, which means it binds water through hydrolysis. That means water is always present in cement. If anhydrite is put into direct contact with cement, there will be a reaction. The reaction won't be as severe with gypsum, as it's already hydrated most of the anhydrite (the dihydrate part), but there is still some present.

Thus, if you want to tile onto such products, you will need toseparate them entirely. This is best done with a products which seals, and which is also water resistant, such as acryllic dispersions.
Even if you use water resistant "PVA", the separation will deteriorate with time, due to the chemical reaction between the cement, which is alkaline, and the polyvinyl acetate. If the bond of the cement onto the substrate hasn't already been compromised because of that, the formation of ettringite crystals will very likely cause complete debonding.

Rapidly curing cements may have some gypsum added when manufactured. It accelerates the curing, but does not affect the integrity of the product, because it's present in such small quantites, and during the early stages of curing.


Final Notes

PVA is not suitable as a primer, sealer, impregnator, or admix. The uses of PVA may be many, but they do not include anything tiling related. Use proper manufacturer approved primers and additives instead. Using PVA will likely cause liability issues when problems arise, and that is bound to cost alot more than buying proper materials to begin with.

[chalkyt]

Thanks Batpig and HeavyTrevy
I was probably a bit loose when I said PVA. I wasn't referring to just any old PVA wood glue (aquadhere etc) but more specifically the products like Cemstick and Bondcrete that are flogged as additives (I must admit that their MDS indicate that they are often only second cousins to PVA and so may address some of the issues that you mentioned), and also the specific tile primers etc that seem to be similar.

I didn't know that a bit of cement patching or tiling was so technically and chemically complex. Always educational this forum!!!

[Gaza]

if i am doing concrete patching or ardit i will prime the surface with boncrete or pva added with water, this seals the concrete to stop it sucking the waterout of the topping and drying too quick & cracking.

i would not seal a surface with boncrete or PVA before using tile adhesive or slimar, infact this week we were doing a job where we had to glue acouctic pads down over a slab with a polyurthane my guys wanted to seal the slab with bondcrete to kill the dust but as i told them we can not do that as the polyurthane is meant to stick to the slab not the bondcret and the failure would occur between the two.