packing out steps

[Dave]

A post by sWe.....

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, are alkaline. 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 through hydration, 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.

[brian c]

Originally Posted by whitebeam

Is it external work to the steps

yes they are external Whitebeam.

[Spirit]

http://www.avenuesupplies.co.uk/getimage.php?id=504&type=3&format=1

[Spirit]

From the horses mouth

Cemstik®, PVA bonding agent and admixture for cement.

[Spirit]

Brian, for external use, the use SBR or EVA. PVA in recommended for internal use.

[brian c]

i wont be using pva guys,never have done and i dont think i ever will.

[whitebeam]

The paper work can say what it likes, Through years of experience and having to remove works that have used pva on external repairs, It dos,nt work and never will. Sbr is the dogs danglis

[Dave]

Originally Posted by Spirit

Whitebeam, for external use, the use SBR or EVA. PVA in recommended for internal use.

So would you use it as a primer for tiling then..?..

[Spirit]

I must admit that I always use Limelite Easy-Bond but a lot of people find it a bit pricey. As you will be laying 65mm thick concrete, is is unlikely to move anyway on a keyed surface so a high quality bonding agent is unlikely to be of great benefit.

All of the PVA failures that I have heard about have been because of misuse of the product, incorrect dilution, dusty surface etc. This can happen with any product. I would imagine that PVA fails more than other products is that it is used probably 10 times more than all of the others put together.

[Spirit]

Originally Posted by Dave

So would you use it as a primer for tiling then..?..

No, but I wouldn't use SBR either.

[whitebeam]

Originally Posted by brian c

yes they are external Whitebeam.

Use sbr, Put 50-50 with water with whatever mix you use and put a slurry mixed with cement onto the surface that your working to that will never let go