What makes tile cement set?

[Ron Lowe]

I've just re-tiled an area in the shower , having changed the shower itself. I just used a tub of ready-mixed Bostik Tile Adhesive, and it worked very well.

But it set me wondering:

What conditions exist once I've spread it on the wall that don't exist in the tub, so that it remains gloopy in the tub, yet sets solid on the wall?

Is it contact with the air?

In the tub, there's a plastic film over the stuff, but there's a substantial volume of airspace in there too. There wasn't a mega-vacuum-sook when I removed the lid.

Or contact with moisture in the air?

Is the air volume in the tub super-dry?

Either way, I larded the stuff on fairly thick ( say 3mm ), and so I can't see how I fully aerated the full volume of material I placed on the wall. So the setting mechanism ( if its based on contact with air ) needs to propogate from the briefly-exposed surface through to the unexposed interior. So why does the whole lot not set in the tub by the same mechanism?

[Phil L]

It contains suppressants to stop it setting, once these suppressants are removed by moisture removal (into the wall and the tile) the cement sets as normal. You can get premixed building mortars of any colour delivered on site which contain the same chemicals, they can be delivered on a Monday and not get used until Wednesday, and they are as fresh as 'just mixed' stuff, provided they are kept covered obviously.

[Ron Lowe]

OK, so the mechanism is esentially one of 'drying out'. In the tub, the moisture content is kept at a constant-ish high.

Is that a fair summary?

[Phil L]

Basically, yes...the same principal applies with gloss paint, the solvents stop the paint from setting in the tin - once the solvents have gone, you are left with the hardened paint

[Stuart Noble]

I think tile adhesive contains microspheres or glass bubbles, which give the illusion of it being wetter than it really is. They act rather like a plasticiser does in cement, imparting sloppiness without adding water. IIRC rheology is the science of making liquids behave the way you want, enabling you to turn water into hair gel by adding 0.1% of a particular thickener

[Andy Wade]

That's not the case. Oil based (alkyd) paints "dry" by oxidation of the base material. That's why the paint in a part-used tin skins over, and the emptier the tin, the thicker the skin.