h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I realise you wern't being 100% serious when you wrote:

"'Wet' stone walls are built with mortar. Therefore they are best built in places that are dry in order that the cement can set"

But I'd just like to point out that cement sets by chemical reaction not by drying. It will actually set underwater.

I'd love to see you get cement to set under water!

Cement "sets" by a complex process of hydration at molecular level, bonding onto material in it's close proximity.
The strength of concrete is seriously depleted when too much water is present BEFORE it has reached "initial set" after some 24 hours. Once "initial set" has been reached, the presence of additional water will improve it's strength by reducing internal cracking as it dries.

Ask your mum to show you how jelly sets - too much water and it stays liquid. Cement is similarly affected.
Thus endith the lecture.

OK I admit I have never tried it. But concrete is used for underwater construction and repairs. I will also concede that they use anti-washout admixtures such as mellose, but never-the-less it is concrete and it does set.

I've just come across this again after a three-month or so. I sound like a pompous twit in it! Forgive me, but I claim new-boy-on-the-block freshman status for the commentary!

I speak from the experience of an Architect who had a contractor's workman push a wheelbarrow of fresh concrete over the edge into a client's swimming pool! The entire wheelbarrow-load turned the pool water a dirty grey-brown. Over the next few days, a lazy foreman tried to get the client's automatic pool-cleaner to suck the load of concrete out. The pool filter clogged, burned out the electric motor and we had to get in outside help. The pool was drained using a sludge pump and all we found when empty was a bucket or two of gravel and a solid but soft lump of set concrete about the size of an ostrich egg. Presumably this was from the centre of the pile at the bottom of the pool!

Yes I'm sure the Engineers can get concrete to set in moulds below water level, but I suspect they use a rapid-hardening variety of cement that will achieve the "initial set" I spoke of within an hour or so, whereafter the submersion will only improve the strength. But just poring it into water won't work.

I think the thing here is moderation.

Throw a wheelbarrow of cement into a swimming pool and there is too much water. On the other hand, if you allow cement to dry out before it sets you get a flaky affair that needs reworking.

If you read a DIY manual on laying cement (as I had occasion to do several years ago) and it will recommend covering the cement in damp sacks for 24 hours or so. (In my case I did not have enough, so I used a watering can to keep it damp every few hours.)

I expect that nobody will read this for years, but....

The romans used concrete as mortar which set underwater, I saw a demonstration where the guy mixed it up, put it into wooden mould put it in a tank of water and the next day, it had set!
Also when joseph basilgete (I cannot spell that) was building the large network of sewers under london, he used portland cement because it was strong and it set under water.

I've read it in just three weeks!

Jolly interesting too. Especially as it kind of backs me up.

SK