On Topic: Funny story, for the chemists; and a question

[Another John]

I wanted to modify a concrete bird bath that we have -- to partly fill in the deep channels in it, so that bird crap doesn't accumulate so badly.

My plan was (is) to make a sloppy mix using fine sand, and pour it into the birdbath, so that the channels fill uniformly.

Like almost all people here, I store various bags of sand, gravel, aggregate etc in one of the sheds, in whatever bags come to hand. I took a few scoops of fine sand from one of them, and mixed it thoroughly with cement -- completely dry to start with of course.

Then I added the water. Things started to smell funny.

I poured the mix -- perfect plan for filling the channels btw -- and things smelt even funnier.

Then the mix turned greeny black and some kind of scum formed on the top. I could smell ammonia - very strongly.

Turns out that I had used a bag of lawn sand, which contains "nitrogen, iron, and ferrous sulphate". (That will teach me to hoard. Not.)

The question: What did I make, when I added cement to lawn sand? And is it poisonous?

[3 days later, it hasn't set -- it was merely a VERY smelly sludge. I've had to scrape it all out, and have left the birdbath soaking for a while.]

Cheers John

[Chris Hogg]

Lawn sand usually contains ferrous sulphate to kill moss, probably partially oxidised to ferric sulphate by now. The nitrogen will be from ammonium sulphate to encourage the grass to grow over the patches of dead moss. Cement is a complex combination of calcium oxide, silica and alumina, with lesser amounts of iron and magnesium oxides. It sets by hydration and recrystallisation. That yours hasn't set suggests the ferrous sulphate and/or ammonium sulphate have inhibited the recrystallisation stage. Either that, or the cement had 'gone off' during storage. I see no reason to think that the mix you made is any more poisonous than the ingredients, although I wouldn't want to consume either, and the water in the bird-bath might be quite alkaline and harmful to birds, so I'd give it a good hose down to wash it clean (assuming you've still got water!).

[tabbypurr]

cement/sand in thin layers breaks off very readily. So I doubt your plan would work.

NT

[Andrew]

Has it turned a nice shade of orange yet, as the iron salts 'rust' ?

[Another John]

The scum on the top had elements of orange, but it was mostly black/green; the "mortar' was black/grey sludge as I said. And the smell of ammonia was overpowering if you got too close.

I've just re-done it with proper sand :-) See how it works.

J.

[whisky-dave]

So you've recreated the illusive green.