[David]

David asked the Naked Scientists:

The lime that was used in buildings, as a sort of cement or part of the mix, does this come from lime stone.  How is it made and is dangerous?

What do you think?

[Atomic-S]

I believe lime comes from limestone, and is derived from it by roasting. The reaction would be CaCO₃ --> CaO + CO₂.  The resulting CaO is then mixed with H₂O to give Ca(OH)₂, common lime. Common lime is strongly basic, and as such will tend to corrode living tissues, which is why you should not handle it a lot.

[chris]

So why is cement "sticky" and how does it glue bricks together?

[lightarrow]

So why is cement "sticky" and how does it glue bricks together?

It's "sticky" because it's made of a very fine powder and of chemicals (e.g. calcium aluminates, sulphate and silicates) wich forms strong hydrogen bonds with water.
It glues bricks together because, with time, calcium aluminates becomes hydrated with water (first phase, quick) and then calcium silicates becomes hydrated with water (second phase, hardening, slow) forming a net of chrystals with all the bulk, making it very rigid, with a structure similar to that of a rock.

Cement is made, essentially, by fine powdering and homogenizing clay + limestone and then heating this powder at 1400°C to eliminate CO₂ and to form calcium aluminates and silicates.

[Bored chemist]

There's more than one sort of cement.
The simple lime mortar  used for old buildings was made from slaked lime  (Ca(OH)2) and sand.
This hardens slowly on exposure to air because it reacts with CO2 to form calcite.

The "portland cement" made from lime and clay hardens because of a different set of reactions (as Lightarrow says). Because it doesn't require CO2 to set it will still work under water.

The whole story is rather complicated; here's some useful data on it.
http://en.wikipedia.org/wiki/Cement

[Pumblechook]

B & Q ?

[lyner]

Pricey there - even on Wednesdays.