Norman Wells wrote:
The Natural Philosopher wrote:
Paul Martin wrote:
In article ,
Steve Thackery wrote:
That one's been already cracked. It's called a tree.

Nope, doesn't work, and one of the greatest green myths of all time.
Everybody knows that trees absorb CO2 when they grow, converting it
into plant mass.

Unfortunately, every living tree eventually dies, rotting away. As
it rots, it releases all the CO2 back into the atmosphere again. The
same is true if you burn it, of course.

The only way a tree can make a lasting contribution to CO2
reduction is if we cut it down when it is fully grown, and then
either store it in such a way it can never rot, or drop it into a
subduction zone so that it releases the carbon so deep in the earth
it can never escape again.

OK, so you store it, just like you store the CO2 captured by other
means, eg. building materials. How much CO2 does roasting limestone
for cement liberate?

I think a lot goes back into it when the cement sets again..

Er, no it doesn't.

Limestone (CaCO3) is heated in a furnace to drive off the CO2 leaving
calcium oxide CaO which is the active part of dry cement. In use that
combines with water (H2O) to form calcium hydroxide (Ca(OH)2) which is
what set cement is. The CO2, which has been locked away by those
helpful little molluscs for millions of years, now swans around the
atmosphere like there's no tomorrow, which might be right.

calcium hydroxide will not stay calcium hydroxide long in the presence
of carbonic acid dear.

And cement is a damned site more complex than that.