[Sally Le Page (OP)]

Paul wants to know:

"Could a vehicle engine be invented which uses CO₂ as a fuel with an output of Calcium Carbonate or something similar which could be defecated periodically at a petrol station and which could then be used as a building material, mixed with something else for strength? This is just a thought that occurred to me this morning. I haven't researched it and my chemistry classes were over 50 years ago so forgive me if the suggestion is absolutely stupid."

What do you think?

[Bored chemist]

Sort of.
But it's a whole lot easier to do carbon capture at a power station, and run the car on electricity.

[alancalverd]

The most obvious and economical source of a CO2 absorber is CaO or Ca(OH)2.

But these do not occur free in nature and are manufactured by heating CaCO3, which has been used directly  (marble, limestone, clunch) and indirectly (CaO lime mortar) as a building material for thousands of years.

Problem is that the best way of heating chalk is by burning wood or fossil fuel, so you end up generating at least twice as much CO2 as you can absorb!

It's interesting to note that all the marble, chalk and limestone was formed from atmospheric carbon dioxide. There must have been an awful lot of it around in the past!

[Bored chemist]

There must have been an awful lot of it around in the past!

But not all in the air at the same time.

[alancalverd]

Are you suggesting that it came out of the ground for a billion years or so, just fast enough to keep pace with its sequestration? So why has it stopped?

[Bored chemist]

Are you suggesting that ...

No.

[alancalverd]

So it wasn't all there at once, and didn't appear gradually either. Magic?

[evan_au]

The most obvious and economical source of a CO2 absorber is CaO or Ca(OH)2.

I would have thought that the most obvious and economical absorber of CO₂ is wood...
- And this has been used as a building material, in many countries.

It's interesting to note that all the marble, chalk and limestone was formed from atmospheric carbon dioxide. There must have been an awful lot of it around in the past!

Earth's plate tectonics is always burying carbon, to have it reappear as CO₂ in volcanic plumes.
- But humans are now actively digging up carbon and releasing CO₂ into the atmosphere at rates far faster than is being released in volcanic plumes
- It is this rapid rate of change in concentration which is impacting surface temperatures, ocean acidification and sea level rise, with impacts on many different ecosystems.
- It is thought that the presence of volcanoes and lack of plate tectonics in the crust of Venus that contributed to the current predominance of CO₂ in the atmosphere of Venus.

See: https://en.wikipedia.org/wiki/Carbon_cycle#Geosphere

[CliffordK]

There must have been an awful lot of it around in the past!

But not all in the air at the same time.

What we know is that the Carbon Dioxide levels haven't been constant over time. 

I'm seeing notes that the CO₂ was as high as 8,000 PPM (or about 20x today's levels) 500 to 600 million years ago. But, it would have been even higher a few billion years ago before the "Great Oxidation Event". 

Stalactites and stalagmites are still forming indicating we continue to get calcium deposits.

[Bored chemist]

So it wasn't all there at once, and didn't appear gradually either. Magic?

Did you forget what you had written?

just fast enough to keep pace with its sequestration

There's no reason to expect the two rates to have been identical.