Energy and Electricity - Introduction to Carbon Capture and Sequestration - page 35

Similarly, CSH present in concrete can liberate CaO from Ca(OH) 2 once
it has converted, again forming carbonate:

H 2 CO 3 + CaO → CaCO 3 + H 2 O

A simplified way to write this calcination process is to combine the
reaction of CaO and CO 2 to give:

CaO + CO 2 → CaCO 3

Let's imagine that we have an unlimited source of CaO. Each mole
of CaO can absorb one mole of CO 2 according to the reaction above.
Current global production of cement is about 3.3 billion metric tonnes
per year. Given that typical concrete is composed of four parts aggre-
gate to one part cement by weight, the global production of concrete
consumes approximately 13.2 billion metric tonnes of aggregate per
year. Assuming this is composed completely of CaCO 3 , this is the equiv-
alent of 5.8 Pg per year. We conclude that sequestering CO 2 emissions
into the aggregate in concrete would absorb about 19% of our annual
CO 2 emissions.
It turns out that concrete can sequester even more CO 2 . The cement
that is used as the binder in concrete is comprised mainly of CaO. The
cement that has been placed into roads, buildings, dams, etc., will
undergo chemical reactions as the concrete ages, and CO 2 from the
atmosphere will diffuse into the concrete and react with CaO to form
CaCO 3 . This is a natural part of the concrete curing process and acts to
increase the strength of the material. CaO accounts for about 63% of the
cement by weight. Assuming this all eventually reacts with CO 2 in the
atmosphere, cement curing and aging could account for an additional 1.6
Pg per year of CO 2 sequestration, or a total of 24% of global CO 2
emissions.
In summary, these calculations indicate that current global concrete
production could absorb only 19% to 25% of net CO 2 emissions. These
calculations are predicated on the availability of an unlimited supply of
CaO. Given that the CaO for cement must be produced by driving the
CO 2 off CaCO 3 with large quantities of heat, usually supplied by fossil fuel
combustion, this process would be infeasible even if the scale of con-
crete production was greater than CO 2 emissions.