CO2 and carbonation
During the cement-making process, and specifically the clinker burning process, gases with Global Warming Potential (GWP) are emitted. Carbon dioxide (CO2) accounts for the main share of these gases. Other climatically-relevant gases, such as dinitrogen monoxide (N2O) or methane (CH4), are emitted in very small quantities only. CO2 emissions are both raw material-related and energy-related. Raw material-related emissions are produced during limestone decarbonation (CaCO3) and account for about 60% of total CO2 emissions. Energy-related emissions are generated both directly through fuel combustion and indirectly through the use of electrical power.
Emissions of climately relevant gases are also associated with
the transport of materials, fuels and final product (within
the industry and to customers), the crushing of rock to form
aggregate, the production of steel reinforcement and the
energy used to process the other raw materials or to power
manufacturing operations.
Concrete carbonation as a CO2 sink
The manufacture of one tonne of UK cement generates
approximately 0.822 tonnes of direct CO2 emissions
(British Cement Association, 2005). As explained above
approximately 60% of this results from the thermal decomposition of
calcium carbonate (CaCO3), otherwise known as
calcination. Emissions of CO2 associated with calcium
carbonate decomposition are not only distinct in terms of the
process that generates them; they are also partly reversible
through the process of carbonation.
In engineering terms, carbonation is the process by which the
pH of concrete is reduced from around 12 to below 9 to 10 through
the absorption of CO2. In Portland cement concrete the
pH is maintained at a level of at least 12.6 and this minimises the
potential for corrosion of steel reinforcing bar which can result
from carbonation; at a pH above 9 to 10 a protective surface oxide
layer forms on the surface of reinforcing bar. It is therefore
important that carbonation is limited to the surface of high
strength structural concrete and prevented from reaching depths
where reinforcing is located. British Standard(s) for specifying
concrete ensures that this can be achieved across a broad range of
applications.
Whilst carbonation during the service life of high strength
structural concrete is intentionally kept to a minimum, there is a
much greater uptake at the end of its life when it is crushed and
CO2 is more readily absorbed due to the significant
increase in surface area. In low strength concrete such as blocks,
and cementitious materials such as mortar, carbonation is much more
rapid as CO2 can permeate the material more easily. This
does not present a problem since no reinforcing bar is
present.
Preliminary calculations examining the key UK cement
markets/applications show that around 19% of the direct
CO2 emissions from the manufacture of cement are
reabsorbed over its lifecycle i.e. during its service life and
secondary life following crushing and reuse. Whilst the carbonation
process cannot be said to diminish the CO2 emission to
air during the manufacture of cement, a 19% uptake will
ultimately reduce their environmental impact from an initial 822 kg
of CO2 per tonne (2005 direct emissions) to around 670
kg/t over its life.
Carbon monoxide (CO) and total carbon
The exhaust gas concentrations of CO and organically-bound
carbon are a yardstick for the burn-out rate of the fuels utilised
in energy conversion plants, such as power stations. By contrast,
the clinker burning process is a material conversion process that
must always be operated with excess air for reasons of clinker
quality. In concert with long residence times in the
high-temperature range, this leads to complete fuel burn-up.
The emissions of CO and organically-bound carbon during the
clinker burning process are caused by the small quantities of
organic constituents input via the natural raw materials (remnants
of organisms and plants incorporated in the rock in the course of
geological history). These are converted during kiln feed
preheating and become oxidised to form CO and CO2. In
this process, small portions of organic trace gases (total organic
carbon) are formed as well. In case of the clinker burning process,
the content of CO and organic trace gases in the clean gas
therefore does not permit any conclusions on combustion
conditions.
Links to further information
References
Cement and concrete sector report, Carbonation Scoping Study, 2006