Bioinspired Concrete - Biotechnologies and Biomimetics for Civil Engineering - page 300

13.4 Environmental Challenges with Cements
and Concrete
Portland cement contains mineral compounds that are formed during high tem-
perature processing. Portland cement is produced, in the simplest of terms, by
heating limestone with clay and calcium sulfate compounds to high temperatures
(1,100-1,600 C). During the production of Portland cement 0.8 ton of carbon
dioxide is produced for each ton of cement produced. This carbon dioxide is
resulting from the fuel burned as well as the CO 2 that is produced by limestone
(CaCO 3 ) reducing to CaO + CO 2 . Current methods of off-setting these exhausts
and pollutants are by focusing on lower Portland cement containing mix designs.
Carbon sensitivity and awareness are growing, however; and industries are
changing. The carbon sensitivity is mainly associated with Portland cement due to
its high volume use. One method of reducing the amount of Portland cement in a
mix design, without sacrificing performance, is by the addition of limestone to the
cement during the final stages of milling. This final cementitious product is then
termed a limestone interground cement and is commonly specified and applied in
Europe, North America, and other countries. Newer technologies exist applying
membrane methods to CO 2 capture and mineralization techniques to form 'syn-
thetic limestone'. These synthetic limestone materials generated from these pro-
cesses can be used interchangeably with natural calcium carbonate limestone.
Unless specifically introduced in the form of a limestone interground cement, or
as limestone aggregate, carbonate is only found in concrete after long periods in
small quantities. This arises from the aging of concrete in environments that contain
gaseous carbon dioxide and is only introduced to the hydraulic composition in small
amounts via the aqueous transport of carbon dioxide in the carbonate equilibria
established with atmospheric carbon dioxide and the aqueous solution used to
hydrate the cement. For instance, lime cements and later Portland cement.
13.4.1 Carbon Sequestration in Concrete
The global cement (3.6 billion tons) and aggregate (37.4 billion tons) markets are
the only product-related markets that have the ability to sequester the mass and
volume characteristic of the global carbon epidemic. With ever increasing CO 2
levels (Francey 2013 ) it is important to usefully store the carbon in a societal
relevant form. Cement and aggregate are typically mixed with ground limestone or
pozzolans, which in some cases are inert and occasionally reactive. Understanding
the reaction of ordinary portland cement (OPC) with various carbon negative
aggregates, pozzolons and additives are increasingly important to the successful
inclusion of carbonated anthropogenic CO 2 in the human built environment.
Much can be leaned by astutely observing the biological world around us in
an ever more accurate fashion. To solve the impending carbon question of the