Carbon dioxide uptake during concrete life cycle - State of the Art
by Björn Lagerblad, CBI, Sweden (www.cbi.se)
Carbonation results when carbonate ions from dissolved carbon dioxide react with the Ca ions of the cement paste and precipitate calcium carbonate. By time all Ca-bearing cement hydrates will decompose and form calcite. The end product will apart from calcite be silica gels, metal hydroxides and clays. Carbon dioxide and water can be found in almost every environment and thus all concretes will be subjected to carbonation. The cement paste will in the course of time go back to the basic components in cement production. Therefore, the question is not if concrete and other cementitious products will carbonate, but how fast they will carbonate. In geological terms the cement paste tums into marly limestone and the concrete into marly agglomerate. Old Roman concrete structures are basicaIly such a rock.
Carbonation is a process from the surface, i.e. the amount of carbonated material is related to exposure time and surface. Surfaces in direct contact with carbon dioxide and water will carbonate rapidly but a shell of already carbonated concrete will slow down the carbonation of the interior. Thus, to be able to calculate the CO2 uptake we must know the transport mechanism of carbon dioxide and carbonate ions through the already altered product. The process of passing a shell of already carbonated concrete is complex. The speed of carbonation is apart from the amount of CO2 in the environment also governed by the size and geometry of the porosity, the degree of water saturation, the type of cement/binder, the temperature, etc. Even concrete submerged in water or buried in soil will carbonate but at a slow speed due to biological degradation and the slowness of exchange reactions between water and the gases in the atmosphere.
To be able to calculate CO2 uptake one must consider the microclimate at individual concrete surfaces, concrete qualities and cement/binder types in a time frame. Thus approximations are needed. In the general case assuming a similar environment and concrete quality the carbonation rate slows down with the square root of time. By choosing the most common types of concrete structures, estimating the exposed surfaces in different environments and concrete qualities it is possible to get a good estimate ofthe rate of carbon dioxide uptake.
As a consequence of the rapidly decreasing rate of carbonation one can assume that most of the carbonation of concrete structures takes place during the first 50 years and after demolition as this will increase the surfaces dramatically. One must, however, also consider that the types of cement and quality of the concrete have changed and will change over time. Thus, there will be a difference between how much is taken up today and how much that will be taken up in 50 years from now. Concrete is a fairly modem material and most concrete structures still remain but we can expect the amount of demolished concrete to increase in the future. A guess is that a 100-year perspective most concrete structures that exist today will probably be demolished and most of the carbonate rock calcinated during cement production will be back as a carbonate rock.
To be able to calculate the carbonation rate some simplifications are needed. In this report concrete strength is used as a substitute for porosity and from literature data constants for different environmental classes are selected. The influence of different cements and additions is handled by correction factors.