In this article we analyse recarbonation in concrete production as a means of reducing carbon dioxide emissions. Solutions from the Mapei RE-CON range can help transforming waste into carbon dioxide-absorbing aggregates, potentially reducing the environmental impact and costs of concrete production.

Recarbonation of transformed waste streams from ready-mix concrete production

We explain the concept of recarbonation and how it can be used to give a better comparison of a building’s total carbon footprint over its service life. Finally, we give examples of CO2 savings potential by using RE-CON line solutions, such as RE-CON ZERO EVO and RE-CON DRY WASHING, to transform waste streams, such as returned concrete and cementitious residues in concrete trucks, into concrete aggregates. When used in new concrete, they increase even more the total potential of absorbed CO2.


Concrete as a building material: pros and cons

Concrete is the second most used material in the world, after water. It is also well known that cement (or more correctly Portland clinker cement) is a major contributor to global CO2 emissions. Compared with wood as a building material, concrete is “front heavy” because of these emissions. Unlike wood, which binds CO2 through the growing of forests, cement and concrete start their service life in a building with a larger carbon footprint than wood. Concrete, however, is still the preferred building material because of its higher strength, fire resistance and durability compared to wood. We simply cannot build everything we need in society solely from one of those two materials. The best material for this purpose should also be chosen from a technical, economic and environmental perspective. It is important, therefore, to consider all the environmental aspects of concrete. It is true to say that concrete has a higher initial carbon footprint compared to wood, but the durability of a building made with a concrete structure is higher than the equivalent massive timber structure. Higher durability means a longer service life. This means the total emissions over 100 years needs to be compared. In this timeframe, a wood structure will require more energy and generate higher emissions from maintenance and renovation over a 100 year life span. In addition to this, one important factor is overlooked in a 100 year perspective of carbon footprint comparisons: the capability of concrete to re-absorb CO2 from the atmosphere. A report by Stripple et al punto1 concludes that up to 20% of CO2 emitted from the calcination process can be reabsorbed into a concrete structure due to re-carbonation, i.e. limestone being formed from the reaction between calcium hydroxide and CO2 in the concrete structure. Figure 2 illustrates this cyclical process.

Figure 1 - An example of a concrete structure: the Heydar Aliyev Center in Baku designed by Zaha Hadid

Calcination: burning limestone into cement

The chemical process whereby carbon dioxide is driven out of the limestone raw material (calcium carbonate) is called “calcination”. For every ton of pure Portland clinker produced by the burning of limestone rock, around 500 kg of CO2 is released through the calcination process:

CaCO3 + Heat → CaO + CO2

In addition to this there is, of course, the energy consumed to produce the heat in the cement kiln, but this is a variable unconnected to this circle model.


Concrete reaction

The mixing of cement and water (with sand, aggregates and admixtures) into concrete forms a new step in the carbon circle: calcium hydroxide.

CaO + H20 → Ca(OH)2

Figure 2 -The carbon circle of concrete

Secondary cementitious materials (SCM)

Basically, what all SCMs have in common is that they contain silica dioxide (SiO2). This silica must
also be amorphous, i.e. reactive. In nature, silica dioxide as a rock mineral has a stable crystalline
structure that does not react easily with other substances. But if the SiO2 has been formed through
heating and then rapid cooling, like a volcanic eruption directly cooled by air or through industrially
produced materials like fly ash or slag, it has a less rigid crystal structure and can, therefore, react with

the calcium hydroxide from the previous step in the process. The reaction is described as:

Ca(OH)2 + SiO2 → CaSiO3 + H2O

and is called a “secondary pozzolanic reaction” from the term “Pozzolan” which comes from the town
of Pozzuoli in Southern Italy whose natural volcanic sand was already being used in ancient times by the
Greeks and Romans to make concrete. It is an interesting observation that we too, almost 2000 years
after the construction of buildings such as the Pantheon in Rome, have now started to use more and
more of these secondary cementitious materials in our modern concrete to replace Portland clinker.

Closing the circle: limestone formation in concrete

When concrete is exposed to CO2 in the atmosphere, this chemical reaction of carbonation takes place:

Ca(OH)2 + CO2 → CaCO3 + H20

This closes the circle and limestone is formed again from calcium hydroxide reacting with carbon dioxide. The process involving carbonate and hydroxide ions occurs in several steps, as described by Stripple et al. The speed of the process (carbonation rate) depends on many factors, such as the strength class (water/cement ratio) of the concrete. Water plays a major role in the chemical reaction, so moisture and ambient humidity are important. Finally, the level of exposed surface of raw concrete in the structure also determines how much of the total carbonation potential is utilized during the service life of the concrete.


Taking advantage of recarbonation in concrete

We know a lot about the recarbonation process in concrete because it can be negative from a durability aspect. Before concrete recarbonates, it has a high pH which acts as protection against corrosion of steel reinforcement. The carbonation front moves from the surface towards the center at a rate of millimeters per year. Carbonated concrete has a lower pH and, therefore, has lower protection of the steel against corrosion. It is important to design a covering of the steel reinforcement which is thick enough to last for the duration of the designed service life of the concrete structure. If the correct measures are taken, or if concrete is reinforced with a material other than steel (e.g. synthetic fibers), recarbonation will make a positive contribution into lowering the long-term carbon footprint of concrete without shortening its service life. The rate of carbonation depends on several factors. The strength class (water/cement ratio) and exposure factors combined with the environment are the main ones. The optimum combination of access to atmospheric CO2 and ambient humidity determines the conditions for recarbonation speed in mm/year. From these basic technical conditions and known calculation methods, a prediction can be made of how much CO2 a concrete structure can absorb in a given period of time. Löfgren2 explains how this can be inserted into a Life Cycle Assessment (LCA) model for a concrete structural element. By using the guidelines from industry standard EN 16757, annex BB and further details from CEN/TR 17310:2019, he illustrates an example of recarbonation potential shown in Figure 3. For example, the yellow curve represents an interior concrete wall made from C30/37 concrete. As much as 34% of the CO2 emissions stemming from the production of the cement for that wall will be absorbed in 100 years if the wall thickness is 100 mm and the concrete remains unpainted and exposed to air on both sides.

Figure 3 - Carbon dioxide uptake in percent of emissions from the cement from production of indoor wall after 100 years, without surface (“open”) and with painted surface, for different wall thicknesses and single or double sided exposure.

Re-con line solutions: taking recarbonation one step further

The solutions in the Mapei RE-CON line help concrete producers transform waste streams of returned concrete and truck washout slurry into recyclable aggregates. This means considerable cost savings from reduced purchase of virgin material, transport in and out of the concrete plant, high operational costs for washing and reclaiming and, finally, reduced landfill costs for residual waste. Figure 4 shows how waste streams are replaced by the RE-CON line products and methods. The dry, dust free and low noise method RE-CON ZERO EVO reclaims returned concrete by powder into a granular material. RE-CON DRY WASHING utilizes this material for cleaning dirty concrete trucks by absorption rather than washing with large amounts of water. The fines are agglomerated into aggregates rather than becoming a waste disposal problem for the concrete producer. Finally, the liquid admixtures in the RE-CON AGG range mitigate the challenging properties of recycled aggregates and make it possible to produce concrete with recycled aggregates without increased water demand and increased cement dosage
Figure 4 - Schematic view of the use of RE-CON line solutions and how they contribute to reducing the cost and negative effects of waste stream handling

Reclaimed aggregates from waste with increased carbonation potential

The RE-CON ZERO EVO and RE-CON DRY WASHING processes yield a reclaimed aggregate material with higher cementitious material content. This material has been studied in a major research project in Norway. Led by Mapei, the RECONC project focused on studying the carbonation potential of the aggregates directly after the RE-CON ZERO EVO process and after 8 cycles of RE-CON DRY WASHING. The number of cycles you can re-use the aggregates in the RE-CON DRY WASHING process depends on local conditions but is generally between 10-20. To study the exact potential, particles of a different age in the process were placed in carbonation chambers and subjected to accelerated levels of CO2 in gas form: 4000-5000 ppm compared to a normal atmospheric conditions of 400 ppm. This was carried out to simulate long term exposure over 50-100 years. As shown in figure 5, the RE-CON ZERO aggregates and RE-CON DRY WASHING aggregates showed different levels of CO2-uptake during these tests. The RE-CON ZERO EVO aggregates were only subjected to cementitious material once in the treatment process. The RE-CON DRY WASHING aggregates were subjected 8 additional times, which explains the higher CO2 uptake. The results showed recarbonation potential of the RE-CON ZERO EVO aggregates of 30 kg CO2 per tonne of material and the same aggregates after 8 cycles of dry washing had an additional 7,5kg of CO2 per tonne of recarbonation potential.
Figure 5 - Carbon Dioxide uptake as studied in accelerated carbonation chambers.

Implementing lab test results in lca simulation

With the first lab results available, the project team decided to use the data in a Life Cycle Assessment (LCA) simulation of a concrete mix design where a part of the aggregates was replaced with RE-CON DRY WASHING aggregates, using the nomenclature and methodology described in EN 15804. As shown in Figure 6, at the Production phase, there is a small decrease in the total GWP100 value per cubic meter of concrete, because RE-CON DRY WASHING aggregates have a lower GWP100 per ton of aggregate compared to virgin raw aggregate. During the 100-year use phase, the C35/45 concrete containing natural aggregates has a carbonation potential of 100 kg of CO2. The actual amount would be reduced by the conditions of exposure etc., as described earlier in this article. Simulations using 40% or 100% replacement of aggregates shows an increased theoretical potential of recarbonation after 100 years. The increase is significant and could contribute to making a concrete mix design more competitive from a total lifetime carbon emissions perspective.
Figure 6 - Recarbonation levels analyzed in a Life Cycle Assessment simulation of a concrete mix design.

Cost savings and CO2 reductions: a win win situation

The RE-CON line offers a methodology to reduce waste from concrete production by transforming it into a CO2 absorbing aggregate. Compared to other solutions, RE-CON line has no need for expensive equipment or complex mechanical processes. It is a low-cost, low-energy and dust free method for greener and more cost-effective concrete production. It will be interesting to follow the development in the industry in coming years. Will concrete producers be able to declare the recarbonation potential in the LCA Use phase of a certain mix design? Or will new accelerated carbonation technologies make it possible to recarbonate the aggregates from RE-CON ZERO EVO and RE-CON DRY WASHING so that a “carbon-negative” aggregate is achieved already in the LCA Production phase? At Mapei, we follow this development closely and contribute continuously with new products, solutions and research and development.

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