Concrete Rebirth: Building a Sustainable Future through Recycling

Recarbonation of rubble from demolition waste

With the objective of carbon neutrality set by the concrete sector of California, one interesting avenue can be explored: recarbonating rubble. This process consists of leaving the rubble from demolition in the open air, or by subjecting it to accelerated treatment to recarbonate this rubble. CO2 is then reabsorbed into the rubble, which will harden and form better quality aggregates. This way less dumpsters are needed to send construction waste to the local landfills.

This phenomenon of concrete trapping is very important, because it makes it possible to balance the carbon footprint of concrete a little more, even if we do not necessarily achieve carbon neutrality. Moreover, this phenomenon also occurs when the concrete is in the form of a structure (building, work of art, etc.), but the reaction is less important, because it takes place on small surfaces where there is an interaction between air and concrete.

However, when the concrete is demolished and in the form of rubble, the exchange surface with the air is even greater and the recarbonation reaction accelerates.

How does carbonation work

During the manufacture of concrete, the water and cement mixture forms hydrates which will contribute to the strength of the concrete. When these hydrates come into contact with CO2 in the air, an irreversible and slow carbonation reaction occurs. The CO2 will penetrate the concrete through its numerous pores and will dissolve. A drop in the concentration of hydroxide ion HO- then occurs, stabilizing the pH of the medium around 8. The hydrates will dissolve.

Dissolution of hydrates

The released calcium ions will then precipitate with the carbon dioxide, giving calcium carbonate:

Calcium ions precipitating with carbon dioxide

Then, when the calcium hydroxide is completely consumed, the environment will become acidic, which could lead to corrosion of the reinforcements in the case of reinforced concrete. We can then delay the moment when the carbon dioxide comes into contact with the first layer of reinforcement. The material would then capture between 10 to 15% of the CO2 initially released during the manufacture of cement.

Schematic of carbonation

Carbonation does not only apply to concrete, but also to all construction materials. It is beneficial for concrete, because it reduces its porosity and improves its mechanical characteristics.

A new state research project was launched in 2020 in order to work on the storage of CO2 by carbonation of recycled concrete, which would facilitate the reuse of recycled concrete aggregates in construction.

Change in properties between recycled concrete and conventional concrete

Recycled aggregates and sand modify the physicochemical characteristics of concrete. Beyond an incorporation of 10% of recycled aggregates, there may be a reduction in the mechanical compressive strength, Young’s modulus and tensile strength by splitting. Additionally, it can modify the drying kinetics and gas permeability. The fresh concrete would thus be more manageable.

Hardened concrete may undergo the formation of thaumasite (sulfate reactions) and swelling. On the other hand, there is almost no cracking and the adhesion of the concrete to the metal reinforcements is correct.

As for frost resistance, this will depend on the original concrete of the recyclates, and for fire resistance, recycled concretes are more resistant, because they have lower internal temperatures.

Experimental projects (compression slabs, parking lot paving, sails, bridge elements) were carried out in order to consolidate the thesis of the use of recycled aggregates. The sites, with more or less high substitution rates (up to 100%), and carried out for the most part with C25/30 concrete, and did not present any particular problem in terms of implementation and durability, even if we observe a slight drop in resistance.

However, additional studies are still necessary, particularly concerning the structural behavior of beams, columns and slabs.

What are the objectives of the concrete sector

For years, the concrete sector has aimed to increase the recycling of deconstruction concrete in the production of new concrete and to reduce the share of non-recycled deconstruction concrete:

it set up a recycling branch in the 1990s
it encourages research projects such as RECYBETON, carried out in recent years
it changes the standards: in 2008, introduction of an aggregate standard, in 2011, the NF P18-545 standard, specifying the characteristics and properties of recycled aggregates, in 2012, the concrete standard authorizing the use of recycled aggregates from deconstruction for the manufacture of concrete

The state of California had set the threshold of 80% for the recovery of construction waste for 2030, then setting a new objective for 2035: reaching 90 %.

The state aims to deepen knowledge on the reuse of materials from deconstructed concrete as elements of new concrete, in order to reduce the use of dumpster rentals. This research has made it possible to radically increase the rate of incorporation of recycled aggregates into concrete, for example, in order to preserve natural deposits.

The question of standards

Following the research mentioned above, standards for building and civil engineering as well as standard for structural concrete for buildings and structures could evolve. Indeed, the standards allow 30% recycled aggregates and 20% recycled sand, while in reality, following numerous studies carried out, it is possible to use at least 50% recycled aggregates and 20-30% sand.

Demolition waste

Today, following numerous research projects, the construction industry has bounced back to meet the various objectives of the ecological transition, particularly in terms of recycling and valorization of concrete. Thanks to the reuse of aggregates from deconstruction concrete, and in particular in the manufacture of new concrete, the sector tends to evolve from a linear economy to a circular economy, reducing the use of raw materials, but also CO2 emissions.