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To what extent should authorities promote the use of recycled aggregates for concrete?

Over the last ten years, politicians and stakeholders alike have focused on better production and use of energy. Nevertheless, energy is not the only resource which can have environmental, economic and geopolitical impacts. All raw materials can play a role, albeit to a different extent. For example, whilst rare earths are scarce in Europe, many other raw materials, such as limestone and aggregates are abundantly available.  In my view, it is therefore wrong to consider that a strategy which is valid for one material can apply to all others. In addition, to consider recycling as the objective for “resource efficiency” would also lead to erroneous decisions. This is why policies should promote the efficient use of locally available materials for a desired purpose.
Due to its multitude of positive attributes, concrete is the second most used material in the world (after water). As a result, it is a raw material intensive construction product. Every year in Europe more than 3200 million tonnes of both primary and secondary aggregates are produced, of which more than a third are destined for concrete production, and the rest for foundations and road sub-base (for example).  In terms of waste, around 350Mt of concrete construction and demolition (C&D) waste is produced. These figures show that, if 100% of concrete were to be recycled, it would cover around 12% of the total amount of aggregates needed. As a result, primary aggregates are still needed to produce concrete. As mentioned, aggregates are abundantly available in Europe: aggregates are a natural and inert material, which pose neither scarcity nor security problems for Europe. This clearly demonstrates that the use of primary and secondary aggregates go hand in hand.

But the question which now remains is how and where recycled concrete can be used most efficiently.

The first element to be taken into account is transport. If concrete waste has to be transported first of all to a centralised recycling plant and then on to a concrete manufacturer, the impact of the energy consumed and emissions resulting from such transportation would probably be higher than those of the same amount of natural aggregates extracted close to the concrete plant. Thus a first priority should be to minimise this distance to ensure an efficient use of concrete C&D waste.

The second element is the quality of the secondary aggregate used in concrete. The necessary grinding, retrieving and sorting secondary aggregates can be an energy intensive, mechanical operation. Concrete manufacturers need a raw material of a sufficiently good quality to ensure that the final product meets certain mechanical and safety properties.  Concrete is indeed very easy to recycle and reuse. Precast concrete factories can be considered as “closed loop factories”; because they can easily use concrete recycled from waste produced in-house (meeting both the transport and quality criteria outlined above).

Impurities are another problem which needs to be tackled.  They occur due to contamination from other materials during the demolition phase. They can pose problems in the manufacturing of new concrete and often result in more binders having to be added to the mix, which is neither an economic nor environmental solution! It is therefore important that concrete is not treated as a “recycling bin” for all C&D waste (including other materials). Correct dismantling of components at the demolition site itself can ensure that the reuse of concrete is simple, economic and environmental friendly.

As a result, very often secondary recycled aggregates are used as a road sub-base or foundation (for example), for both financial and environmental reasons. Using recycling aggregates in this way require less transportation and mechanical operations. They also offer the added benefit of stability when used in this way, thanks to their residual binding properties.

In conclusion: whilst concrete can be 100% recycled, we must make sure that we recycle it wisely.

Tags: resource efficiency, sustainable construction, recycling, reuse

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