Behind the scenes of decarbonated concrete

Reinforced concrete, kerbstones or trench concrete… There’s a formula for every purpose! Because behind the apparent simplicity of this universal material lies a veritable alchemy of materials.

At Néolithe, our thinking starts well in advance of the worksite. What if the decarbonisation of concrete could be achieved through a new generation of aggregates made from non-recyclable waste? That’s the idea behind Néolithe’s project.

At the heart of this recipe is an essential ingredient: aggregates made from non-recyclable waste, known as final waste.
How can we ensure the strength, compactness and durability of this decarbonated concrete?

We spoke to Martin Redonnet, Aggregates Applications Technician, and Ronan Hamelin, Aggregates Sales Manager.

“Our aggregates are made from non-recyclable waste’ explains Ronan Hamelin. “It’s a real ecological lever, but it also imposes requirements in terms of formulation.”

Called Anthropocite®, this aggregate is the result of the Accelerated Fossilisation^® process developed by Néolithe. Intended for non-structural concrete, it meets precise criteria, starting with a minimum density of 1.5 t/m³, which is essential to guarantee its performance. And not only does it replace some of the natural aggregates, it also stores carbon.

To incorporate an aggregate of this type, it first has to be finely characterised. “We start with water absorption tests to measure its porosity’ explains Martin Redonnet. “This has a direct influence on the amount of water needed to formulate the concrete. An aggregate that is too porous will absorb too much water, which will distort the dosages.“

Once the properties of the aggregate are known, the adjustment work begins. “Each concrete has its own function and its own requirements, whether it’s reinforced concrete, kerbstones or trench concrete,’ explains Ronan. Incorporating up to 20% Anthropocite^® aggregates, while ensuring mechanical performance and workability, requires a tailor-made formulation.

This process involves the production of test specimens tested at different ages (7, 14 and 28 days) to check the concrete’s compressive strength. ‘We also measure the consistency of the fresh concrete using the Abrams cone,’ adds Martin. “This tells us whether the concrete is fluid enough or too firm to be used correctly.”

At the heart of this formulation is the water/cement ratio, on which both the strength and workability of the concrete depend. “Too much water, and the concrete loses strength. Not enough, and it becomes unmanageable“, Martin sums up. Finding the right balance is a delicate art.

Each formulation undergoes a battery of tests in the laboratory and then in the field. ‘The air occlusion test is particularly important,’ adds Martin. “It enables us to anticipate the resistance of concrete containing Anthropocite® to freeze/thaw cycles, a crucial criterion for its durability.”

There’s no substitute for in situ testing on pilot sites. ‘This allows us to observe how the concrete actually behaves,’ stresses Martin. “How it positions itself, how it reacts to weather conditions, whether there are signs of segregation*… These are elements that we can’t always anticipate in the laboratory.” This feedback from the field is essential for finalising a formulation before it goes to scale.

* Concrete segregation refers to a processing defect that manifests itself in the separation of the various constituents of fresh concrete, mainly the coarse aggregates (pebbles) and the cement paste (a mixture of cement, water and fines).

The moment of pouring sets the seal on all the technical choices made. And it’s also the moment when real conditions (rain, heat, wind) can call into question the balances found upstream.

‘This is when we see whether all the choices made in the laboratory were the right ones,’ says Martin. The quality of the aggregates then becomes decisive. “Their size, shape and cleanliness all influence the fluidity of the concrete, its compactness, and therefore its final strength.” A well-calibrated aggregate prevents segregation, ensures good cement distribution and guarantees a durable finish.

Far from being a simple substitute, Anthropocite^® embodies a new approach to the manufacture of decarbonated concrete, with no compromise on quality, in the context of its uses.

And the approach doesn’t stop there. “We’re still exploring new sources of waste and studying the use of our aggregates in structural concrete,’ concludes Martin. “This will require further tests and adjustments. But it’s at the very heart of our mission: to move concrete towards a more resilient future!“