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Déchets verre, waste

An oasis of waste reconverted into ceramic materials

Transforming industrial waste and unused by-products could make it possible to respond to issues of scarcity for civil engineering resources, recyclability and even reducing use of fossil fuels. Doan Pham Minh, process engineering researcher at IMT Mines Albi, explains several avenues for recycling and reusing materials explored by his work.

One man’s trash can be another man’s treasure. Turning rubbish into resources is the aim of the circular economy. And it is also the issue at the heart of the Innovative Ceramic Materials for Energy Storage and Construction (MACISEB)[1] project, launched in 2019, with the participation of researchers from IMT Mines Albi[2]. “Our objective is to transform inorganic, industrial waste and by-products, which can be found around us, into something that is useful for society,” describes Doan Pham Minh, process engineering researcher. The solutions identified as part of the project will then be transferred to companies in the Occitania region. From finding other uses for unrecyclable waste to replacing raw materials that are running out, the principle of ‘second life’ can be applied to a large range of unexpected situations.

Sand reserve seeks replacement for time to rest and recuperate

The French Agency for Ecological Transition (Ademe) reports that between 27 and 40 billion tons of sand are extracted each year around the world. It can be found in our buildings and windows, as well as our computers. “The demand for this resource is even more critical than that for noble metals. And the reserves are running out so quickly that they are arriving at breaking point,” emphasizes Pham Minh. Extracted from quarries or taken from riverbeds, natural sand is formed by the lengthy process of erosion. Too long, therefore, to meet society’s needs. However, this material is indispensable for the civil engineering sector (its main consumer) and therefore the economic stability of many countries.

Read more on I’MTech: Sand, an increasingly scarce resource that needs to be replaced

This is why the MACISEB project is seeking sand replacement products from inorganic by-products, i.e. industrial waste that is not currently being used. “The idea is not to completely change our means of manufacturing, but to replace a critical raw material using a circular economy approach,” specifies Pham Minh. With his team, the researcher has created resource maps for the entire Occitan territory. He identified and located deposits with high potential and similar properties to sand. He also ensured that these products are sustainable, by noting the quantity and availability of this waste. In this way, multiple candidates were selected, including glass residue.

During the recycling process, glass is ground up into grains fine enough to be reused by glass factories. However, a portion of this glass, too fine, coarse or contaminated, is not reused. “We are recovering this leftover glass to replace part or all of the sand needed to make ceramic bricks or tiles,” specifies Pham Minh. Sand from foundries, slag from blast furnaces, and ashes from biomass thermal power plants are also promising.

Using these materials, researchers have suggested formulas to create bricks and tiles with the same mechanical and thermal properties as those made with clay and natural sand. Moreover, the formulas comply with industrial specifications. The products are therefore guaranteed to be able to be manufactured using equipment that companies already possess, without extra investment. The first bricks will be made in 2022, and then tested by the Scientific and Technical Center for Building (CSTB).

From wind to heat: reusing wind turbine blades

The operating lifespan of a wind turbine is estimated at around twenty years. This means that the first French facilities are now arriving at end-of-life, and there will have to be more dismantling in the coming years. In short, recycling is becoming a major challenge for the wind energy industry. While the parts made from metal (pole and rotor) and concrete (base) recycle well, the blades – made from glass fiber mixed with organic resin – are not so lucky. Another part of the MACISEB project involves researchers recycling this waste into thermal storage materials. “Our objective is to reuse glass fiber from the blades to develop ceramics used by concentrated solar power (CSP) plants,” explains the researcher. This means of energy production transforms solar energy first into heat, then electricity. To do so, it uses systems made up of mirrors that concentrate the sun’s rays at one point, generating extremely high temperatures (from 200 to 1,500°C). The heat is transported by fluid, to propel the turbines and produce. It can be stored in ‘thermal batteries’, to later be released during the night to ensure continuity of service.

At present, thermal power plants store heat using molten salt – a mixture of potassium nitrate and sodium nitrate. “These compounds can also be found in agricultural fertilizer. There is therefore a conflict of use between the two sectors. However, there is currently no commercial alternative that is economically and environmentally viable,” explains Pham Minh. Transforming turbine blades into ceramics would therefore provide a new solution for this sector. With this in mind, researchers are developing materials capable of handling intense, repeated cycles of heating and cooling for multiple years. This solution would eventually make it possible to reuse a waste product that promises to grow. But it will also give a technological boost to the thermodynamic solar energy sector, which could allow it to establish itself in the renewable energy market. As part of the MACISEB project, this research is being undertaken by the PROMES laboratory, a partner of the project and academic reference body in the area of thermal storage. ART-DEV, partner and social sciences laboratory, is also looking into the social conditions for recycling wind turbine blades and the possibility of implementing a recycling ecosystem for the blades at a regional scale.

Industrial fumes: an idea to get the turbines going

Another application could make use of ceramic materials made from inorganic waste to capture heat. At present, the industry squanders over 30% of the energy it consumes in the form of so-called waste heat, released into the atmosphere in industrial fumes. Researchers at IMT Mines Albi are collaborating with company Eco-Tech Ceram, specialist in thermal storage, in order to recover this energy, store it and use it to supply industrial processes. For example, ceramicists and metal-working factories use high-temperature ovens, often running on natural gas. Reusing the heat captured from their fumes would make it possible to partially heat their equipment and therefore reduce their fossil fuel consumption.

Like for thermodynamic solar, the challenge is therefore to develop ceramic materials adapted for companies’ conditions of use. “Nevertheless, here another issue arises: industrial fumes contain pollutants. Such acidic, corrosive gases accelerate the aging of ceramics and therefore alter their performance,” explains the researcher. Moreover, the composition of fumes varies according to the industrial operations. The first thing to do will therefore be to characterize the kind of fumes, their temperature, etc. sector by sector, in order to develop sustainable materials while keeping costs under control[3].

Anaïs Culot

[1] Project funded by the European Regional Development Fund (ERDF), part of European policy aiming to strengthen economic, social and territorial cohesion in the European Union by supporting development in regions such as here, in the Occitania region.
[2] The project brings together researchers from the RAPSODEE center, the PROcesses, Materials and Solar Energy (PROMES) laboratory, the Actors, Resources and Territories in Development (ART-DEV) laboratory and the company Eco-Tech Ceram.
[3] This is part of the objectives of certain projects, Eco-Stock® solutions to recycle complex industrial waste heat (SOLUTEC, launched in 2021) and developing monolithic materials from local clay blends to reuse industrial waste heat in Occitania (CHATO, launched in 2021), led by IMT Mines Albi.

3D printing, a revolution for the construction industry?

Estelle Hynek, IMT Nord Europe – Institut Mines-Télécom

A two-story office building was “printed” in Dubai in 2019, becoming the largest 3D-printed building in the world by surface area: 640 square meters. In France, XtreeE plans to build five homes for rent by the end of 2021 as part of the Viliaprint project. Constructions 3D, with whom I am collaborating for my thesis, printed the walls of the pavilion for its future headquarters in only 28 hours.

Today, it is possible to print buildings. Thanks to its speed and the variety of architectural forms that it is capable of producing, 3D printing enables us to envisage a more economical and environmentally friendly construction sector.

3D printing consists in reproducing an object modeled on a computer by superimposing layers of material. Also known as “additive manufacturing”, this technique is developing worldwide in all fields, from plastics to medicine, and from food to construction.

For the 3D printing of buildings, the mortar – composed of cement, water and sand – flows through a nozzle connected to a pump via a hose. The sizes and types of printers vary from one manufacturer to another. The “Cartesian” printer (up/down, left/right, front/back) is one type, which is usually installed in a cage system on which the size of the printed elements is totally dependent. Another type of printer, such as the “maxi printer”, is equipped with a robotic arm and can be moved to any construction site for the direct in situ printing of different structural components in a wider range of object sizes.

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Pavilion printed by Constructions 3D in Bruay-sur-l’Escaut. Constructions 3D, provided by the author

Today, concrete 3D printing specialists are operating all over the world, including COBOD in Denmark, Apis Cor in Russia, XtreeE in France and Sika in Switzerland. All these companies share a common goal: promoting the widespread adoption of additive manufacturing for the construction of buildings.

From the laboratory to full scale

3D printing requires mortars with very specific characteristics that enable them to undergo rapid changes.

In fact, these materials are complex and their characterization is still under development: the mortars must be sufficiently fluid to be “pumpable” without clogging the pipe, and sufficiently “extrudable” to emerge from the printing nozzle without blocking it. Once deposited in the form of a bead, the behavior of the mortar must change very quickly to ensure that it can support its own weight as well as the weight of the layers that will be superimposed on it. No spreading or “structural buckling” of the material is permitted, as it could destroy the object. For example, a simple square shape is susceptible to buckling, which could cause the object to collapse, because there is no material to provide lateral support for the structure’s walls. Shapes composed of spirals and curves increase the stability of the object and thus reduce the risk of buckling.

These four criteria (pumpability, extrudability, constructability and aesthetics) define the specifications for cement-based 3D-printing “inks”. The method used to apply the mortar must not be detrimental to the service-related characteristics of the object such as mechanical strength or properties related to the durability of the mortar in question. Consequently, the printing system, compared to traditional mortar application methods, must not alter the performance of the material in terms of both its strength (under bending and compression) and its longevity.

In addition, the particle size and overall composition of the mortar must be adapted to the printing system. Some systems, such as that used for the “Maxi printer”, require all components of the mortar except for water to be in solid form. This means that the right additives (chemicals used to modify the behavior of the material) must then be found. Full-scale printing tests require the use of very large amounts of material.

Initially, small-scale tests of the mortars – also called inks – are carried out in the laboratory in order to reduce the quantities of materials used. A silicone sealant gun can be used to simulate the printing and enable the validation of several criteria. Less subjective tests can then be carried out to measure the “constructable” nature of the inks. These include the “fall cone” test, which is used to observe changes in the behavior of the mortar over time, using a cone that is sunk into the material at regular intervals.

Once the mortars have been validated in the laboratory, they must then undergo full-scale testing to verify the pumpability of the material and other printability-related criteria.

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Mini printer. Estelle Hynek, provided by the author

It should be noted that there are as yet no French or European standards defining the specific performance criteria for printable mortars. In addition, 3D-printed objects are not authorized for use as load-bearing elements of a building. This would require certification, as was the case for the Viliaprint project.

Finding replacements for the usual ingredients of mortar for more environmentally friendly and economical inks

Printable mortars are currently mainly composed of cement, a material that is well known for its significant contribution to CO₂ emissions. The key to obtaining more environmentally friendly and economical inks is to produce cement-based inks with a lower proportion of “clinker” (the main component of cement, obtained by the calcination of limestone and clay), in order to limit the carbon impact of mortars and their cost.

With this in mind, IMT Nord-Europe is working on incorporating industrial by-products and mineral additives into these mortars. Examples include “limestone filler”, a very fine limestone powder; “blast furnace slag”, a co-product of the steel industry; metakaolin, a calcinated clay (kaolinite); fly ash, derived from biomass (or from the combustion of powdered coal in the boilers of thermal power plants); non-hazardous waste incineration (NHWI) bottom ash, the residue left after the incineration of non-hazardous waste, or crushed and ground bricks. All of these materials have been used in order to partially or completely replace the binder, i.e. cement, in cement-based inks for 3D printing.

Substitute materials are also being considered for the granular “skeleton” structure of the mortar, usually composed of natural sand. For example, the European CIRMAP project is aiming to replace 100% of natural sand with recycled sand, usually made from crushed recycled concrete obtained from the deconstruction of buildings.

Numerous difficulties are associated with the substitution of the binder and granular skeleton: mineral additions can make the mortar more or less fluid than usual, which will impact the extrudable and constructable characteristics of the ink, and the mechanical strength under bending and/or compression may also be significantly affected depending on the nature of the material used and the cement component substitution rate.

Although 3D printing raises many issues, this new technology enables the creation of bold architectural statements and should reduce the risks present on today’s construction sites.

Estelle Hynek, PhD student in civil engineering at IMT Nord Europe – Institut Mines-Télécom

This article has been republished from The Conversation under a Creative Commons license. Read the original article (in French).