I worked for more than a decade in the circular economy and I realized a surprising fact: 500 million tonnes of construction waste are produced each year in the EU. That’s enough trash to fill the whole nation of the Netherlands. It motivated me to find greener building methods and environmentally friendly materials.
One solution is an emerging trend called 3D concrete printing (3DCP). There are several names for it, including additive manufacturing and digital manufacturing. Concrete 3D printing is done by layer-by-layer extrusion. It is a unique field that brings together multiple fields, such as chemists, software developers, designers, structural engineers, automation experts and above all creative visionaries, to formulate the innovations of tomorrow.
The circular economy must also be implemented in this new area. In general, 3D concrete consists of cement, aggregates, water and admixtures. In principle, the substitution of these components by secondary raw materials will eventually be possible. However, most commercial 3D concrete does not use any industrial lateral flow, but there is great potential. OPC (Ordinary Portland Cement) is the most commonly used binder, fine sand with a size of up to 2 mm is used as aggregate.
In the 3DGREEN-CON consortium and R&D project, co-funded by the European Innovation Council (EISMEA), we industrially developed Finland’s first green 3D concrete. Its ecological character comes from the substitution use of recycled aggregates.
Finland’s first green 3D concrete
In my opinion, the use of OPC can be reduced by the use of so-called SCM (Substituted Cementitious Materials). On an industrial scale, the main secondary raw materials (industrial waste), for example blast furnace slag, silica fume and fly ash, can be used. All of these are available in large quantities, especially in developing countries. Ready-to-use cements with these by-products are also available and are found under the names CEM II and CEM III cements. Other SCMs with a stable supply can be limestone and calcined clay. Limestone derived as an industrial by-product is of particular interest for application given the potential for circularity.
Limestone powder has long been used as a filler component in the binder. The effect of limestone powder on rheology depends on the physical characteristics of the particles, eg fineness and surface roughness. To improve the workability of fresh cementitious materials, an appropriate amount of limestone powder with a similar or coarser grain size compared to Portland cement can be added to the mixture. The filler effect is the main phenomenon influencing cement hydration. Replacing a minor amount of Portland cement with limestone could accelerate hydration at early age due to increased nucleation sites provided by the surface of the limestone particle.
Calcined clay SCM in Portland cement could bring many benefits to 3DCP. Perhaps the most important factor is the abundance of accessible clay reserves. Kaolin clay, technically the most suitable, is rich in tropical and subtropical environments, that is to say in India and Southeast Asia]. The calcination temperature for the production of calcined kaolinitic clay is 700-850°C, which is considerably lower than that for the production of portland cement clinker (1250-1450°C). However, more research is needed for commercial viability.
OPC contains clinker and gypsum. Virgin gypsum can be replaced with another industrial by-product of flue gas desulphurization in power plants.
The world of alternative green cements for 3D concrete is even more exciting, especially given the sustainability narrative. One of the most sought after green cements is CSA (Calcium Aluminate Cement).
CSA has 49% less CO2 emissions during its production compared to OPC. The production of CSA clinker takes place at a temperature of 1250°C, which is 200°C lower than the production of OPC. Additionally, CSA clinkers formed at low temperatures are much easier to grind, resulting in additional energy savings. The recent trend to produce CSA from industrial sectors rich in Calcite, Silica and Alumina acts in favor of the circular economy.
CSA gives an interesting rapid setting property. CSA can be used as a 100% substitute for OPC or as a partial substitute depending on the system. A combination of OPC with 7% CSA substitution was used to create 3D concrete. This area still needs considerable research for commercial viability.
In my 7 years in green construction, we have studied many of these cement systems, from producing green cement using industrial waste to different routes of application with large companies in various sectors such as UPM, Stora Enso, Fortum, etc. raw materials.
Other interesting alternative binders are CSAB (Calcium Sulpho Aluminate Belite) and MgO reactive cements. They need significant research into Concrete 3D printing applications to determine their unique application potential.
Once the 3D printed object reaches end of life, there are several alternatives. It can be crushed and used as an aggregate in concrete, that is, enter the circular loop. Circular materials change every aspect of construction and affect the entire life cycle of a building
Circular materials change every aspect of construction and affect the entire life cycle of a building
During my teaching sessions at a German university, I was lucky enough to see that there was enough popular interest in these materials, with young talents finding an interesting field in them. Not only did they understand the underlying concepts of technology and sustainability that I taught, but they actively promoted them by going further and questioning the possibility of creating 3D printed habitats using local circular materials. It really is a step in the right direction.
3D printed habitats using local circular materials
Considering the EU Green Deal and the need to decarbonize, use waste and adopt a new manufacturing system, we need to think in terms of circular economy from the start, especially for new trends like 3D concrete. I am optimistic that our joint efforts would create an inclusive and sustainable society.
I invite European research and technology organisations, universities, companies, public bodies and funding bodies to collaborate and boost the innovation, implementation and application of this new emerging trend in technology. industry 4.0.