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Geopolymer instead of cement: New recipe for CO₂-free mortar for 3D printing

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Geopolymer instead of cement: New recipe for CO₂-free mortar for 3D printing

Geopolymer instead of cement: New recipe for CO₂-free mortar for 3D printing

The production of concrete causes around eight percent of global CO2 emissions. 3D printing of buildings promises – at least in theory – an improvement: since the walls and ceilings can be designed to be more load-bearing, they require less material.

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However, there is also an opposite effect: Since the mortar has to set very quickly with 3D printing, it needs a higher proportion of cement that hardens quickly – and this is particularly harmful to the climate. The bottom line is that 3D-printed concrete often causes more greenhouse gases than conventional ones.

“The critics of 3D printing are currently still right,” says Thorsten Stengel, professor of building materials and construction chemistry at Munich University of Applied Sciences. “But the research is definitely going in the direction of using fewer binding agents.”

An example of this is the Texas start-up Hive3D. It has presented a low-carbon mortar for 3D printing with its partners Eco Material and Green Cement. It is said to reduce the CO2 footprint by 93 percent compared to conventional mortar – and at the same time be cheaper and more robust.

The special thing about the mortar: The binder used is not lime-based “Portland” cement, which releases a particularly large amount of CO2 during its production, but a so-called “geopolymer” called pozzolan. This is a mixture of different, mostly silicon-based minerals. It is named after the town of Pozzuoli, near Naples. Even the ancient Romans extracted pozzolana from the volcanic ash there and mixed it with their “Opus caementicium”. In doing so, they created buildings like the Pantheon that have survived for thousands of years.

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Even today, pozzolana is a widely used additive for cement. However, it is usually no longer obtained from volcanic deposits, but from the fly ash from coal-fired power plants or from blast furnace slag from steelworks. On average, around half of the cement in Europe consists of such climate-neutral residues. Higher proportions of Portland cement are only used in the case of particularly high requirements – such as with 3D printing.

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However, Portland cement cannot usually be completely replaced. “If you mix pozzolan powder with water, nothing happens at first,” says Thorsten Stengel. “In order for it to react, it needs an activator – for example in the form of a strong lye.” This is exactly what Portland cement provides: When mixed with water, it releases the strongly alkaline calcium hydroxide (CaOH). Only now can the minerals in the pozzolan network.

However, Hive3D and its partners are now reporting that they have developed mortar for 3D printing entirely from pozzolan – completely without Portland cement. How is that possible?

“The basic material was originally developed by our partner for quick road repairs,” the start-up replies when asked. “But we modified it extensively to make it a 3D printable mortar.”

Apparently, the correct pre-treatment of the fly ash is a decisive factor in this. “We use a unique and innovative process that increases the reactive surface of the particles and optimizes their size distribution,” writes Eco Material on its website (in this case, however, still based on a type of cement that only contains up to 60 percent fly ash consists). And Hive3D shares via email: “We can use most types of fly ash, but they need the right particle properties for their cement-like properties to come out.” This does not work with all fly ash, but with most it does.

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Another adjustment screw are the additives. After the fly ash has been processed and provided with additives, it can be activated “just with water”, explains Hive3D. “The additives contain the necessary chemicals to make it hyper reactive. It has a setting time of 45 seconds.” Thomas Bier, Professor of Construction Chemistry at the Bergakademie Freiberg, comments: “This is interesting because one of my doctoral students has just submitted a thesis in which the additives are also mixed in powder form, so that afterwards only water has to be added.”

In order for the geopolymer to become printable mortar, sand has to be added. Here, too, Hive3D is not particularly picky. “We can use any type of dry sand,” writes the start-up. In Texas, Hive3D has already printed some 40 to 80 square meter houses with the new geopolymer cement.

Other advantages of the geopolymer: When fully cured, the mortar is denser, less prone to cracking and correspondingly more robust, writes the manufacturer. In addition, it can be produced at normal ambient temperatures and does not have to be fired at high temperatures like Portland cement. This reduces energy consumption and costs accordingly. Overall, the cement replacement is only a third as expensive as comparable commercial solutions.

“The concept doesn’t sound stupid at first,” says Thomas Bier. Because it is precisely the properties that tend to slow down geopolymers in conventional concrete construction that could be helpful in 3D printing: A rather viscous consistency and a short processing time often meant that the geopolymer concrete was not evenly distributed in the formwork. With 3D printing, however, both are desirable. The high pH value, which Bier says makes handling “very uncomfortable”, is also not such a big problem with 3D printing because people have less contact with the material.

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But one problem remains: to the extent that coal-fired power plants are shut down, the supply of fly ash also falls. Hive3D is aware of this: “Our partners can also harvest old fly ash from heaps, so supplies will not run out for a long time. We are also experimenting with volcanic ash, which is readily available.”

Update, 8/3/2023, 9:55 am: Addition to the additives used.

(hrm)

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