Thixotropy, Nanoclay and the Optimal Parameters of 3D Printed Concrete

In ‘The Effect of Material Fresh Properties and Process Parameters on Buildability and Interlayer Adhesion of 3D Printed Concrete,’ international authors strive to understand more about materials and parameters in relation to concrete—specifically in terms of buildability and inter-layer adhesion properties.

3D printing with concrete is being used by companies and researchers around the globe today as they hope to harness all the classic benefits of 3D printing, from affordability to greater speed in production, and the ability to create more complex geometries—often meaning that products and prototypes not possible before can now be fabricated. And although there is a growing sense of acceptance of the technology, the researchers explain that there is still little data regarding materials and process parameters.

“A soft, flowable material is easy to extrude,” explained the researchers. “On the other hand, a stiff material can sustain the weight of more layers, although it needs more pressure for the extrusion. However, the stiffness of a cementitious material changes with time due to ongoing hydration. As a result, it may not be extrudable after its dormant period. In addition, this sometimes results in poor interface bond strength due to lack of moisture, mechanical contacts, and the presence of pores in the substrate.”

A lack of surface moisture can affect inter-layer strength, as well as process parameters. Inter-layer adhesion is a common challenge, and researchers have suggesting numerous fixes, from increasing mechanical contract between the layers or even spraying cement paste in the affected zone.

Demonstration of 3D printability of (a) control mix (CM); (b) 0.5% clay modified NM mixture.

For this project, the researchers added a ‘small amount of nanoclay’ to add both strength and thixotropy. The nanoclay showed anisotropic qualities, and showed the greatest compressive strength when tested in the direction of the layer deposition.

“Nanoclay carries a negative charge on its faces and a positive charge on its ends. During the material flow, it tends to separate from each other by the electrical repulsion between similar charges. On the other hand, at rest, it flocculates by oppositely charged ends, while increasing the yield stress and viscosity,” stated the researchers, pointing out that nanoclay is not helpful in increasing the stiffening rate.

Schematic of rheology modification for 3D concrete printing

The researchers noticed macro pores, theorizing that they were weakening the 3D printed, increasing with materials higher in thixotropy.

“Reducing the standoff distance below the nozzle opening size (width) had a positive impact on improving the bond strength of the NM mix, which can be explained by the decreased porosity in the interface zone,” concluded the researchers. “The impact of the nozzle standoff distance was found to be more pronounced for the material with a higher yield stress value.

“In future work, other parameters such as structuration rate, roughness, environmental conditions, and the effect of chemical additives will be studied to gain new insights into interlayer bond strength in the concrete printing process.”

3D printing with concrete continues to rise in popularity, as users—many in the construction world—innovate with projects such as 3D printed bridges, lightweight structures, and hardware for disaster relief. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.

(a) static yield stress; (b) tensile bond strengths of CM and NM mixtures.