The world of construction is undergoing a seismic shift with the advent of digital fabrication. One particular innovation that has gained significant attention over the past two decades is 3D concrete printing. 3D concrete printing holds the promise of reducing construction costs, enhancing production rates, expanding architectural possibilities, and improving safety on construction sites. In recent years, various techniques have emerged, including selective binding particle-bed technology, smart dynamic casting, and the widely recognized extrusion-based technology, commonly referred to as 3D concrete printing. Despite its transformative potential, the practical implementation of 3D concrete printing faces two critical challenges: achieving optimal material performance and precise control over the stiffening rate of the concrete. These factors are pivotal for the seamless progress of 3D printing. Effective transportation of concrete in 3D printing necessitates high fluidity and a sufficiently long open time without premature setting. Simultaneously, the concrete must possess early-age strength to support the weight of subsequent layers. Striking the right balance between fluidity and stiffening rate is a complex undertaking. Without additional measures, the maximum yield stress achievable often limits the height of structures that can be printed, typically around three-quarters of a meter. Although highly thixotropic mixtures can extend this limit, they pose challenges during pumping. Numerous methods have been explored to control concrete properties and influence the stiffening rate, including mechanical, chemical, thermal, magnetic, electric, and microwave-based interventions. Among these, chemical admixtures, particularly accelerators, have shown great promise in accelerating the stiffening process. However, the precise dosing of accelerators into fresh cementitious materials remains a significant challenge.
Efforts to develop dosing systems for 3D concrete printing have been made, but many have fallen short in achieving homogeneity and operational simplicity. For instance, nozzle reactors with rotating pins and dynamic mixers with additional motors have been attempted, but they often result in unmixed regions and complex maintenance requirements. An alternative approach involving static mixers, which lack moving components, has gained traction.
Static mixers, a standard equipment in process industries since the 1970s, have shown promise in dosing accelerators and other admixtures for 3D concrete printing]. Among various types of static mixers, the helical static mixer is the most commonly used, characterized by its housed-baffles design, where adjacent elements have opposite rotational directions. This design ensures continuous blending of processed flows, eliminating the need for moving parts. The use of static mixers offers several advantages, including improved mixing homogeneity, reduced maintenance complexity, and the avoidance of dead zones in the print head.
The innovation in dosing accelerators comes in the form of a carrier fluid, specifically a limestone powder suspension. This approach offers several advantages over traditional methods. First, it ensures a similar viscosity ratio of two streams, enhancing mixing homogeneity in the static mixer. In contrast, injecting a low-viscosity accelerator directly would lead to inhomogeneous mixing. Moreover, the use of a limestone powder suspension dilutes the accelerator concentration at the inlet position, reducing the risk of blockage due to rapid stiffening. Importantly, the limestone powder suspension, in combination with the accelerator, maintains a long open time as they do not react with each other.
The core of this innovative approach lies in the twin-pipe pumping (TPP) technology strategy. It leverages the limestone-based suspension as an admixture carrier to address the conflicting requirements of fluidity during transportation and early-age strength development for stability during printing. The TPP strategy involves pumping two distinct mixtures through separate pipes: a cement-based mixture without accelerator and a limestone-based mixture with a high dosage of accelerator. These two mixtures are then combined in a motionless helical static mixer positioned at the nozzle. The cement-based mixture is formulated to maintain high fluidity and a long open time, while the accelerator in the limestone-based mixture remains largely inert without the presence of Portland cement. As these two mixtures pass through the helical static mixer, the blending elements ensure a homogeneous combination. This results in a combined mixture that exhibits a rapid stiffening process and a smooth transition from a liquid to a solid state. To further enhance stability, viscosity modifying admixtures (VMA) are added to the limestone-based mixture, preventing segregation and improving cohesion.
A recent study published in Journal Cement and Concrete Research conducted by Dr. Yaxin Tao, Dr. Karel Lesage, Professor Kim Van Tittelboom, Dr. Yong Yuan, and Professor Geert De Schutter from Ghent University in Belgium, brings forth a groundbreaking approach that has the potential to overcome the limitations currently faced in 3D concrete printing.
In their study, the researchers set out to resolve the challenges associated with transitioning from pumping to depositing 3D printable concrete. They designed and tested custom mixtures, regulated the substitution rate of limestone powder, and adjusted accelerator dosages to optimize the rheological behavior of the combined mixtures. The authors examined the effect of different limestone powder substitution rates on compressive strength. It was observed that, regardless of the curing age, increasing the substitution rate led to a decrease in compressive strength. This decrease was attributed to the dilution effect, resulting from the reduction in Portland cement content and subsequent decrease in hydration products. However, even with a 25% substitution rate, the mixture achieved compressive strength above 50 MPa at 28 days, suitable for most 3D printing applications.
Accelerators play a crucial role in achieving a fast-stiffening rate, which is essential for 3D concrete printing. The study investigated the impact of accelerator dosage on flow diameter, setting time, heat evolution, and compressive strength. With increasing accelerator dosage, the flow diameter of combined mixtures decreased, reflecting improved fluidity. Accelerator addition reduced initial and final setting times, supporting the rapid stiffening process. Heat evolution curves showed that higher accelerator dosages led to increased heat release rates during early hydration, facilitating faster stiffening. While early-age compressive strength was enhanced with the addition of accelerator, later age strength decreased. Nevertheless, even with high accelerator dosages, the compressive strength remained within acceptable limits for practical applications.
For the pumping process, it is crucial to maintain high fluidity in both the cement-based and limestone-based mixtures. The authors examined the flow diameter of these mixtures, as well as the combined mixture. The results indicated that the flow diameter of the combined mixture remained consistently low, regardless of variations in sand ratio. This low flow diameter aligns with the goals of the TPP strategy, emphasizing high pumpability and buildability.
The research conducted by Ghent engineers presents a groundbreaking approach to address the challenges currently faced in 3D concrete printing. The innovative TPP strategy, combined with the use of a limestone-based suspension as an admixture carrier, has the potential to revolutionize the field. By achieving a balance between fluidity and stiffening control, this approach opens up new possibilities for the construction industry. It is a significant step forward in realizing the full potential of 3D concrete printing for efficient, cost-effective, and sustainable construction projects.
Yaxin Tao, Karel Lesage, Kim Van Tittelboom, Yong Yuan, Geert De Schutter, Twin-pipe pumping strategy for stiffening control of 3D printable concrete: From transportation to fabrication, Cement and Concrete Research, Volume 168, 2023, 107137,