In-line activation of cementitious materials for 3D concrete printing

In-line activation of cementitious materials by introducing the accelerators at the print head is a promising solution to attain the conflicting rheological requirements in 3D concrete printing. However, this technique is associated with many challenges including, difficulty in achieving good mixing homogeneity, formation of dead zones etc. This study investigates a method to attain in-line activation of cementitious materials by encapsulating the accelerator and introducing in the initial mixing. This is followed by thermal intervention at the print head to melt/dissolve the encapsulation for mixing the accelerator with cementitious materials. A gelatine based capsule was used as the shell material for the encapsulation. The encapsulated accelerator was used at varying dosages, and the fresh and hardened properties were assessed. The results showed that the in-line activation of the encapsulated accelerator at 2.5% rapidly transformed from a highly pumpable state to stiffened state with high yield strength and elastic modulus development. For instance, the yield strength of concrete after activation was increased fivefold, compared to control printable mix (CPM). Meanwhile, before activation, the fresh 3D printable mix with encapsulated accelerator showed excellent flowability for long durations. The mechanical properties of 3D printed concrete containing encapsulated accelerator, including the compressive strength and interlayer bond strength, were also improved. Besides, the ultrasonic pulse velocity test revealed that the pore homogeneity reduces with the increasing capsule dosage due to the floating tendency of capsules. The development of micro-capsules would assist in improving the homogeneity.

Automated summary

In-line activation of cementitious materials by encapsulating and introducing accelerators has been studied in this research. The results showed that the encapsulated accelerator rapidly transformed the concrete from a pumpable state to a stiffened state with improved mechanical properties. The flowability of the fresh printable mix was maintained before activation. The study also found that the homogeneity of the concrete reduced with increasing capsule dosage.