Encapsulation of sodium silicate to attain on demand buildability enhancement in concrete 3D printing

This study investigates the encapsulation of a buildability enhancing additive using a phase change material (PCM) as a thermo-responsive additive for concrete 3D printing. The encapsulated additive is mixed with printable mixes and activated at the print head. The printhead activation via the heating process dissolves the capsules and releases the buildability enhancing additive to attain the required rheological properties for printing. A sodium silicate-based set accelerator was used as the buildability enhancing additive and encapsulated using a paraffinic PCM. The comprehensive experimental study was conducted to understand the effect of encapsulated sodium silicate on the pumpability of concrete followed by buildability after print head heating. It was demonstrated that the smaller addition of encapsulated sodium silicate (5 %) followed by print head activation resulted in the static yield strength (SYS) of 122 kPa after 25 min of placement compared to 8 kPa observed for mixes containing nanoclay as a thixotropic additive. Furthermore, the dissolution process of sodium silicate was assessed via an analytical method using optical technology to determine the diffusion coefficient of sodium silicate in the printable concrete. The proposed method was validated by printing a thin vertical wall with the optimised mix design developed during the study. Consequently, the mechanical properties of the printed specimen were investigated. The mixes containing the encapsulated additive showed a compressive strength reduction by up to 47 % for mould cast and 3D printed specimens and this was correlated to increased porosity of the mixes.