Quantitative characterization of bubble stability of foam concrete throughout extrusion process: From yield stress, viscosity and surface tension point of view

Abstract

Foam concrete (FC) is suitable to be used as printing ink for drones in extreme environment because of its light weight, it can reduce the load of drones and improve printing efficiency. Furthermore, since the FC density and thermal insulation performance can be flexibly changed by changing the bubble content, it can be used to print functional gradient components and special-shaped insulation walls. The stability of bubbles is crucial as it directly impacts the performance of 3D printed FC (3DPFC). Here, we examined the bubble destabilization and deformation of FC throughout the mixing process, resting period prior to extrusion, and extrusion process based on three parameters, i.e., yield stress, viscosity, and surface tension. The results indicate that increasing the yield stress from 1406 Pa to 13379 Pa of the precursor leads to a decrease in bubble volume fraction after mixing from 38.26 % to 27.24 %, while increasing viscosity from 2.16 Pa s to 6.65 Pa s and decreasing surface tension from 72.4 mN/m to 33.5 mN/m are favorable for improving the sphericity of bubbles with the diameter between 300 and 800 μm in FC. In the resting stage, the yield stress of the interstitial paste is the primary factor controlling bubble stability. When the initial yield stress of the equivalent interstitial paste is 5212 Pa, the bubble volume fraction decreases by only 0.8 % within 60 min. During extrusion, high yield stress leading to bubble deformation and instability, whereas viscosity and pore solution surface tension act as sources of bubble compression resistance. There exists a suitable diameter interval for bubble pressure-bearing limit under different paste environment during extrusion.