Effect of nano-silica sol dosage on the properties of 3D-printed concrete

Extruded laminate-based 3D printing (3DP) of concrete does not require formwork support during construction and can be used to construct buildings with different appearances. However, 3DP requires strict control over the setting time and mechanical and deformation properties of concrete materials. In this paper, we designed and formulated 3DP concrete with different nanosilica sol (NSS) admixtures to investigate how NSS modified the fluidity, extrudability, setting time, mechanical properties, and microstructure of 3DP concrete. The results showed that NSS thickened the admixture, shortened the setting time, and changed the thixotropy of the 3DP concrete. The dosage of NSS should not exceed 1.5 %, above which the workability range decreased significantly. In addition, an appropriate amount of NSS greatly improved the seven-day early strength of 3DP concrete when the dosage was controlled within the range of 0.5-1.5 %. Under the optimal dosage of 1 %, the compressive flexural strength was improved by 10 %. Scanning electron microscopy (SEM) was used to observe the microscopic pore structure, and the hydration products were analyzed using X-ray diffraction (XRD). The results demonstrated that the pore structure became denser after mixing NSS, and the main hydration products were Ca(OH)2 (CH) and CaCO3. It can be assumed that NSS accelerated the hydration reaction, which produced more CH and CaCO3. Therefore, the flocculation rate of the mixture was increased, and the reduced porosity increased the structural density.