The mechanical properties of 3D-printed mortars are determined in terms of their compressive and direct tensile bond strengths. To determine such properties using existing methods, a preliminary experiment was conducted. The compressive strength of the printed mortar was compared to mold-casted specimens and it was found that the compressive strength decreased by ~30%. Among the fabrication variables, an increase in nozzle height negatively influenced the direct tensile bond strength. For the same conditions and age, the direct tensile strength decreased by as much as 16–29% when the number of layers increased from 2 to 6. When the specimens were fabricated using a specially designed stainless steel frame and core drill, followed by extraction and the application of physical impact, the 28 days compressive strength of the specimen decreased by ~50%.

Three-dimensional concrete printing (3DCP) materials require a relatively low water-to-binder ratio (W/B) of 0.3 or less to ensure their buildability and flow properties are sufficiently maintained after mixing. In this study, the rheological properties of 3DCP materials with W/B 0.28 were evaluated up to 60 min after mixing, and the yield stress and plastic viscosity were analyzed over time. A gradual decrease in flow rate with time was observed during the transport of 200 kg of material per batch through a 20 m hose. To examine the time-dependent changes in flow rate and layer volume, a 2200 mm × 1000 mm test specimen was printed. The dependence of the layer width over time during the printing process was measured and analyzed. The experimental analyses showed that the flow rate and layer volume of the 3DCP material gradually decreased with time after mixing, which was correlated with the rheological properties.

This study evaluated the setting time, mechanical properties and microstructure of Portland cement (OPC) by adding SiO₂ nano- and micro-particles. The setting time was reduced due to the pozzolanic reaction of the nano- and micro-SiO₂, and the compressive strength was increased through a reduction in the porosity of the microstructure. When nano- and micro-SiO₂ were used alone, micro-silica was the most effective in reduced the initial and final setting times and developing compressive strength. When two or more nano- and micro-SiO₂ were used, a micro-sized binder and a small amount of nano-silica effectively improved performance as the setting time was reduced to 50–52% of that of ordinary Portland cement (OPC). It appears that a small amount of nano-silica could reduce the setting time and increase compressive strength because it caused the pozzolanic reaction and because the nanoparticles filled the pores between the silica fume and cement, which were composed of relatively large particles. This result could also be derived from compressive strength and microstructure analysis. Cement paste containing to nano- and micro-silica increased the strength by approximately 112% compared to OPC. Because nano-binders may cause a reduction in flow due to their large specific surface area, adding chemical admixture needs to be considered during mix design. In addition, the particle size distribution must be considered when nano- and micro-materials are used because an imbalance in particle size distribution can increase the pore size in the microstructure.

In this study, we experimentally analyzed the deformation shape of stacked layers developed using three-dimensional (3D) printing technology. The nozzle traveling speed was changed to 80, 90, 100, and 110 mm/s when printing the layers to analyze its effect on layer deformation. Furthermore, the cross-sectional area and the number of layers were analyzed by printing five layers with overall dimensions of 1000 (w) × 2200 (l) × 50 (h) mm (each layer was 10 mm high) using Vernier calipers. Moreover, we analyzed the interface and cross-sectional area of layers that are difficult to confirm visually using X-ray computed tomography (X-ray CT) analysis. As a result of measuring the deformation at the center of the layer, it was confirmed that the deformation was greater for lower nozzle traveling speeds. Consequently, the X-ray CT analysis verified that the layer had the same cross-sectional area irrespective of the layer printing order at the same nozzle travel speed, even if the layer was deformed.

Silicate-based nano- and micro-sized binders were used with ordinary Portland cement to evaluate their influence on the setting time, activation energy, mechanical properties, and microstructure. It was found that the setting time was reduced due to the pozzolanic reaction of the silicate-based binders and the densification of the microstructure. However, there is a lack of research on nano-sized pozzolanic materials. Therefore, in this study, research on activation energy and microstructure was conducted. The compressive strength increased owing to a reduction in the porosity in the microstructure, and activation energy also tended to decrease. Moreover, using both micro-silica and a small proportion of nano-silica was more effective in reducing the setting time and activation energy than using any of them individually. The study established that adding a small proportion of nano-silica could reduce the setting time and increase the compressive strength because it positively influenced the pozzolanic reaction and filled the pores between micro-silica and cement, which were composed of relatively larger particles, with smaller particles. Because nanomaterials may degrade flowability due to their large specific surface area, it is deemed necessary to consider the addition of chemical admixtures during mix design. A characteristic has been revealed when nanomaterials are used, and special attention to the particle size distribution characteristics is required because the imbalance in particle size distribution may increase the porosity inside the microstructure. Therefore, it is recommended to use micro-sized pozzolanic materials together when using nano-sized pozzolanic materials.