Lightweight Concrete 3D Printing by Selective Cement Activation – Investigation of Thermal Conductivity, Strength and Water Distribution

The 3D printing technology Selective Cement Activation (SCA) is a particle-bed-based additive manufacturing method in which a dry mixture of sand and cement is spread in thin layers and solidified with water. SCA allows the production of complex and high-resolution components without the necessity for additional support structures. One drawback of concrete 3D-printing for free formed facade elements has been the need for additional thermal insulation to fulfil relevant building requirements. This causes an additional economic and ecological effort to create custom-built insulation to fit the 3D facade. Therefore, this paper discusses the fabrication of lightweight concrete through SCA by replacing the sand (S) with lightweight aggregates (LA, expanded glass beads) in order to decrease the thermal conductivity. However, the open pore structure of the lightweight aggregate could also change the water distribution behavior between the layers which would positively affect the hydration process. Test series with different w/c-ratios (0.3, 0.4 and 0.5) and type of aggregate (S and LA) were produced. Additionally, the test series with LA were fabricated without and with methylcellulose to further increase water absorption. The strength (compressive and flexural) and thermal conductivity were measured direction dependent to take anisotropic effects into account. Additionally, the water distribution perpendicular to the layers was determined using¹H-NMR. Specimens produced with LA showed a 3.5 to 4.4 times lower thermal conductivity compared to the specimens produced with S. However, using LA affected the density and thus the strength values of the material. Additionally, the open pore structure of the LA affected the water distribution between the layers which lead to an increase in strength with decreasing w/c-ratio. This is contrary to previous experiments using SCA and sand where strength has improved with increasing w/c-ratio [1–3].