Effect of Recycled Fine Aggregate and Clay Brick Powder on Properties of 3D Printed Concrete

In order to reduce the amount of natural aggregates and cementitious material in 3D printed concrete, this study used recycled fine aggregate (RFA) to partially replace natural fine aggregate and clay brick powder (CBP) to replace cement. The fluidity and compressive strength test of cast-in-situ concretes with RFA only (0, 25%, 50%, 75% and 100% replacement ratio), with CBP only (0, 5%, 10%, 15%, 20% and 30% replacement ratio) and with both of them were firstly carried out to obtain the optimal replacement ratio of RFA and CBP. Then, it investigated the effect of the addition of 50%RFA and 10%CBP and the corresponding mix proportion adjustment methods (adding extra water and increasing water reducer dosage) on the fresh and harden properties of 3D printed concrete. The test results indicate that the decreasing amplitude of the compressive strength of cast-in-situ concrete is within 10% when the replacement ratio of RFA is less than 50%. With the increase of CBP replacement ratio from 0 to 30%, the compressive strength of cast-in-situ concrete increases first, then decreases and then slightly increases. The specimen with CBP content of 10% exhibits the highest compressive strength. Compared to the concrete with 50%RFA only, the concrete with 50%RFA and 10%CBP has higher strength, while the fluidity is almost kept unchanged. For 3D printed concrete, the addition of 50% recycled fine aggregate and 10% brick powder and additional water can keep the initial expansion of 3D printed concrete unchanged and improve the buildability of concrete, but it can decrease the slump, opening time, compressive and split tensile strength and increase intensity anisotropy. However, increasing water reducer dosage in the mixture can not only significantly improve the fluidity, opening time and compressive strength of 3D printed concrete, but also reduce the intensity anisotropy.