Recycling of aggregate micro fines as a partial replacement for fly ash in 3D printing cementitious materials

Abstract

3D printing cementitious materials (3DPC) with fly ash usually presents problems with retardation of hydration reaction and low strength development at early ages, which does not match the requirements of the 3D printing process. Incorporating limestone aggregate micro fines (AMF) is a way of gaining both the performance and environmental benefits from the technology of 3DPC. This paper investigates the influences of AMF, mainly at forms of calcium carbonate (CaCO3), on the fluidity, extrudability, shape stability, green strength, hydration heat, and strength development of 3D printing mortar with FA. It is shown that the mini-slump and extrudability of mixtures are decreased with the increase of AMF contents. However, the shape stability, green strength, and Young’s modulus of mixtures containing AMF in the process of 3D printing are found to be higher than those of reference mixture without AMF, and these properties also increase with increasing AMF contents. The increased shape stability, green strength, and Young’s modulus can be ascribed to the irregularmorphology of AMF particles. The additional hydration products due to the accelerating effects of AMF also contribute to improving the load-bearing internal skeleton. In addition, while 3D printing mortar with FA and AMF is characterized by low compressive strength at early ages, their long-term strength typically meets or exceeds that of reference mortar due to the synergy effects between FA and AMF. This study provides a feasible solution, satisfying the fresh and hardened performance requirements, for the use of AMF in 3D printing mortar.