On rheology of mortar with recycled fine aggregate for 3D printing

• 100% content of RFA is proved to be applicable in 3DMP from the perspective of rheology. • The combined use of RFA and sodium gluconate contribute to the continuous printing. • A dimensionless parameter is defined to describe the buildability with higher accuracy. This study explores the feasibility of utilizing recycled fine aggregates (RFA) in 3D mortar printing (3DMP) by evaluating the properties of 3D printing (3DP) mixtures with different contents of RFA (0%, 50%, and 100%) as well as sodium gluconate (0%, 0.6%, and 1.2%) before and after high-rate shearing from the perspective of rheology, failure pattern and buildability. The results showed that, the incorporation of RFA highly increased the 3DP mixture’s initial value and time-varying rate of the static yield stress, viscosity and area of thixotropy hoop, while adding sodium gluconate decreased them respectively. Meanwhile, since high-rate shearing were found to highly decrease the mentioned rheological parameters of 3DP mixtures, a feeding system which can provide high-rate and continuous shearing was recommended in practical printing to prolong the open time of printing mixtures. Besides, a dimensionless buildability parameter λ was proposed and defined as the ratio of accumulated offsets (whether compression or tension) to the theoretical height to describe the buildability of 3DP mixtures, which exhibited much higher accuracy than the maximum printing layer, maximum printing height and average vertical strain, respectively.