Rheological rejuvenation and structural build-up of 3D printed manufactured sand concrete via secondary mixing protocol

The application of sustainable manufactured sand (MS) in 3D printed concrete is currently severely restricted by the “pumpability-buildability” conflict, primarily characterized by the high internal friction and angularity of MS particles. To overcome this bottleneck, this study proposes a novel process-oriented strategy: a secondary mixing protocol featuring a static resting period and delayed superplasticizer addition. This approach is explicitly designed to induce a “rheological rejuvenation” effect in stiff MS mixtures. Quantitative results demonstrate that this protocol reduces the static yield stress of high-volume MS ink (T-3) by approximately 39% (from 1,516 Pa to 924 Pa) and significantly diminishes thixotropic hysteresis, thereby successfully reopening the effective printing window to approximately 65 min. Although increasing MS content accelerates the structural build-up rate, the optimized 3D printed manufactured sand concrete (T-3) achieves a 28-day compressive strength of 61 MPa, representing a 15% enhancement over the river sand reference group (T-0). These findings confirm that the secondary mixing protocol is a robust solution capable of unlocking the potential of high-volume MS in sustainable digital construction.