Toward Sustainable 3D Concrete Printing: Assessment of SCM–Superplasticizer Interactions on Rheology and Buildability

Three-dimensional concrete printing (3DCP) requires mixtures that develop sufficient early buildability while preserving open time for reliable interlayer bonding. This study investigates the time-dependent evolution of static yield stress for printable concretes incorporating three supplementary cementitious materials—metakaolin (MK), silica fume (SF), and biochar (BC)—used with either a polycarboxylate ether- (PCE) or naphthalene-based superplasticizer. Static yield stress was measured at 15, 30, and 45 min of concrete age using the stress-growth method with a shear vane apparatus. Performance targets were τs (15 min) ≤ 2.8 kPa, reflecting extrudability/pumpability; τs (30 min) ≤ 3.1 kPa, representing printability/open time; and τs (45 min) ≥ 3.4 kPa, representing buildability. Pooled Type-II ANOVA showed a highly significant SP effect (p < 0.001), a significant SCM × SP interaction (p = 0.031), and a significant time effect (p = 0.005), whereas SCM (p = 0.709) and SCM% (p = 0.914) were non-significant once interaction and time were included. Across SCMs, SNF–PCE gaps are ~0.2–0.8 kPa at 30 min (+7–30%) and ~0.4–1.3 kPa at 45 min (+12–45%), with the largest gaps in SF, intermediate in MK, and smallest in BC.