This study quantifies how interlayer time gaps and mixture rheology jointly influence interfacial integrity and mechanical performance in 3D concrete printing (3DCP). Two mixtures incorporating identical binders, but differing limestone fineness (4 and 50 μm) were printed with interlayer delays of 0, 2, and 5 min, corresponding to stacking rates of approximately 9, 0.5, and 0.2 m/h. The finer limestone mixture exhibited higher plastic viscosity (13.2 vs. 11.5 Pa s) and faster structural buildup (58 vs. 38 Pa/min), resulting in accelerated early stiffening. Deposition yield stress derived from buildup measurements remained low for 0–2 min (1400–1500 Pa for the higher-thixotropy mixture; 1100–1500 Pa for the lower one) but increased markedly at 5 min to 4100 and 2900 Pa, respectively. Six single-wall prints were evaluated via surface defect imaging, layer-resolved ultrasonic S-wave velocity mapping, and 28-d compressive testing of specimens extracted from lower (layers 1–5) and upper (6–10) regions. Defect density rose with both delay and height, reaching 14.5% in the upper layers of the higher-thixotropy mixture at 5 min, while the 0-min condition remained ≤ 2%. S-wave velocity declined from 2100 to 2150 (0 min) to 1,800–1,870 m/s (5 min), accompanied by a strength reduction from 50 to 25–30 MPa. The findings define a practical deposition window, yield stress of 1100–1500 Pa, that minimizes interlayer defects under realistic delays, and demonstrate that layer-wise S-wave monitoring sensitively captures vertical stiffness gradients, providing a viable process-control metric for large-scale additive construction.

This study examines the performance of electrically conductive cement composite (ECCC) heating blocks as an innovative method for indirect heat curing of alkali-activated fly ash (AAFA) binders. The heating performance of the ECCC heating blocks, powered by a 15 V DC voltage applied for 24 h, and the AAFA composites positioned between them for curing was assessed through thermal imaging, surface temperature measurements, and tunneling electrical resistivity analysis. The performance of one-day-cured AAFA using ECCC heating blocks was then compared to conventional steam curing at 90°C and 99 % humidity, utilizing microstructural characterization techniques (TGA, XRD, MIP) and compressive strength testing. Results showed that smaller AAFA samples achieved superior average surface temperature due to their reduced size. ECCC blocks provided uniform heat distribution, with stable tunneling electrical resistivity during the first 5 h and subsequent increases due to thermal expansion and the disruption of conductive pathways. ECCC heat-cured composites exhibited higher total porosity (43.2 %), with 32.7 % attributed to transition pores (10–100 nm). In contrast, steam-cured composites had a comparable total porosity (41.7 %) but with 30.4 % comprising micropores (<10 nm). Despite these differences, compressive strength testing demonstrated promising performance, with cellulose microfibers (CMFs) enhancing strength by 13 % in steam-cured samples (46.3 to 52.2 MPa) and 12 % in ECCC heat-cured samples (36.1 to 40.3 MPa).These findings establish ECCC heating blocks as a sustainable and energy-efficient alternative to steam curing, offering consistent hydration and adaptability for diverse AAFA precast applications. • ECCC heating blocks provide a sustainable alternative to steam curing, enhancing early strength in AAF binders. • ECCC blocks lower energy use while improving compressive strength, making them efficient for sustainable construction. • Adding 0.3 % CMFs increases compressive strength under both ECCC and steam curing methods. • Compressive strength (MPa): heat-cured (40.3 with CMFs, 36.1 without), steam-cured (52.2 with CMFs, 46.3 without). • ECCC heating blocks support sustainable construction and improve alkali-activated materials.

This study quantifies how interlayer time gaps and mixture rheology jointly influence interfacial integrity and mechanical performance in 3D concrete printing (3DCP). Two mixtures incorporating identical binders, but differing limestone fineness (4 and 50 μm) were printed with interlayer delays of 0, 2, and 5 min, corresponding to stacking rates of approximately 9, 0.5, and 0.2 m/h. The finer limestone mixture exhibited higher plastic viscosity (13.2 vs. 11.5 Pa s) and faster structural buildup (58 vs. 38 Pa/min), resulting in accelerated early stiffening. Deposition yield stress derived from buildup measurements remained low for 0–2 min (1400–1500 Pa for the higher-thixotropy mixture; 1100–1500 Pa for the lower one) but increased markedly at 5 min to 4100 and 2900 Pa, respectively. Six single-wall prints were evaluated via surface defect imaging, layer-resolved ultrasonic S-wave velocity mapping, and 28-d compressive testing of specimens extracted from lower (layers 1–5) and upper (6–10) regions. Defect density rose with both delay and height, reaching 14.5% in the upper layers of the higher-thixotropy mixture at 5 min, while the 0-min condition remained ≤ 2%. S-wave velocity declined from 2100 to 2150 (0 min) to 1,800–1,870 m/s (5 min), accompanied by a strength reduction from 50 to 25–30 MPa. The findings define a practical deposition window, yield stress of 1100–1500 Pa, that minimizes interlayer defects under realistic delays, and demonstrate that layer-wise S-wave monitoring sensitively captures vertical stiffness gradients, providing a viable process-control metric for large-scale additive construction.