Xingang Xu

The insufficient shape stability and uncontrollable rheological behavior of 3D printed cement composites (3DPCCs) still limit their wider application in construction. In this study, graded carbonated steel slag powder (CSS1 and CSS2) was used to adjust the rheological properties of 3DPCCs based on the oscillation shear and controlled shear-rate protocols, providing a sustainable and novel strategy for improved printability and mechanical properties. The results showed that the incorporation of carbonated steel slag into 3DPCCs effectively improved the viscoelasticity and yield behavior. As the carbonation time increased from 0 d to 21 d, the dynamic yield stresses of 3DPCCs with CSS1 and CSS2 increased from 126.08 Pa and 116.38 Pa to 707.29 Pa and 509.29 Pa, respectively. In addition, the structural deformation of 3DPCCs with CSS1 and CSS2 decreased from 20.35% and 21.61% to 9.83% and 11.29%, respectively, while the 3-day compressive strength increased from 8.5 MPa and 6.0 MPa to 14.2 MPa and 13.2 MPa, respectively. In conclusion, the carbonated steel slag shows great application potential in improving the rheological behavior and printability of 3D-printed cement composites.

Repair mortars containing recycled concrete powder (RCP) and ground granulated blast-furnace slag (GGBFS) are promising low-carbon materials for the rapid repair of concrete structures and pavements. However, their practical use is often limited by slow early hydration, insufficient early strength, and weak bonding with existing concrete substrates. In this study, four early-strength admixtures, namely calcium formate, anhydrous sodium sulfate, calcium acetate, and triethanolamine, were incorporated into a P·I 42.5 cement-based repair mortar containing RCP and a low dosage of GGBFS. Their effects on fluidity, flexural and compressive strength, tensile bond strength, drying shrinkage, and hydration characteristics were investigated. The results showed that the suitable dosages of calcium formate, anhydrous sodium sulfate, calcium acetate, and triethanolamine were 1.5%, 1.0%, 0.8%, and 0.05% by mass of total cementitious materials, respectively. Among the four admixtures, calcium formate provided the best balance among strength enhancement, bond performance, workability retention, and dosage tolerance. Compared with the control group, the 3 d and 28 d flexural strengths of the 1.5% calcium formate group increased by 37.0% and 20.3%, respectively. Anhydrous sodium sulfate gave the highest tensile bond strength, with the 14 d value increasing by 33.15% to 1.052 MPa, but its effective dosage range was relatively narrow. Calcium acetate was more effective in reducing drying shrinkage, with a 28 d shrinkage value of 695.14 × 10⁻⁶. SEM and XRD results suggested that the admixtures mainly accelerated early hydration, while no new major crystalline phases were detected. Excessive dosages caused strength loss, bond deterioration, or increased drying shrinkage. These findings are applicable to the specific RCP–GGBFS repair mortar formulation and dosage ranges investigated here. They provide a practical basis for selecting early-strength admixtures for RCP-containing repair mortars used in concrete structure and pavement repair.

To optimize the shape stability of 3D-printed white calcium sulfoaluminate (WCSA) cement composites, oyster shell powder (OSP) and cuttlebone powder (CBP) were introduced as white admixtures to regulate rheological properties and printability. The setting behavior, rheological properties, and shape stability of the WCSA cement composites were evaluated by Vicat setting-time tests, rotational rheological measurements, three-stage thixotropic recovery tests, and structural deformation measurements, together with mechanical strength tests, XRD, and SEM analyses. The results showed that the incorporation of OSP and CBP shortened the setting time of WCSA cement composites. The initial and final setting times decreased from 41 min and 67 min to 17 min and 30 min in the WCSA cement composites with OSP, and from 42 min and 66 min to 20 min and 33 min in the WCSA cement composites with CBP, which improved printing operability. As the OSP and CBP content increases from 0% to 24%, the dynamic yield stress of WCSA cement composites increased from 48.83 Pa to 530.59 Pa and 60.30 Pa to 1085.80 Pa, respectively. The thixotropic recovery degree of WCSA cement composites increased from 57.89% to 86.46%, and 56.60% to 92.14%, respectively. As the OSP and CBP contents increase from 0% to 24%, the structural deformation decreased from 12.39% to 6.91% and 13.29% to 5.12% respectively, which improved buildability of the printed structures. In addition, although OSP and CBP reduced the mechanical strength of WCSA cement composites compared with the control group, the flexural and compressive strengths gradually recovered as the contents increased from 6% to 24% due to the enhanced filling effect and improved particle packing. This study provides a reference for the application of marine calcareous solid wastes in sustainable 3D-printed cementitious materials.

Controlling structural deformation is essential for the structural stability of 3D-printed cement composites. In this paper, carbonated recycled aggregate (CRA) was incorporated into 3D-printed magnesium phosphate cement composites (MPCCs) to control rheology and improve buildability. Experimental results show that CRA increased the static yield stress from 2210.96 to 6238.18 Pa and the storage modulus. When the incorporation of CRA was more than 15%, the phase angle was less than 45°, indicating predominantly solid-like behavior. Additionally, due to the higher porosity, the compressive strength and flexural strength of 3D-printed MPCCs decrease with the increasing CRA content; however, the decline tendency becomes significantly more pronounced when CRA content exceeds 10%. Structural deformation decreased from 14.39% to 6.91%, attributed to the rough surface of CRA, which promotes more uniform stress transfer during stacking. This study demonstrates a simple upcycling route that improves the printing stability and sustainability of 3D-printed magnesium phosphate cement composites (MPCCs).

With the aim of reducing greenhouse gas emissions from energy consumption and advancing the green energy transition, this study employs sodium hydroxide and water glass as activators to facilitate the replacement of fossil fuels with renewable energy sources, with physically activated recycled micro-powder serving as an auxiliary cementitious material to prepare alkali-activated recycled hollow concrete. This study pioneers the application of the coarse aggregate tight-packing theory (bulk density method) to the preparation of alkali-activated recycled hollow concrete containing sand. By integrating Matlab image binarization techniques, we quantitatively analyzed the causes of porosity deviation, achieving precise alignment between target and actual porosity. This work fills a theoretical gap in the quantitative design of porosity for this concrete type. Additionally, the effects of different binder material dosages and pore volumes on the mechanical properties and permeability coefficients of sand-containing porous concrete were evaluated. Experimental results indicate that the calculated pore volume of sand-containing porous concrete prepared using the dense-packing theory (bulk density method) exhibits a smaller average error compared to the actual pore volume. As the amount of cementitious materials increases, the compression strength of permeable concrete gradually increases. When the cementitious material content is 450 kg/m³, and the target porosity is 15%, the concrete’s 28-day compressive strength reaches 21.4 MPa. At a porosity of 15%, the permeability coefficient ranges from 5.2 to 5.7 mm/s.