Rheology control is the most critical determinant of success in 3D concrete printing (3DCP), typically achieved through the hydration control of cement. However, this inevitably leads to overdesign of printed concrete featuring a low water-to-binder ratio (w/b), which is incompatible with its non-load bearing purpose and raises a series of environmental and durability problems, such as high carbon footprint and early-age shrinkage. Herein, we propose a novel rheology control strategy via in-situ polymerization, allowing the mix design of printed concrete with a high w/b ratio of 0.6. The proposed approach consists of two stages: 1) introducing monomers as retarders to extend the open time during pumping, and 2) incorporating initiators into the mixture to trigger polymerization, facilitating the structural build-up after deposition by forming polymer bridges between cement particles. We show that the addition of monomers significantly retards yield stress growth, while the following in-situ polymerization engenders a rapid strength development, satisfying the rheological requirements for 3DCP. Mechanistic experiments reveal that the retarding effect results from the complexation of monomers with aqueous species, such as Ca 2+ ions, thereby hindering the nucleation of hydrates. As polymerization initiates, the impetus for the structural build-up of the cement pastes first originates from the proliferation of polymer bridges due to the gradual formation and adsorption of polymer, and then relies on the reinforcement of these polymer bridges through the formation of chemical bonds or crosslinks. On top of the environmental benefit, the proposed strategy holds the potential in avoiding admixtures conflict, mitigating early-age shrinkage, and improving mechanical properties. Our strategy opens possibilities for a novel technical route to achieve rheology control of 3DCP, and the discovery in this work will be a landmark for revealing the mechanism of 3DCP via in-situ polymerization.
Seawater and sea-sand based ultra-high performance concrete (UHPC) is currently regarded as one of the most innovative and promising construction materials. To provide design guidance for the usage of silica fume (SF) during the production of seawater and sea-sand based UHPC, this work investigated the effect of SF on the later-age macro-performances and micro/nanostructure of seawater ordinary Portland cement (OPC) pastes prepared with a low water-to-binder (W/B) ratio. The results showed that the optimal SF dosage in seawater OPC-SF system was 10 wt% while that for the deionized (DI) water OPC-SF system was 20 wt%. This showed that the usage of SF could be reduced for the production of the seawater based UHPC system. It was found that the later-age compressive strength of seawater OPC-SF system was increasingly degraded with increasing SF dosage when compared with the corresponding DI water OPC-SF system. The difference in the distribution of unhydrated clinkers in the matrix as SF addition increased was the main cause. The seawater OPC-10%SF system exhibited the highest compressive strength and the lowest pore content among all the seawater-OPC systems. Additionally, with the usage of this ultra-low W/B ratio, Friedel's salt was only present in the seawater OPC pastes prepared without SF, but absent in the seawater OPC-SF pastes.
Owing to the low water-cement ratio (less than 0.25) of ultra-high-performance concrete (UHPC) materials, the system exhibits increased viscosity, diminished slurry flow, and challenges in pouring during the construction process, which compromise its workability. Consequently, the development of highly efficient viscosity-reducing polycarboxylate superplasticizer (PCE) is of paramount importance. In this study, we synthesized a series of viscosity-reducing polycarboxylate superplasticizers(JN-PCE) utilizing allyl polyethylene glycol ether (APEG), acrylic acid (AA), and sodium methallyl sulfonate (SMAS) as raw materials through the process of free radical polymerization. Fourier-transform infrared spectroscopy, nuclear magnetic resonance, gel permeation chromatography, and energy-dispersive X-ray spectroscopy were employed to characterize the materials. Subsequent evaluations of surface tension, fluidity, adsorption performance, and Marsh time demonstrated that these superplasticizers exhibited commendable dispersion and remarkable viscosity-reducing properties within a low water-cement ratio system. At a water-cement ratio of 0.18, the Marsh time of the JN-type PCE was reduced by 40% while maintaining the same fluidity, thereby underscoring the effective viscosity-reducing capabilities of the synthesized polycarboxylate superplasticizer.
危险品化学品经营许可证（带存储）