Structural build-up of cementitious paste with nano-Fe3O4 under time-varying magnetic fields

The structural build-up of cementitious paste with nano-Fe3O4 under time-varying magnetic fields was experimentally investigated using small amplitude oscillatory shear (SAOS) technique. Several modes of magnetic fields, such as constant, sudden-changed and linearly-changed, were applied to the cementitious paste. Results showed that the structural build-up of the cementitious paste depended on the magnetizing time and magnetic field strength. Applying constant magnetic fields improved the liquid-like behavior during first minutes and afterwards the solid-like property was enhanced. Both the sudden-increased and sudden-decreased magnetic fields resulted in a sharp decrease in storage modulus. The linearly increasing magnetic field resulted in a slight increase in storage modulus but higher liquid-like behavior. When the magnetic field was linearly decreased from 0.5 T to approx. 0.25 T, the structural build-up was enhanced significantly, and with the continuously decreasing magnetic field from approx. 0.25 T to 0 T, a decrease in storage modulus was observed

Rheological properties of cement paste with nano-Fe3O4 under magnetic field : flow curve and nanoparticle agglomeration

Understanding the influence of magnetic fields on the rheological behavior of flowing cement paste is of great importance to achieve active rheology control during concrete pumping. In this study, the rheological properties of cementitious paste with water-to-cement (w/c) ratio of 0.4 and nano-Fe3O4 content of 3% are first measured under magnetic field. Experimental results show that the shear stress of the cementitious paste under an external magnetic field of 0.5 T is lower than that obtained without magnetic field. After the rheological test, obvious nanoparticle agglomeration and bleeding are observed on the interface between the cementitious paste and the upper rotating plate, and results indicate that this behavior is induced by the high magnetic field strength and high-rate shearing. Subsequently, the hypothesis about the underlying mechanisms of nanoparticles migration in cementitious paste is illustrated. The distribution of the nanoparticles in the cementitious paste between parallel plates is examined by the magnetic properties of the powder as determined by a vibrating sample magnetometer. It is revealed that the magnetization of cementitious powders at different sections and layers provides a solid verification of the hypothesis

Rheological behavior of cement paste with nano-Fe3O4 under magnetic field : magneto-rheological responses and conceptual calculations

The magneto-rheological responses of cement paste with nano-Fe3O4 particles are experimentally investigated. The estimated magneto-dynamic force between two neighboring nanoparticles and equilibrium movement velocity of the nanoparticles in cement-based suspensions are calculated. Results show that the nanoparticles have a potential to move to form magnetic clusters when a magnetic field is applied, which creates a sort of agitation effect breaking down the early C-S-H links between cement particles, and thus the corresponding suspensions exhibit liquid-like behavior immediately after applying the magnetic field. The solid-like property of the studied suspensions becomes more dominant with magnetizing time due to the formation of magnetic clusters and the reconstruction of C-S-H bridges. The rheological properties of paste medium exert significant influences on the magneto-rheological responses of cement paste containing nano-Fe3O4 particles. It is revealed that the calculated magnetic yield parameter and nanoparticle movement velocity are useful relevant indicators to evaluate the magneto-rheological effect of cementitious paste

Quantitative assessment of the influence of external magnetic field on clustering of nano-Fe3O4 particles in cementitious paste

In view of active rheology control of cementitious materials, nano-Fe3O4 can be added as responsive particles. Following the concept of magnetorheological fluids, it is assumed that magnetic nanoparticles will form chains or clusters in cementitious paste following magnetic field lines. A quantitative experimental validation of this assumption is presented herein. The clustering of nano-Fe3O4 particles under magnetic fields is studied by mapping iron (Fe) element distribution in cementitious paste using energy dispersive X-ray spectroscopy. By means of image analysis, the Fe-element patterns are quantified by the deviation of Fe-elements in a unit area from the mean value expected in case of a uniform distribution, as expressed by coefficient of variation (COV). The magneto-rheological responses of cementitious pastes are evaluated using small amplitude oscillatory shear technique. Results show that the magneto-rheological effect exhibits a linear relationship with the relative change of COV, providing a quantitative validation of magnetic clustering in cementitious paste

Active control of properties of fresh and hardening concrete

Concrete mixtures have an optimized mix design in view of attaining desired properties. However, after mixing, during further processing, it is typically not possible to further adjust the performance of the fresh and hardening concrete. A new and emerging approach is to actively control the concrete properties by means of responsive particles or polymers triggered by an externally applied signal. Active control of properties of concrete refers to the concept of on- demand changes of one or more properties of the concrete after mixing by triggering a response to one or more of the constituents using a specific trigger signal (e.g. thermal, chemical, electrical, magnetic...). The on-demand control of properties can focus on the processing stage (including e.g. pumping, casting, 3D printing), the curing and hardening stage (including e.g. control of capillary pressure, shrinkage, setting, and hardening) and even on the hardened stage during service life (e.g. active corrosion control, active crack healing...). Addressing specific obstacles in cementitious environments, ensuring responsive material stability, controlling signal applicability, cost, logistics, and on-site safety is crucial for successful implementation. A RILEM technical committee has been initiated in 2023, working on the concept of Active Control of Properties of Concrete (RILEM TC 317-ACP). The committee will focus on active control of properties of fresh and hardening concrete. This paper gives a short introduction to scope and activities of TC 317-ACP.https://letters.rilem.net/index.php/rilem/indexhttps://letters.rilem.net/index.php/rilemhj2024Civil EngineeringSDG-09: Industry, innovation and infrastructur

Insights into the viscoelastic properties of cement paste based on SAOS technique

Understanding the viscoelastic properties of cement paste is beneficial to pumping, formwork casting and 3D printing of cement-based materials. This paper presents new insights into the viscoelastic properties of cement paste with various compositions. Several parameters, including critical strain, storage modulus and viscoelastic yield stress obtained from small amplitude oscillatory shear (SAOS) test, are selected to characterize the viscoelasticity of fresh cementitious paste. Results reveal that increasing water-to-cement (w/c) ratio reduces the storage modulus at linear viscoelastic region (LVER) and viscoelastic yield stress of cement paste, whereas higher w/c increases the critical strain. The replacement of cement by fly ash has no significant influence on the critical strain of cement paste. Due to the improvement of cohesive bonding between cement particles by nanoparticles, the incorporation of nano-Fe3O4 particles results in an increase in the storage modulus at LVER, critical strain and viscoelastic yield stress. The critical strain of cement paste gradually increases with the concentration of polycarboxylate ether (PCE) superplasticizer, which possibly can be attributed to the interactions and entan-glement of PCE molecules adsorbed onto the solid particles. By contrast, cement pastes with low PCE additions exhibit an increase in the viscoelastic yield stress, while higher PCE additions significantly decrease the storage modulus at LVER and viscoelastic yield stress of cement paste