Active Rheology Control Of Cementitious Materials Using Magnetic Field

The engineering application process of concrete includes transporting,pumping and formwork casting.Many conflicts and contradictions in requirements of fresh concrete properties exist in different operations.Take the pumping as an example,excellent rheological properties such as low yield stress,suitable plastic viscosity and low structural build-up are required to reduce the pumping pressure and ensure the stability of the concrete.In contrast,higher structuration rate is beneficial to reduce formwork pressure during casting.Contradicting requirements of properties also exist in 3D concrete printing.Once the mixture proportion is confirmed,however,the properties of the prepared concrete cannot be further controlled in real time during casting process.This becomes a main barrier to improve the robustness and facilitate application of high-performance concretes.To satisfy future challenges,innovative casting technique or active rheology control method should be proposed.The main purpose of this thesis is to understand the rheological behavior of cementitious paste containing magnetic particles under magnetic field.Typical rheological responses of cement paste with nano-Fe₃O₄ particles to an external magnetic field were elaborated,and the magnetic force between two nei ghboring nanoparticles in cementitious paste and their movement velocity were derived.The influences of paste medium,concentration,and particle size of nano-Fe₃O₄,and magnetic field types on the structural build-up of cement paste under magnetic field were investigated.The clustering of nano-Fe₃O₄ particles in cement-based paste was examined,and the correlation between magneto-rheological effect and clustering of nanoparticles was established.The structural evolution of nano-Fe₃O₄ incorporated suspensions under combining effect of magnetic field and shearing was illustrated.The rheological behavior of cement paste with fly ash or calcium aluminate cement(CAC)was studied.Based on the results,the following conclusions can be reached :(1)Applying an external magnetic field promotes nano-Fe₃O₄ particles to move to agglomerate clusters.Their displacement creates a sort of mechanical micro-agitation effect,probably destroying early C-S-H bridges between cement particles,and further releasing possible entrained water in agglomerated clusters.Therefore,cement paste containing nano-Fe₃O₄ particles shows totally liquid-like behavior immediately after initiation of the external magnetic field.After longer period of magnetization,the cement paste shows increased stiffness owing to the presence of magnetic chains or clusters and the re-generation of C-S-H bridges between cement particles.The magnetic yield parameter indicates the competition between magnetic force and viscous stress of the suspension.The estimated magnetic yield parameter and the movement velocity of the nanoparticles can be used as relevant indicators to describe the extent of magneto-rheological responses of cement paste containing nano-Fe₃O₄.(2)Without superplasticizer(PCE),the magneto-rheological responses of cement paste with nano-Fe₃O₄ are weakened with the decrease of w/c due to the increase in viscoeastic stress and viscosity.At fixed w/c,cement pastes with low dosages of PCE(0.2% and 0.4%)shows negligible magneto-rheological responses.For the cement paste with 0.6% PCE,slight magneto-rheological responses are observed after longer period of magnetization.Increasing nanoparticles concentration increases the intensity of liquid-like properties immediately after initiation of the magnetic field and enhances the stiffness of the paste after undergoing longer magnetization,due to a large number of available nanoparticles contributing to the formation of magnetic clusters.The particle size plays a considerable role in dominating the visc oelastic properties of cement paste in the absence of magnetic field.The responses of cement paste to an external magnetic field are mainly determined by the crystalline structures and magnetic properties of the nano-Fe₃O₄ particles.(3)Weak magnetic fields have little effects on the rheological responses of cement paste.At sufficient high magnetic fields,magneto-rheological effect shows a linear correlation with magnetic field strength.When suddenly apply a magnetic field to a resting cement paste,the viscous behavior is improved due to the micro-agitation effect.After removing the magnetic field,a higher increase rate of structural build-up is observed.Under linear-changed magnetic field,increasing magnetic field from 0 T to 0.5 T leads to improved liquid-like behavior.When the magnetic field decreases from 0.5 T to approx.0.25 T,the chain-like structures keeps intact because of the viscoelastic behavior of paste suspension,resulting in an increase in the storage modulus.With the continuously decreasing magnetic field from approx.0.25 T to 0 T,the chain-like structures gradually disintegrate and thus increases the liquid-like properties.(4)To describe the clustering of nanoparticles quantitatively,the EDX elemental mapping of Fe was converted into the distribution of Fe-element number by using image analysis technique.Results show that when applying a magnetic field to cementitious paste containing nano-Fe₃O₄ particles,the magnetic nanoparticles tend to cluster,moving away from a fairly uniform distribution in case of absence of magnetic field.The coefficient of variation(COV)derived from the EDX elemental mapping,representative for the spatial clusterin g of the nano-Fe₃O₄ particles,shows a linear correlation with the magneto-rheological effect.It is a useful quantitative indicator for the magneto-rheological properties of cementitious paste.(5)After shearing test under magnetic field,large magnetic clusters in cement paste tend to migrate from the periphery of the plate to the center due to the combining effect of magnetic force and shear rate gradients.At constant low-rate shearing,the shear viscosity of nano-Fe₃O₄ incorporated limestone powder suspension increases rapidly upon applying a step-increased magnetic field,and then reaches a steady increase after shearing for a longer period.The shear viscosity after removing the magnetic field recovers to the similar level to that without magne tic field.Linearly increasing magnetic field from 0 T to 0.3 T shows less influences on the evolutions of shear viscosity,probably due to the weak connections between nano-Fe₃O₄ particles.At the magnetic field higher than 0.3 T,the shear viscosity gradually increases.Under linear-decreased magnetic field,more rapid response but smaller increase rate of the shear viscosity occurs.At the magnetic field decreasing from 0.75 T to approx.0.63 T,the shear viscosity reaches a peak.The shear viscosity evolution under multi linear-changed magnetic fields is history-independent.(6)For spherical fly ash,higher saturation magnetization and magnetic fraction indicates more obvious magneto-rheological responses.In the case of non-spherical fly ash with low magnetic properties,the paste exhibits higher magneto-rheological response because of the morphological effect.Under the same volumetric replacement,the magneto-rheological effect has a considerable linear relationship with the saturation magnetization of original fly ash.Applying an external magnetic field plays a greater role in improving the stiffness of CAC-incorporated cement pastes than the liquid-like properties.The rapid improvement of the stiffness can probably due to the high magnetic fraction of the CAC.The rheological behavior of resting or flowing cement paste containing magnetic particles can be altered by applying a magnetic field.It is an effective way to control the structural evolution and rheology of cement-based materials artificially and reversibly using magnetic field.