Rheological Property Evolution in Concentrated Cement-Polyelectrolyte Suspensions

We have studied the rheological property evolution and hydration behavior of pure white portland cement (Type I) pastes and concentrated cement-polyelectrolyte suspensions. Polyelectrolyte species have a marked effect on the initial stability, elastic property evolution ( G '( t )), and hydration behavior of this cement system. Pure cement pastes exhibited an initial G ' value of ∼10⁴ Pa and fully reversible G '( t ) behavior until the onset of the acceleratory period ( t ∼ 2 h), where the pastes stiffened irreversibly. In contrast, cement-polyelectrolyte suspensions exhibited initial G ' values of ∼1 Pa and G '( t ) behavior comprised of both reversible and irreversible features. Their initial G ' values, measured after disrupting the particle network under high shear conditions, grew exponentially with hydration time, where G _i'= G _i,0' exp( t /τ_c) and τ_c corresponds to the characteristic hydration time determined from calorimetry measurements. Our observations of these cement-polyelectrolyte systems suggest that hydration phenomena impact interparticle forces during early stage hydration and, ultimately, lead to initial setting through the formation of solid bridges at the contact points between particles within the gelled network.     

Effect of Aging on Rheological Properties of Lime Putty

The increasingly wider application of lime-based materials for the conservation and restoration of historical buildings has led to a revival of interest in lime putty. One of the most well-known and highly regarded properties of lime putty is its plasticity/workability.
This paper describes the results of a rheological investigation conducted on putties having identical water content but aged for different times, with the aim of providing an objective and quantitative evaluation of plasticity. Rheological measurements have demonstrated that, all other conditions being equal, shear stress increases with aging. We propose here to consider some parameters derived from Tattersall and Bingham models that can be used as a quantitative measure of plasticity directly correlated with aging.     

In Situ Rheological Investigation of the Coagulation in Aqueous Alumina Suspensions

Electrostatically stabilized alumina suspensions can be destabilized by the enzyme-catalyzed decomposition of urea (direct coagulation casting). Depending on the conditions, this reaction can shift the pH of a suspension to the buffer pH of the reaction products or increase the ionic strength at the buffer pH. The coagulation for both mechanisms was investigated using in situ rheological measurements. Using a vane tool in oscillation mode, the measuring conditions were optimized to find a reasonable method for time-dependent measurements. Constant parameters (stress or strain) proved to be unsuitable, because the linear viscoelastic region shifted considerably during the coagulation. Furthermore, the gel structure produced on coagulation via increase of ionic strength (Δ I ) was very sensitive to the oscillation. Therefore, for long-time experiments, a short continuous measurement with a low strain was followed by amplitude sweeps with increased intervals to determine the linear values of G ' and G ". In this way, the increase of the moduli G ' and G " could be followed for longer times, and it was possible to demonstrate two results. First, the final G ' of the network was about 10 times higher for Δ I -coagulated material than for suspensions coagulated via pH shift (ΔpH). Second, particle rearrangement processes took place in Δ I -coagulated networks even after the chemical changes were finished, whereas ΔpH-coagulated samples were "frozen-in" when approaching the isoelectric point and showed no further physical changes afterward.     

Comb Polymer Architecture Effects on the Rheological Property Evolution of Concentrated Cement Suspensions

We have studied the rheological behavior of concentrated cement suspensions in the absence and presence of comb polymers comprised of a polyacrylic acid (PAA) backbone and charge-neutral, poly(ethylene oxide) (PEO) teeth. These species possessed a uniform backbone molecular weight and graft density, with varying teeth molecular weight. Both PAA, a linear polyelectrolyte, and PAA/PEO comb polymers imparted initial stability to concentrated cement suspensions above a critical weight fraction, w * of 4 mg/(g of cement). Cement–PAA suspensions, however, set prematurely. Their rapid, irreversible stiffening stemmed from deleterious interactions between PAA and multivalent counterions in solution. Interestingly, the presence of PEO teeth comprised of only a few monomer units in length mitigated such interactions. The rheological property evolution of concentrated cement–PAA/PEO suspensions exhibited complex behavior ranging from the reversible gel-like response observed at short teeth lengths to a remarkable gel-to-fluid transition observed during the deceleratory period for systems comprised of longer PEO teeth. At longer hydration times, all cement–PAA/PEO suspensions exhibited initial elastic modulus values, G_i '∼ exp( t /τ_c) before the onset of the acceleratory period, followed by initial set. Their characteristic hydration time, τ_c, and set time depended strongly on the concentration of "free" carboxylic acid groups [COO⁻] arising from non-adsorbed polyelectrolyte species in solution.     

Gelation, Consolidation, and Rheological Properties of Boehmite-Coated Silicon Carbide Suspensions

We have studied the gelation, consolidation, and rheological properties of boehmite-coated SiC suspensions. A boehmite-coated SiC suspension consists of SiC particles covered with a boehmite layer of a few nanometers in thickness in the suspension. Similar to boehmite suspensions, the boehmite-coated SiC suspension can gel over time. The gelation, as well as the rheological behavior of the boehmite-coated SiC suspension with respect to pH, is shown to be similar to that of a boehmite suspension. However, because of the particle-size difference, a boehmite-coated SiC suspension gels more slowly than suspensions of smaller boehmite particles. The boehmite coating improves the consolidation density of SiC, increasing the sediment density from 39 to 52 vol% and the centrifuged density from 50 to 60 vol%. It, also, makes the consolidation behavior of a boehmite-coated SiC suspension with respect to pH more consistent with the rheological behavior; i.e., lower suspension viscosity and storage modulus correlate with a higher consolidated density. In contrast, suspensions containing SiC particles partially covered with boehmite and individual boehmite particles in the suspensions show no improvement in the sediment density and no systematic correlation between the consolidation density and the rheological properties. This indicates that the complete coating of boehmite on the SiC particles is critical to the improvement in consolidation density.     

Polyelectrolyte Effects on the Rheological Properties of Concentrated Cement Suspensions

Polyelectrolyte species, known as superplasticizers, dramatically affect the rheological properties of dense cement suspensions. We have studied the influence of sulfonated naphthalene formaldehyde condensate (SNF) and carboxylated acrylic ester (CAE) grafted copolymers of varying molecular architecture on the surface (e.g., adsorption behavior and zeta potential) and rheological properties of concentrated cement suspensions of white portland cement and two model compounds, β-Ca₂SiO₄ and γ-Ca₂SiO₄. The adsorption of SNF species was strongly dependent on cement chemistry, whereas CAE species exhibited little sensitivity. The respective critical concentrations (Φ*) in suspension required to promote the transition from strongly shear thinning to Newtonian flow (flocculated → stable) behavior were determined from stress viscometry and yield stress measurements. Theoretical analysis of interparticle interactions suggested that only colloidal particles in the size range of ≤1 μm are fully stabilized by adsorbed polyelectrolyte species. Our observations provide guidelines for tailoring the molecular architecture and functionality of superplasticizers for optimal performance.     

Rheological Characterization of Synergistic Mixtures of Carrageenan and Locust Bean Gum for Aqueous Gelcasting of Alumina

Previous works have shown that carrageenan can be successfully used in the aqueous gelforming of powders, because carrageenan forms a firm gel, similar to that formed by agarose, but at a much lower cost. In this work, the synergistic effect of carrageenan with locust bean gum is studied. The rheological behavior of 2 wt% solutions of these polysaccharides and their mixtures are measured under mixing conditions (60°C) and by recording the viscosity and elastic modulus on cooling. The effect of the addition of these solutions to 50 vol% alumina slurries up to a concentration of 0.5 wt% is studied. Although gelling time increases, the resulting gels are stronger than for carrageenan alone. Gelcast alumina bodies with green and sintered densities of 57% and 97.6% of theoretical have been obtained.     

Cement Composition Effects on the Rheological Property Evolution in Concentrated Cement–Polyelectrolyte Suspensions

We have studied the rheological property evolution and hydration behavior of white and ordinary portland cement (type I) pastes and concentrated cement–polyelectrolyte suspensions. Cement composition had a marked effect on the elastic property evolution ( G '( t )) and hydration behavior of these suspensions in the presence of poly(acrylic acid)/poly(ethylene oxide) copolymer (PAA/PEO), even though their affinity to adsorb such species was nearly identical. Both white and ordinary portland cement pastes exhibited G '₀ values of ∼10⁴ Pa and fully reversible G '( t ) behavior until the onset of the acceleratory period ( t = 2 h), where the pastes stiffened irreversibly. In contrast, cement–PAA/PEO suspensions exhibited G '₀ values of ∼1 Pa and G '( t ) behavior comprised of both reversible and irreversible features. Interestingly, ordinary portland cement–PAA/PEO suspensions experienced a gel-to-fluid transition on high shear mixing at short hydration times (<1 h), and the particle network did not rebuild until ∼24 h of hydration. In sharp contrast, white portland cement–PAA/PEO suspensions remained weakly gelled throughout the initial stage of hydration even after high shear mixing. At longer hydration times (>1 h), both cement–PAA/PEO suspensions exhibited G ' _i ( t ) ∼ exp( t /τ_c) with τ_c values of 5.6 and 1.3 h for ordinary and white portland cement, respectively. Our observations suggest that hydration phenomena impact interparticle forces during early stage hydration and, ultimately, lead to initial setting through the formation of solid bridges at the contact points between particles within the gelled network.     

Influence of Acidity on the Stability and Rheological Properties of Ionically Stabilized Alumina Suspensions in Ethanol

The ionic stability of alumina particles in moderately concentrated ethanol suspensions is studied. Surface chemistry and interparticle forces are manipulated by controlling the acidity of the suspensions without dispersants. The acidity of ethanol solution is determined using ion transfer functions, wherein the relationships between acidity, alumina particle surface charge, zeta-potential, stability, and suspension rheological behavior are established. Positive isoelectric point (IEP) shift is observed for alumina in ethanol on increasing the solids concentration. However, dilute and concentrated aqueous suspensions of alumina give the same IEP. The viscosity and flow curves for alumina/ethanol suspensions are acidity dependent. The flow curves of the suspensions follow the Casson model, and the Casson yield value is used to evaluate suspension stability.     

Rheological assessment of metakaolin-based geopolymer composites through squeeze flow

Geopolymer (GP) composites show great potential as a replacement for ordinary Portland cement (OPC) in construction material, extrusion-based, and additive manufacturing. The rheological properties of highly viscous and reinforced systems have not yet been well studied, due to limitations in the current state of the art rheometers and viscometers, such as size and torque limits. In this study, the basic rheological properties of highly reinforced, geopolymer composites with potential for 3D printing are innovatively investigated with “squeeze flow” and “flow table” tests commonly used in civil engineering. Squeeze-flow rates of 0.1, 1.0, and 3.0 mm/s were assessed with varying sand weight percentages or basalt fiber lengths and compared to a conventional OPC mixture to differentiate the flow properties and deformation resistance of both materials. It is shown that the deformation resistance as a result of jamming increases with increasing solid reinforcement percentages, but that the overall effect of fiber size is somewhat inconclusive. In addition, the effect of squeeze-flow rate exhibits an increase in load required to initiate flow at lower squeezing rates, but, upon reaching a certain ratio of solids to liquid in the matrix, the results become variable.     

Effects of Boehmite-Coating Thickness on the Consolidation and Rheological Properties of Boehmite-Coated SiC Suspensions

We examined the effect of the boehmite coating thickness on the rheology and consolidation of boehmite-coated SiC suspensions. The thickness of the boehmite coating on SiC was varied by adjusting the boehmite concentration relative to SiC. For boehmite concentrations less than 10 wt%, the coating thickness increased with increasing boehmite concentration. For boehmite concentrations higher than 10 wt%, the coating thickness saturated. Further increase in the boehmite concentration led to the presence of small boehmite particles in the suspensions. All boehmite-coated suspensions gelled near their isoelectric points and the storage moduli of the gels with respect to pH exhibited a maximum near the isoelectric points. Below 10 wt% boehmite, the suspensions had very few small boehmite particles. The maximum storage modulus, G ^'_0,max, of the boehmite-coated SiC gel decreased with increasing coating thickness, t , as G ^'_0,max∝ t ⁻², in good agreement with our earlier theoretical prediction. Meanwhile, the maximum sedimentation densities, φ_max, of the coated suspensions occurred at around pH ∼ 4.0 and increased with increasing coating thickness from under φ_max= 25 vol% with 1 wt% boehmite to above φ_max= 65 vol% with 10 wt% boehmite due to increased zeta potential with increasing coating thickness. Above 10 wt% boehmite, the excess boehmite particles in the suspension increased the maximum suspension storage modulus, G ^'_0,max, and decreased the maximum sedimentation density, φ_max.     

Rheological, Structural, and Stress Evolution of Aqueous Al2O3:Latex Tape-Cast Layers

The rheological, structural, and stress evolution of aqueous alumina (Al₂O₃):latex tape-cast layers of varying composition were studied by shear rheology, direct visualization, and a controlled environment stress measurement device. Their low shear viscosity was nearly independent of the alumina:latex ratio for binary mixtures whose particle size ratio (λ=̄_alumina:̄_latex) approached unity, but varied over an order of magnitude for systems with particle size asymmetry. Direct visualization of these mixtures revealed that particle flocculation occurred as their total solids loading increased. Their structure was characterized at intermittent points during the drying process by imaging freeze-dried samples using scanning electron microscopy (SEM). Their corresponding stress histories exhibited three distinct regions: an initial period of stress rise, followed by a stress maximum, and, finally, a period of stress decay. Pure alumina layers exhibited a maximum stress of ∼1 MPa and a residual stress below 0.01 MPa. Pure latex films exhibited a maximum stress of ∼0.1 MPa and only a slight stress decay. The ceramic phase dominated the initial period of stress rise, while the latex phase strongly influenced the residual stress of composite layers cast from alumina:latex suspensions. Their maximum drying stress increased with decreasing Al₂O₃ particle size, whereas their residual stress increased with increasing latex T _g.     

Nonaqueous Aluminum Nitride Extrusion: I, Die-Entry Flow Behavior

Simultaneous orifice and capillary rheometry at 160°C was used to determine flow parameters of aluminum nitride extrudates plasticized with a polyethylene–mineral-oil binder. Data were analyzed using the Benbow model for square-entry ram extrusion. Variables included the powder concentration, polymer molecular weight and concentration, and lubricant concentration. Die-entry rheology was explained in terms of the mean interparticle separation, polymer spatial requirements, and lubricant adsorption. Particles and polymer were observed to not pack independently. The yield stress of the body increased rapidly as the solids loading approached the critical volume limit of extrudability. An increase in polymer molecular weight or partial decrease of adsorbed lubricant increased the yield stress of the body and produced a satisfactory extrudate.     

SiAlON粉体合成的研究进展

概述了SiAlON材料的组成、结构特点、理化性能和应用领域 ,全面阐述了SiAlON粉体的合成方法以及由天然原料合成SiAlON粉体的研究进展     

Molecular-level composition design for efficient synthesis of SiAlON ceramics

SiAlONs are a class of liquid-phase sintered ceramics with excellent room-temperature strength and toughness, but whose residual grain boundary glass softens at high temperatures, limiting use in extreme environments. For this reason, efforts are made to minimize the volume of the grain boundary glass while still facilitating full densification. This work describes a potential route for the densification of SiAlONs with very low concentrations of liquid-phase sintering additive (e.g., rare-earth oxides such as yttria) by using an organometallic precursor. Solid solution of Al and O in the Si₃N₄ lattice was accomplished through the incorporation of solute atoms via liquid organic precursor aluminum sec-butoxide (ASB). Al₂O₃ powder is conventionally used for this purpose, and the subsequent lattice softening associated with the solid solution helps to facilitate densification. However, a liquid-phase additive is still essential for the full densification of SiAlONs. Higher densities were obtained from SiAlON powder blends utilizing organometallic ASB than those utilizing alumina powder, allowing for greater densification at very low Y₂O₃ concentrations. The thermal decomposition of the organic precursor was investigated by high-temperature scanning electron microscopy, thermogravimetric analysis, and various X-ray diffraction experiments. Immersion density measurements and lattice parameter refinements were performed for samples sintered with varying Y₂O₃ concentrations and/or dwell times. Results indicate that ASB-containing powder blends favor SiAlON formation more strongly than Al₂O₃-containing powder blends and favor densification at very low Y₂O₃ concentration.     

碳热还原氮化法制备SiAlON陶瓷材料

碳热还原氮化工艺是近年来制备低成本高性能SiAION陶瓷材料的一种实用方法．具有产业化生产潜力。本文对碳热还原氮化法制备SiAION的进展进行了综述,归纳分析了不同条件对生成物性能的影响,对今后的研究进行了展望。     

Effect of 2-Phosphonobutane-1,2,4-tricarboxylic Acid Adsorption on the Stability and Rheological Properties of Aqueous Nanosized 3-mol%-Yttria-Stabilized Tetragonal-Zirconia Polycrystal Suspensions

2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA) was evaluated as a dispersant for nanosized 3-mol%-Y₂O₃-stabilized tetragonal-ZrO₂ polycrystal (3Y-TZP) suspensions. The adsorption of PBTCA was characterized using the decolorization method of ferric 5-nitrosalicylate complexes. Maximum adsorption of the dispersant on the 3Y-TZP powder was found to occur at pH 3.0. At pH >3.0, the adsorbed amount decreased with increased pH. Semiquantitative analysis using auger electron spectroscopy showed that PBTCA adsorbed irreversibly on the powder. The surface charge of the powder was evaluated by measuring the zeta-potential in dilute powder suspensions. The suspension was most effectively stabilized at high pH by the high charge induced by the adsorption of PBTCA. Rheological properties of the suspension were evaluated as a function of dispersant amount and solids loading. The optimum amount of dispersant increased with increased solids loading for solids loading >20 vol%. A stable suspension of 35 nm 3Y-TZP particles with a solids loading as high as 32 vol% was obtained using PBTCA as dispersant, in contrast to 28 vol% when using ammonium polyacrylate (NH₄PAA). Theoretical calculations of the interaction between 3Y-TZP particles showed that the stabilization of the suspensions was attributed to a combination of the electrostatic repulsion and a steric barrier caused by the adsorbed PBTCA. Induced coupling plasma analysis showed that PBTCA could be completely burned out during sintering, which confirmed its suitability as a dispersant for 3Y-TZP.