基于稳态流变测试的水泥浆体剪切模式研究

采用RHEORIAB QC型旋转黏度计测定不同剪切方式下水泥浆体稳态流变曲线,并用修正宾汉姆流变模型对其进行拟合,研究了剪切方式对水泥浆体稳态流变测试的影响.结果表明:预剪切对水泥浆体动态屈服应力和塑性黏度影响不大.恒定剪切变形速率小于、等于、大于、偏大于水泥浆体结构抗剪切破坏能力时,剪切应力随时间分别先线性增加接着保持同一值、直接保持恒定值、先减小接着保持同一值、轻微增加后达到平衡.随剪切速率增加,水泥浆体在0.1～100 s-1、100～400 s-1、400～600 s-1三个区段依次呈现出剪切变稀、宾汉姆流体、剪切增稠的流变行为.剪切速率变化范围向剪切变稀或剪切增稠段移动,水泥浆体动态屈服应力减小、塑性黏度增大;单个剪切速率的剪切时间越长,水泥浆体动态屈服应力、塑性黏度均越小.     

纳米SiO2对新拌水泥浆体流变性能的影响

选用RHEOLAB QC型旋转黏度计,分别测试了内掺水泥质量分数0.0％、1.0％、2.0％与3.0％纳米SiO2的水泥浆体在5℃、20℃、30℃时的稳态流变曲线、静态屈服应力、恒定速率剪切下的表观黏度与剪切应力,系统研究了不同环境温度时纳米SiO2对新拌水泥浆体流变性能的影响.结果表明:随纳米SiO2掺量增加,水泥浆体的动态屈服应力与稠度系数均显著增大;随温度升高,掺纳米SiO2的水泥浆体的稠度系数均增大,纳米SiO2掺量为0％与1.0％的水泥浆体的动态屈服应力增大,而纳米SiO2掺量为2.0％、3.0％的水泥浆体的动态屈服应力基本不变;随纳米SiO2掺量增加,水泥浆体的静态屈服应力与其在10～120 min的增长速率均增大;20℃、30℃时,随纳米SiO2掺量增加,水泥浆体的剪切变稀程度增大;5℃与30℃时,随纳米SiO2掺量增加,水泥浆体的触变指数增大;当纳米SiO2掺量为1.0％时,水泥浆体的活化能低,温度敏感性弱,热稳定性好.     

多孔SiC中黏弹性流体的流变性数值研究

为研究黏弹性流体在多孔SiC中流变行为,将多孔SiC的骨架建成十四面体结构单元的阵列,利用POLYFLOW计算流体力学软件,黏弹性流体的本构方程采用PTT模型,主要研究黏弹性流体流经多孔SiC的法向应力及剪切应力随松弛时间、多孔介质孔径以及流速变化规律,以及剪切速率的分布特点。黏弹性流体弹性越强,孔道结构对其作用的应力越大,从而使聚合物分子链发生断裂的几率越大。     

氧化石墨烯对新拌水泥浆体流变性的影响

随着纳米材料和技术不断发展,关于氧化石墨烯对水泥基材料改性作用的相关研究越来越受到重视。在此背景下,通过Anton Paar Rheolab QC型旋转黏度计研究了氧化石墨烯对新拌水泥浆体流变性的影响,并测试了浆体静态屈服应力、动态屈服应力和黏度系数以及触变环面积。结果表明:相同氧化石墨烯掺量下,随浆体静置时间延长,低剪切速率下浆体的静态屈服应力逐渐增大;随氧化石墨烯掺量增大,浆体结构建立参数、动态屈服应力和黏度系数先增大后减小,而触变滞后环面积则呈现相反的变化趋势。     

基于扩展度动态测量的水泥浆体流变参数表征

建立了侧面滑移模型,并推导出水泥浆体剪切应力和剪切速率的计算公式.将公式应用到水泥浆体扩展度的动态变化过程,获得浆体扩展度随时间线性变化阶段的“剪切应力-剪切速率”曲线,求得不同水灰比水泥浆体的塑性粘度和屈服应力,并与旋转粘度计的测量值进行了比较.结果 表明,随时间延长,水泥浆体扩展速率表现出先慢后快再变慢的特点,并存在快速变化的线性段.扩展度快速变化线性段的塑性粘度与屈服应力经比例系数修正后与旋转粘度计测定值吻合良好.由此说明,对最终扩展度在一定范围(180 ～ 320 mm)的水泥浆体,可以通过扩展度动态测量表征浆体的流变参数.     

水泥-白云石粉浆体流变性能研究

试验采用旋转黏度计测定了水泥-白云石粉浆体剪切应力和塑性黏度随剪切速率变化的规律,对所测浆体T-γ曲线采用Bingham流体模型进行拟合,得到浆体屈服应力和塑性黏度,采用Power Law流体模型拟合出浆体的流变指数,并用触变环面积表征浆体的触变性.研究结果表明:在0 ～ 30％掺量范围内,随白云石粉掺量的增大,水泥浆体的屈服应力、塑性黏度和触变性均逐渐增加;白云石粉细度对浆体屈服应力和塑性黏度影响较小.     

一个胶凝原油的蠕变模型

为了研究胶凝原油的启动屈服过程,确定胶凝原油的蠕变模型至关重要。基于不同剪应力下胶凝原油的蠕变特性,建立了具有黏性流的黏弹性固体流变模型来描述胶凝原油的黏弹流变特性,通过试验验证该模型能精确描述胶凝原油的黏弹性蠕变过程;若施加的剪应力高于胶凝原油的塑性屈服应力,胶凝原油的蠕变将从稳定流变阶段很快达到加速流变阶段,表现为典型的黏弹塑性特征,将非线性黏塑性体和具有黏性流的黏弹性固体流变模型串联起来得到一个非线性黏弹塑性剪切流变模型,该模型能充分反映胶凝原油的加速剪切蠕变过程,并与试验结果吻合的较好。     

大理石粉颗粒丛特性对水泥浆体流变性能影响

试验研究了水泥-大理石粉浆体剪切应力和塑性黏度随剪切速率变化的规律,使用Bingham流体模型和PowerLaw流体模型分别对浆体低剪切速率和高剪切速率阶段的剪切应力进行拟合,得到浆体屈服应力,塑性黏度系数.探究了水泥-大理石粉颗粒丛特性即颗粒分布宽度、颗粒数密度、总比表面积和Zeta电位与浆体屈服应力和塑性黏度的关系.研究结果表明:大理石粉比表面积小于640m2/kg时,起到稀化浆体的作用,大于640m2/kg时起稠化作用,且稀化或稠化的作用随其掺量的增加而增大.浆体中颗粒间接触点数量和固-液相接触面积增大使浆体屈服应力,塑性黏度提高.大理石粉降低了水泥浆体Zeta电位,浆体中粒子间静电作用力减弱,对流动性起到削弱作用.     

水泥浆体屈服应力和黏度与其测试分析影响因素研究

采用RHEOLAB QC型旋转黏度计,分别以转速与剪切速率为控制变量测定了水泥浆体稳态流变曲线,分别用Couette转换与流变模型研究了水泥浆体屈服应力和黏度与测试及分析影响因素之间的关系.结果表明:Couette转换能反映测试条件变化对屈服应力和黏度的影响;增大试样筒与转子的间距,由转速转化的剪切速率增大,颗粒迁移作用增强,动态屈服应力递减,塑性黏度递增;增大试样筒外壁的摩擦系数,浆体与外筒壁间相对运动情况不发生变化,对动态屈服应力与塑性黏度基本无影响;依据拟合相关系数,流变模型中Herschel-Bulkley模型的吻合程度最好;依据结果标准差,Herschel-Bulkley模型的动态屈服应力结果稳定性最好,Bingham与改进Bingham模型的塑性黏度结果的稳定性相差不大.     

橡胶剪切静动态特性试验及建模分析

设计了橡胶剪切试验夹具和试验方法,测试橡胶在不同压缩比下的剪切静动态特性。提出一种橡胶剪切模型:剪切应力由弹性应力、粘性应力和摩擦应力叠加,其中粘性应力表征橡胶的频率相关性,摩擦应力表征橡胶的振幅相关性。弹性应力模型和摩擦应力模型参数由准静态剪切试验识别,粘性应力模型参数由动态剪切试验识别,并通过曲线拟合得到各模型参数随橡胶压缩比变化的函数关系。     

Method for calculating dynamic yield stress of fresh cement pastes using a coaxial cylinder system

The calculation of the rheological parameters of fresh cement pastes plays a key role in understanding the rheology of cement-based mixes. Because cement paste is not a simple Bingham fluid, a suitable nonlinear model must be found for characterizing its flow. A test system in which the rotational speed or shear rate can be changed in multiple steps is regarded as a suitable rheological test protocol because the paste reaches a steady state. Furthermore, theoretical derivations show that the solution of the Couette inverse problem corresponding to the modified Bingham model and the Herschel–Bulkley (H-B) model is complex. However, a comparative analysis revealed that the yield stress of fresh paste could easily be obtained through a calculation process based on a Parabolic model. This study presents the complete calculation procedure for this model. The influence of the plug flow is considered, and test points with low minimum shear stress (τ_min) are excluded. Finally, the accuracy of the proposed method is verified through comparisons with the results obtained using mini-cone slump tests. These results show that the dynamic yield stress calculated using the expression of the Couette inverse problem based on the Parabolic model in consideration of the plug flow is very close to the yield stress obtained using the mini-cone slump flow test. This proves that the proposed method could precisely characterize the dynamic yield stress of cement pastes.     

A new look at the measurement of cementitious paste setting by Vicat test

The Vicat test is a standard test for measuring the setting times of cement paste and mortar. The physical background of the test is based on the resistance of a paste to dynamic penetration by a rod with a certain weight and shape (shear strain). The information obtained (initial and final set time) is very useful to compare cement setting properties. This study shows that it is possible to obtain more fundamental information about the setting property kinetics with only one modification of the testing procedure.The apparent mass of the static full immersed needle is measured.Due to the deformation of the cement paste at rest, the needle apparent mass varies with time. We show that the variation of the stress mobilized at the plate surface is related to the increase of yield stress during the setting period. The results of these experiments are discussed and compared with the traditional Vicat test for cement paste.     

Use of creep recovery protocol to measure static yield stress and structural rebuilding of fresh cement pastes

In this study, a creep recovery shear rheological protocol was applied to fresh cement pastes. A viscosity bifurcation behavior was observed through applying a range of creep stresses. When applied stress is sufficiently low viscosity increases and the material yields, exhibiting viscoelastic solid-like behavior. Beyond a critical stress viscosity decreases and the material flows, exhibiting viscoelastic liquid-like behavior. Through examining this bifurcation behavior we found that the transition of viscosity occurs at very low strains. The strains at which this transition occurred were compared with critical strains measured through low amplitude oscillatory shear. Results provided support that the solid-liquid transition occurs beyond the critical stress measured through creep, thereby tying it to static yield stress. The protocol was implemented to probe pastes modified with attapulgite clays, a highly thixotropic system, and was found to be effective in characterizing static yield stress and thixotropic rebuilding.     

Estimating time and temperature dependent yield stress of cement paste using oscillatory rheology and genetic algorithms

A controlled shear stress–shear rate rheometer was used to determine the viscoelastic behavior of cement paste incorporating various superplasticizers and subjected to prolonged mixing at high temperature. At a low applied shear stress range, the oscillatory shear strain/stress curve of cement paste was characteristic of a linear elastic solid; while the higher stress range was characteristic of a viscous liquid exhibiting a linear strain increase with increasing applied shear stress. The transition from solid-like to liquid-like behavior occurred over a very narrow stress increment. This transition stress corresponded to the yield stress parameter estimated from conventional flow curves using the Bingham model. The yield stress from oscillatory shear stress tests was estimated using the intersection between the viscous part of the oscillatory shear strain/stress curve and the oscillatory shear stress axis. In this study, equations describing the variation of shear strain versus shear stress beyond the solid–fluid transition for cement pastes incorporating various superplasticizers at different ambient temperatures and mixing times were developed using genetic algorithms (GA). The yield stress of cement pastes was subsequently predicted using the developed equations by calculating the stress corresponding to zero strain. A sensitivity analysis was performed to evaluate the effects of the mixing time, ambient temperature, and superplasticizer dosage on the calculated yield stress. It is shown that the computed yield stress values compare well with corresponding experimental data measured using oscillatory rheology.     

Effect of coarse particle volume fraction on the yield stress and thixotropy of cementitious materials

In order to help in modelling the yield stress of fresh concrete, we study the behavior of suspensions of coarse particles in a thixotropic cement paste. Our aim is to relate the yield stress of these mixtures to the yield stress of the suspending cement paste, to the time passed at rest, and to the coarse particle volume fraction. We present here procedures that allow for (i) studying a homogeneous and isotropic suspension, (ii) comparing the yield stress of a given cement paste to that of the same cement paste added with particles, and (iii) accounting for the thixotropy of the cement paste. We observe that the yield stress of these suspensions of cement paste with coarse particles follows the very simple Chateau-Ovarlez-Trung model [X. Chateau, G. Ovarlez, K.L. Trung, Homogenization approach to the behavior of suspensions of noncolloidal particles in yield stress fluids, J. Rheol. (2008) 52 489-506.], consistently with the experimental results of Mahaut et al. [F. Mahaut, X. Chateau, P. Coussot, G. Ovarlez, Yield stress and elastic modulus of suspensions of noncolloidal particles in yield stress fluids, J. Rheol. (2008) 52 287-313.] obtained with many different particles and suspending yield stress fluids. This consistency between the results obtained in various yield stress fluids shows that the yield stress of the suspension does not depend on the physicochemical properties of the suspending yield stress fluid; it only depends on its yield stress value. This shows that studies of suspensions in model yield stress fluids can be used as a general tool to infer the behavior of fresh concrete. Moreover, we show that the thixotropic structuration rate of the interstitial paste (its static yield stress increase rate in time) is not affected by the presence of the particles. As a consequence, it is sufficient to measure the thixotropic properties of the constitutive cement paste in order to predict the thixotropic structuration rate of a given fresh concrete. This structuration rate is predicted to have the same dependence on the coarse particle volume fraction as the yield stress.     

Creep and relaxation of cement paste caused by stress‐induced dissolution of hydrated solid components

The creep and relaxation of cement paste caused by dissolving solid hydration products is evaluated in this work. According to the second law of thermodynamics, dissolution or precipitation of solid constituents may be altered by the change in stress/strain fields inside cement paste via alteration of the stress power or strain energy. Thus, it is hypothesized that stress‐induced dissolution can affect the overall creep/relaxation behavior of cement composites. A novel, fully coupled thermodynamic, mechanical, and microstructural model (TM²) that uses the finite element method was developed to predict the time‐evolving properties of cement paste under prescribed strains and to test the hypothesis. In the model, the strain energy was incorporated to accurately predict the effect of stress and strain fields on cement microstructure change. From the simulation results, depending on the stress/strain levels and the choice of the domain (over which the thermodynamic equilibrium is enforced), stress‐induced dissolution of solid constituents can lead to significant creep/relaxation.     

Measuring permeability and stress relaxation of young cement paste by beam bending

When a saturated rod of a porous material is deflected in three-point bending, two types of time-dependent relaxation processes occur simultaneously: hydrodynamic relaxation, caused by the flow of liquid in the porous body, and viscoelastic (VE) relaxation of the solid network. By measuring the decrease in the force required to sustain a constant deflection, it is possible to obtain the permeability from the hydrodynamic relaxation function, in addition to the VE stress relaxation function of the sample. We report the early-age evolution of permeability, elastic modulus, and stress relaxation function for Type III Portland cement paste with water–cement (w/c) ratios of 0.45, 0.50, and 0.55. The stress relaxation function is shown to preserve its shape during aging; that function is numerically transformed into the creep function.