低场核磁共振技术在水泥基材料中的应用

近年来,核磁共振技术已经成为表征水泥基材料水化进程、非化学结合水量和孔结构的一种重要手段。简单介绍了核磁共振基本原理和低场核磁共振主要参数;阐述了低场核磁共振技术在水泥基材料测试中的应用,主要包括水化进程、孔结构、水分迁移和扩散等方面的内容;展望了低场核磁共振技术在水泥基材料研究中的应用前景。     

低水灰比对硅酸盐水泥水化程度的影响

研究了低水灰比硅酸盐水泥的水化程度，并利用XRD和SEM分析了其水化产物的微观结构。结果表明在低水灰比条件下，水泥的水化程度较低，其硬化水泥浆体中存在较多的未水化水泥；同时由于自身的密实性增强和体系的低孔隙率，使水泥水化产物的结晶、生长情况也受到影响。     

硬化水泥浆体弹性模量细观力学模型

应用复合材料力学理论和有孔介质力学(Poromechanics)理论建立了一个描述硬化硅酸盐水泥浆体弹性模量的细观力学模型, 将硬化水泥浆体从不同尺度上划分为4个层次, 即C-S-H凝胶、 水泥水化产物、 水泥浆体骨架和水泥浆体, 分别应用不同的细观力学模型予以描述: 将C-S-H视为饱和的有孔介质; 应用Mori-Tanaka模型描述水泥水化产物的弹性性质; 应用三相模型(Three-phase model)模拟水泥浆体骨架的有效弹性模量; 最后, 再次应用Mori-Tanaka模型和有孔介质理论, 计算水泥浆体的排水和不排水弹性模量(Drained and undrained elastic moduli)。该模型所需要的参数为水泥浆体各个组成部分的自身弹性性质, 使用方便。通过预测文献中的实测结果, 证明了该模型的有效性。      

普鲁兰多糖对水泥净浆性能的影响

主要研究一种新型微生物多糖——普鲁兰多糖(Pullulan)对新拌水泥浆体性能的影响,对比分析了不同掺量的普鲁兰多糖对水泥净浆标准稠度用水量、凝结时间、流动度和Zeta电位的影响,以及对硬化水泥浆体力学性能的影响.研究表明:普鲁兰多糖增大了水泥浆体标准稠度用水量,延长了水泥的凝结时间;随着普鲁兰多糖掺量的增加,新拌水泥浆体的初始流动度降低,但随着时间的延长,流动度不降反升;普鲁兰多糖的掺入降低了水泥浆体的Zeta电位,0～13 min内,Zeta电位值极度不稳定,然后趋于平稳状态;普鲁兰多糖与减水剂(PC)复掺后,随着普鲁兰多糖掺量的增加,水泥浆Zeta电位发生了复杂的变化过程;普鲁兰多糖对硬化水泥浆体的抗压强度无明显不利影响.     

弯曲荷载对硬化水泥浆体和砂浆碳化过程的影响

研究了弯曲荷载对硬化水泥净浆和砂浆试体碳化过程的影响.结果表明:弯曲荷载会加速水泥浆体和砂浆的碳化过程,在弯曲荷载作用下水泥浆体和砂浆的碳化动力学过程可用式x=C0eaηtb描述,其中b值均接近1/2;碳化时间对砂浆碳化深度的影响略小于对水泥浆体的影响,而应力水平对水泥浆体和砂浆碳化的影响大致相同.     

冻融作用下硬化水泥浆体和混凝土结构的变化

用压汞法和扫描电子显微镜研究了冻融循环作用下硬化水泥浆体和混凝土宏观与细观结构的变化.结果表明,冻融以后硬化水泥浆体和混凝土中砂浆的孔隙率,特别是大于50 nm的毛细孔体积和微裂纹增大较多,导致硬化水泥浆体由密实体向松散体发展,最后导致粗骨料与砂浆分离.提出了快速冻融破坏机理与自然条件下缓慢冻融的破坏机理存在差异的观点.     

固体核磁共振技术在水泥及其水化产物研究中的应用

固体核磁共振作为一种十分理想的强大的分析手段,已在水泥及水泥水化产物研究领域得到了广泛的应用。本文在介绍核磁共振基本原理以及固体核磁共振技术的基础上,重点介绍了~(27)Al和~(29)Si核磁共振在水泥熟料矿物分析、水泥及其他水泥掺合料的水化产物分析、水化过程研究中的应用情况,评价了目前固体核磁共振技术在水泥领域存在的问题,并展望了其在无机非金属材料领域的发展趋势。     

废水泥浆对水泥胶砂性能的影响研究

研究了废水泥浆澄清液、浆体以及干燥后的粉体对水泥胶砂流动度以及抗压强度的影响,为废水泥浆在预拌混凝土生产中的的回收利用提供技术基础,解决废水泥浆的环境污染和资源化利用问题。     

铜离子掺杂对水泥水化的影响机理

本文研究了铜离子对熟料矿相种类、结构和水化进程的影响机理。结果表明:铜离子对水泥水化进程的缓凝抑制作用按照影响机理分为两个阶段:第一阶段,铜离子掺杂导致水化体系中Ca~(2+)浓度降低,早期氢氧化钙(Cu(OH)_2)结晶形成受阻,破坏了反应溶液的高碱性环境,且钙矾石(Aft)的形成速率减小、生成量减少,诱导期延长,水化产物间无法快速搭接形成网状结构,水泥浆体凝结硬化受阻;第二阶段,铜离子造成C_3S晶粒异常长大,水化反应活性降低,水泥浆体结构变得疏松、孔隙率增大,从而导致其后期强度显著降低,且孔隙中分布晶体尺寸较大的钙矾石,相互穿插,形成水泥浆体中的薄弱联结。     

碳化与氯盐复合作用下水泥浆体的微结构演变

为了阐明碳化与氯盐复合作用下硬化水泥浆体的微结构,基于X射线计算机断层扫描成像(X-CT)和电子探针微区分析(EPMA)技术探明了氯盐对水泥浆体碳化速率的影响,测定了碳化作用下水泥浆体内Cl、S和Na元素的浓度分布。结果表明:氯盐可细化养护龄期为28d的水泥浆体孔结构,提高其密实度并减缓碳化速度;碳化作用下水泥浆体的碳化区易出现裂缝,二氧化碳气体通过这些裂缝扩散到水泥浆体内部进行碳化,致使水泥浆体碳化深度不均匀;碳化过程中Cl、S和Na元素向非碳化区迁移和浓缩,初始均匀分布的元素在碳化区含量减少,在非碳化区含量升高。所得结论为混凝土结构耐久性设计提供科学的理论依据。     

NMR,XRD, IR and synchrotron NEXAFS spectroscopic studies of OPC and OPC/slag cement paste hydrates

This work aims to determine the fundamental similarities and/or differences between OPC and OPC/slag paste hydrates. OPC and 35% slag pastes are investigated using five techniques: ²⁹Si NMR, ²⁷Al NMR, X-ray diffraction (XRD), infrared (IR) and synchrotron near edge X-ray absorption fine structure (NEXAFS) spectroscopy. ²⁹Si NMR provides valuable information related to the formation of the C–S–H gel, the main hydrated phase of the cement paste. ²⁷Al NMR is a useful tool to characterize calcium aluminates and aluminate hydrates such as ettringite and monosulphate hydrate. XRD identifies polycrystalline phases of the hardened cement paste, including ettringite, monosulphate and CaOH₂. Vibrational frequencies in IR assist in identifying the silicate, sulphate and carbonate phases of the cement paste. As far as we are aware, Si K-edge NEXAFS has never been applied in cement research and its advantages and disadvantages are discussed. Using these techniques, a comparison between OPC and 35% slag paste hydrates is made, shedding light on differences in the amount and form of hydrated phases present, especially the absence of ettringite in the 35% slag paste.     

Effects of sand aggregate on ultrasonic attenuation in cement-based materials

Ultrasonic wave attenuation measurements have been successfully used to characterize the microstructure and mechanical properties of inhomogeneous materials; these ultrasonic techniques have the potential to provide for the in-situ characterization of microstructure changes in cement-based materials due to damage. Recent research has applied acoustic scattering models to quantitatively predict ultrasonic attenuation for evaluating the air void content in hardened cement paste. The objective of the current research is to investigate the influence of sand aggregate on the ultrasonic attenuation as a first step towards a full simulation of more realistic microstructures in real concrete. Hardened cement paste samples containing sand aggregate of varying volume fractions are considered. The research employs an independent scattering model and a self-consistent effective medium theory to predict the scattering-induced attenuation of longitudinal ultrasonic waves by the sand inclusions distributed in the cement paste matrix. The predicted attenuation coefficients are compared with measured ones. It is observed that at low volume fractions, both models provide a good estimate of the total attenuation in the specimens. These results indicate that it is possible to use a physics-based model to quantify the effect of sand aggregate on ultrasonic attenuation.     

Solid state NMR investigations on the role of organic admixtures on the hydration of cement pastes

The influence of different inorganic additives and organic admixtures on the hydration and hardening of Portland cement (CEM I 42.5R) were studied on a nanometer scale by advanced solid state NMR methods. Added quartz was found to be partially attacked by the alkaline media of the cement paste. Even small amounts of organic admixtures strongly influence the hydration and crystallization process of the cement paste. Methyl cellulose, poly(vinyl acetate co vinyl alcohol), poly(ethylene oxide), poly(acrylic acid) and poly(acrylamide) modify the hydration of the calcium aluminum oxides. Major changes in the inorganic structure were detected for low amounts of citric acid and tartaric acid which suppress silicate condensation and strongly alter calcium aluminum oxide hydration. Within this study several solid state NMR methods like 1D magic angle spinning (MAS), 2D exchange and 2D double quantum NMR were applied for the detection of ¹H, ²⁷Al and ²⁹Si nuclei. Thus, cement pastes, inorganic additives and organic admixtures could be monitored individually. The findings on a molecular level as provided by NMR are related to changes in the mechanical properties of the cement pastes.     

低应力水平下混凝土中氯离子扩散行为多尺度分析方法

混凝土的扩散渗透性能与其微观结构（包括细观尺度上粗骨料颗粒与砂浆之间的界面过渡区及砂浆本身的微观结构等）密切相关。在微观尺度上,砂浆和界面过渡区均可视为由无孔砂浆基质和孔隙水夹杂相组成的两相复合材料,二者的主要区别表现为孔隙率不同。在外荷载作用下,砂浆和界面过渡区的毛细孔隙率及孔隙连通性会发生改变,从而改变混凝土的扩散渗透性能。基于此,该文建立了低应力水平下混凝土中氯离子扩散行为多尺度理论分析方法,获得了混凝土表观扩散系数与外荷载（以体应变表征）及砂浆和界面过渡区当前孔隙率的定量关系。分析所采用的主要参数为砂浆和界面过渡区的毛细孔隙率、无孔砂浆基质和骨料相的力学参数、骨料相和界面过渡区的体积分数、外荷载等。与已有文献数据对比知,该文分析结果与之吻合良好,表明了理论分析方法的合理性与准确性。此外,基于该方法,探讨分析了混凝土微/细观结构对其宏观扩散性能的影响。     

Area of lineal-path function for describing the pore microstructures of cement paste and their relations to the mechanical properties simulated from μ-CT microstructures

The pore distribution of a cement paste strongly affects its mechanical behavior such as its stiffness and strength. Porosity is an influential parameter that can be used to identify the complex pore microstructures of cement paste, but it has limitations as a scalar parameter. In this study, the lineal-path function, a low-order probability function, is investigated as a supplement or an alternative parameter for describing the microstructural characteristics of cement paste microstructures. In particular, the area of the lineal-path function is used as a measure of the pore microstructural characteristics, which can be linked with its properties.A relatively new method for simulating crack propagation, the crack phase field model, is used to evaluate the stiffness and tensile strength of cement paste microstructures and the evaluated properties are linked to the proposed characterization parameters. The evaluation is performed on virtual specimens obtained from micro-level computerized tomography (μ-CT) images of real cement paste specimens. The validity of the microstructure-property relations obtained from the proposed characterization parameters and the crack phase field model are confirmed through the statistical analysis of dozens of specimens.It is concluded that the correlation between the area of the lineal-path function and the mechanical properties is very strong. The parameter could potentially be used as a supplementary or an alternative parameter to describe the pore microstructures of cement paste.     

荷载作用下饱和水泥浆体中氯离子扩散性能研究

混凝土类水泥浆复合材料中各种尺度的孔隙,如凝胶孔、毛细孔、掺入的气体气泡以及微裂纹等影响着氯离子的扩散性能。孔隙结构参数(如孔隙率)在外荷载作用下会产生变化,进而影响了水泥浆体中氯离子扩散性能。外荷载作用对氯离子扩散行为的影响,可以等效为外荷载所引起的孔隙率的改变对氯离子扩散性能的影响。从微观角度出发,将饱和水泥浆体看作由水泥浆体基质(其孔隙率为零)和孔隙水夹杂相所组成的两相复合材料介质。基于弹性力学理论推导并获得了饱和水泥浆体达到其强度前(即未产生新裂纹前)当前孔隙率与材料初始孔隙率及体应变之间的定量关系,得到了水泥浆体中氯离子扩散系数与这些参数的定量关系。基于Fick第二定律分析了外荷载(体应变)和孔隙率变化对氯离子扩散性能的影响。研究表明:氯离子在饱和砂浆中的扩散系数随孔隙率增大而显著增大;氯离子在砂浆中的扩散系数随压缩体应变的增大而减小,随拉应变增大而增大。     

Coupled AC impedance and thermomechanical analysis of freezing phenomena in cement paste

Freezing and thawing properties of cement paste were studied using an alternating current impedance spectroscopy (ACIS) technique coupled with the thermomechanical analysis (TMA). The measurements from the ACIS were analyzed in association with the length change of the cement paste determined by TMA. The micro-scale behavior of cement paste observed by the ACIS technique was well correlated with the residual expansion of cement paste after the freezing and thawing cycle. The efficacy of the ACIS technique for assessing the durability of cement paste to frost action was confirmed.     

Drying/hydration in cement pastes during curing

As concrete cures in the field, there is a constant competition for the mixing water between evaporation and hydration processes. Understanding the mechanisms of water movement in the drying/hydrating cement paste is critical for designing curing systems and specialized rendering materials, as well as for selecting repair materials and methodologies. In this work, X-ray absorption measurements indicate that fresh cement paste dries uniformly throughout its thickness, as opposed to exhibiting the sharp drying front observed for most porous materials. Furthermore, in layered composite cement paste specimens, water always flows from the coarser-pore layer to the finer one, both when coarser pores are produced by using an increased water-to-cement ratio (w/c) and when they are present due to using a cement with a coarser particle size distribution at a constant w/c. Conversely, no clear differential water movement is observed between layers of cement paste and mortar of the same nominal w/c. Based on the results of these experiments, drying has been introduced into the NIST CEMHYD3D cement hydration and microstructure development model, by emptying the largest water-filled pores present at any depth in the model specimen at a user-specified (drying) rate. With this addition, the CEMHYD3D model produces results in good agreement with experimental observations of both the drying profiles and the hydration kinetics of thin cement paste specimens.     

跨尺度模拟硬化水泥净浆力学性能:微观物相的影响

采用纳米压痕实验测量硬化水泥净浆中未水化物、水化产物和孔隙等微观物相的力学性能,并基于背散射电镜（BSE）图像的灰度分析计算各微观物相的含量。在得到各微观物相含量和力学性能的基础上,针对水泥净浆的弹性模量进行均一化建模,并讨论各微观物相及其模量的选取对跨尺度模拟硬化水泥净浆力学性能的影响。通过微米压痕的实测净浆模量验证模型及参数选取的可靠性。提出在硬化水泥净浆力学性能的多尺度模型中,需要选取12 GPa作为孔隙有效模量,并将水化产物划分为低密度的水化硅酸钙凝胶（LD-CSH）和高密度的水化硅酸钙凝胶（HD-CSH）两种物相,而不同种类的未水化物可被视作一种物相。在此基础上,使用Mori-Tanaka模型或自洽模型计算得到的净浆模量与实测净浆模量吻合。     

NMR relaxometry during internal curing of Portland cements by lightweight aggregates

The hydration of an ordinary Portland cement of low water-to-cement ratio under the influence of internal curing by addition of water-saturated lightweight aggregates (LWA) is investigated by non-destructive low-field NMR relaxometry. The methodology, which was recently developed to investigate internal curing by addition of a water-saturated polyelectrolyte gel, is applied to follow the transition of water from the mineral aggregates into hydrating cement paste and to detect the time dependence of the water consumption by the hydration reaction. By the changes of the transverse relaxation times of the physically bound water, the compaction of the pore space of the hardening cement is estimated qualitatively. The water retention potential of two types of LWA and the temporal moisture requirement of the cement during internal curing are determined without and with superplasticizer.