水泥老化过程中水动态的准弹性中子散射(QENS)谱分析

准弹性中子散射(quasi-elastic neutron scattering, QENS)实验是研究水泥老化过程中水动态的一种新颖的实验方法.本文利用老化时间分别为7, 14和30 d水泥样品的QENS谱实验数据, 通过应用四个高斯项的和的能量分辨函数R(Q, E)代替一个高斯项的能量分辨函数来改进经验扩散模型(empirical diffusion model, EDM), 再进行非线性最小二乘拟合.由此导出水泥样品中水动态的相关物理参数: 不动水数密度A, 自由水指数FWI=B₁/(A+B₁+B₂), 洛伦兹函数的半高宽Γ, 移动水跳跃之间的平均停留时间τ ₀及自扩散系数D_t, 而且可得出更为精准的QENS谱拟合曲线.拟合得到的物理参数可定量描述水泥老化过程中水动态过程, 从而为QENS实验在水泥老化过程中水动态研究的应用提供一种合理实用的谱分析方法.     

基于准弹性中子散射谱分析水化硅酸钙(C-S-H)中受限水的动态

研究水化硅酸钙(C-S-H)中受限水动态的一种重要手段是准弹性中子散射(quasi-elastic neutron scattering, QENS)实验. C-S-H样品的QENS谱数据可通过跳跃扩散和转动扩散模型进行分析拟合, 进而导出C-S-H样品微纳孔中水动态的相关物理参数: 不动水指数C、转动扩散系数D_r、均方位移< u² >、自扩散系数D_t及平均停留时间τ₀. 本文对水与水泥质量比为30%的C-S-H样品, 测量温度为230-280 K的QENS谱进行了分析, 得到的拟合参数可定量描述C-S-H样品内不同尺度的微纳孔中受限水随温度变化的动态过程. 转动扩散系数D_r随散射矢量Q的变化可知, Q值较大时, 水分子的转动对QENS谱影响较大. 均方位移 < u² > 随Q值的不同而变化, 其拟合值可区分C-S-H样品中的不动水、强受限水和受限水. 在Q较小时, D_t 和τ₀ 的拟合值随温度而变化, 并分别在230和240 K突然增大, 由此揭示温度为230-240 K 时, C-S-H 样品中受限水分子的动态特性发生了转变.     

Factors Affecting Early-Age Hydration of Ordinary Portland Cement Studied by NMR: Fineness,Water-to-Cement Ratio and Curing Temperature

The aim of the present study was to apply nuclear magnetic resonance (NMR) relaxation measurements for understanding the microstructure evolution of cement paste during hydration. Ordinary Portland cement powder was mixed with double-distilled water, and hydration process was analyzed via ¹H proton NMR spin–spin relaxation time. In order to induce strong modification of the rate of hydration, water-to-cement ratio, curing temperature and cement fineness were varied. The evolution of the NMR spin–spin relaxation time, T ₂, of hydrating water versus the hydration time was monitored from the very first few minutes after the mixing up to several hours. Authors' address: Marcella Alesiani, Department of Physics, University La Sapienza, Piazzale Aldo Moro 5, 00185 Rome, Italy     

Theoretical and experimental study of structures and properties of cement paste: The nanostructural aspects of C-S-H

Ordinary Portland cement (OPC) paste with water to cement ratio 0.5 has been studied by FTIR spectroscopy and TEM/SAED techniques. The progressive shift in frequencies and change in intensity of characteristic peaks in the FTIR spectra of cement powder indicate polymerization of silicates due to hydration reaction. The appearance of new peaks around 975-985 and 3640-3650 cm⁻¹ in FTIR spectra of cement paste has been attributed to the formation of two principal hydration products (C-S-H) and Ca(OH)₂, respectively. There is a strong resemblance of peak shape at 980 cm⁻¹ of cement paste with that of synthesized 1.1 nm Tobermorite. TEM/SAED techniques reveal the presence of three distinct types of C-S-H nanostructures viz. spherical particles, fibres and bundle of fibres, and nano tubes in cement paste. Theoretical study of two C-S-H clusters by MNDO and Density Functional Techniques shows excellent agreement between computed structural data and reported experimental results of Tobermorite and Jennite. The computed interplanar Ca-Ca distance (11.162 Å) of C-S-H cluster (Ca/Si=1.125) indicates greater structural similarity of this cluster to 11 Å Tobermorite. There is a strong similarity of IR profile of C-S-H cluster (Ca/Si=1.125) with 1.1 nm Tobermorite. Above study indicates that this cluster is probably the most likely basic unit of C-S-H phases formed during the early stage of OPC cement hydration.     

The dynamical crossover phenomenon in bulk water, confined water and protein hydration water

We discuss a phenomenon regarding water that was until recently a subject of scientific controversy, i.e. the dynamical crossover from fragile-to-strong glass-forming material, for both bulk and protein hydration water. Such a crossover is characterized by a temperature T(L) at which significant dynamical changes occur, such as violation of the Stokes-Einstein relation and changes of behaviour of homologous transport parameters such as the density relaxation time and the viscosity. In this respect we will consider carefully the dynamic properties of water-protein systems. More precisely, we will study proteins and their hydration water as far as bulk and confined water. In order to clarify the controversy we will discuss in a comparative way many previous and new experimental data that have emerged using different techniques and molecular dynamic simulation (MD). We point out the reasons for the different dynamical findings from the use of different experimental techniques.     

Interactions between chloride and cement-paste materials

The durability of cement-based materials with respect to exterior aggressions is one of the current priorities in civil engineering. Depending on their use, the cement-based materials can be exposed to different types of aggressive environments. For instance, damages to concrete structures in contact with a saline environment (sea water on bridges, deicing salts on roads, etc.) are of utmost importance. Upon exposure to saline water, Cl- ions penetrate into the structures and subsequently lead to reinforcement corrosion. Chloride attack is often combined with other aggressive influences such as temperature (e.g., freezing) or the ingress of other ions (e.g., sulfates in sea water). We therefore aim to explore the effect of sodium chloride (NaCl) on the structural chemistry of cement paste. Existing studies about reinforcement corrosion by chloride have focused on the penetration of Cl- ions and the comparison between "free" ions (water-soluble ions) and bound ones. However, little is known about the fixation mechanisms, the localization of Cl in the cement matrix and the structural interaction between Cl and the silicate and aluminate hydrate phases present in cement paste. We present here results of a multinuclear nuclear magnetic resonance study on the fixation of chloride in the hydration products and the characterization of new phases potentially appearing due to chloride ingress.     

Glass transition in biomolecules and the liquid-liquid critical point of water

Using molecular dynamics simulations, we investigate the relation between the dynamic transitions of biomolecules (lysozyme and DNA) and the dynamic and thermodynamic properties of hydration water. We find that the dynamic transition of the macromolecules, sometimes called a "protein glass transition," occurs at the temperature of dynamic crossover in the diffusivity of hydration water and also coincides with the maxima of the isobaric specific heat C_{P} and the temperature derivative of the orientational order parameter. We relate these findings to the hypothesis of a liquid-liquid critical point in water. Our simulations are consistent with the possibility that the protein glass transition results from crossing the Widom line, which is defined as the locus of correlation length maxima emanating from the hypothesized second critical point of water.     

Dynamic behaviors of water contained in calcium-silicate-hydrate gel at different temperatures studied by quasi-elastic neutron scattering spectroscopy

The dynamic behaviors of water contained in calcium-silicate-hydrate(C-S-H) gel with different water content values from 10%to 30%(by weight),are studied by using an empirical diffusion model(EDM) to analyze the experimental data of quasi-elastic neutron scattering(QENS) spectra at measured temperatures ranging from 230 K to 280 K.In the study,the experimental QENS spectra with the whole Q-range are considered.Several important parameters including the bound/immobile water elastic coefficient A,the bound water index BWI,the Lorentzian with a half-width at half-maximum(HWHM) Γ_1(Q) and Γ_2(Q),the self-diffusion coefficients D_(t1) and D_(t2) of water molecules,the average residence times τ_(01)and τ_(02),and the proton mean squared displacement(MSD)(u~2) are obtained.The results show that the QENS spectra can be fitted very well not only for small Q(≤1 A~1) but also for large Q.The bound/immobile water fraction in a C-S-H gel sample can be shown by the fitted BWI.The distinction between bound/immobile and mobile water,which includes confined water and ultra-confined water,can be seen by the fitted MSD.All the MSD tend to be the smallest value below 0.25 A~2(the MSD of bound/immobile water) as the Q increases to 1.9 A~1 no matter what the temperature and water content are.Furthermore,by the abrupt changes of the fitted values of D_(t1),τ_(01),and Γ_1(Q),a crossover temperature at 250 K,namely the liquid-to-crystal-like transition temperature,can be identified for confined water in large gel pores(LGPs) and/or small gel pores(SGPs) contained in the C-S-H gel sample with 30% water content.     

The influence of hydration time on the Ni uptake by cement

In this study, Ni uptake by hardened cement paste has been investigated with the aim of improving our understanding of the immobilization process of Ni(II) in cement and the influence of the hydration time on the Ni speciation on the microscopic scale. Information on the Ni distribution and speciation of the Ni phases formed in the cement system has been gained by employing synchrotron-based μ-X-ray fluorescence (μ-XRF) and μ-X-ray absorption spectroscopy (μ-XAS). The Ni-doped cement samples were prepared at a water/cement ratio of 0.4 and with a metal loading of 5000 mg/kg using a sulphate-resisting Portland cement. The samples were hydrated for six hours and one year to account for the chemical environment in a fresh and aged cement paste, respectively. The μ-XAS measurements reveal that a mixture of Ni phases is formed at single regions of interests, independent of the hydration time. Data analysis further indicates that Ni(II) is predominantly immobilized in a layered double hydroxide (LDH, Ni-Al LDH) and only to a minor extent precipitates as Ni-hydroxides. A comparison of the results from this micro-spectroscopic investigation with those from an earlier macro-spectroscopic study, indicate that the same Ni phases are formed both on the macro- and micro-scale.     

Cracks and pores – Their roles in the transmission of water confined in cementitious materials

Cement paste is formed through a process called hydration by combining water with a cementitious material. Concrete, the worlds most versatile and most widely used material, can then be obtained when aggregates (sand, gravel, crushed stone) are added to the paste. The quality of hardened concrete is greatly influenced by the water confined in the cementitious materials and how it is transmitted through cracks and pores. Here we demonstrate that the water transport in cracks and capillary pores of hardened cement pastes can be approximately modeled by simple equations. Our findings highlight the significance of arresting the development of cracks in cementitious materials used in repository barriers. We also show that neutron scattering is an advantageous technique for understanding how water transmission is effected by gel pore structures. Defining measurable differences in gel pores may hold a key to prediction of the reduction of water transport through cement barriers.     

矿物掺合料作为砂浆细集料的水化产物与光谱性能研究

粉煤灰、锂渣和钢渣作为工业废渣,等质量替代水泥时其利用率往往较低,为了大量地使用这些工业废渣。采用X射线单晶衍射仪、同步热分析仪、傅里叶红外光谱仪和电镜扫描分析方法,研究了锂渣、粉煤灰和钢渣替代细集料后砂浆的水化产物、光谱特性、微观形貌,并探讨了砂浆抗折/抗压强度随替代率(0%,30%,50%,70%和100%)增长的变化规律。研究结果表明：四种浆体的水化产物主要为CSH凝胶、Ca(OH)₂、少量的AFt和未水化的颗粒(Al₂O₃,SiO₂),其中水泥-锂渣浆体、水泥-粉煤灰浆体、水泥-钢渣浆体中的未水化颗粒还含有一定的Li₂O·Al₂O₃·SiO₂,Ca_1.56SiO_3.5·xH₂O和RO phas。四种浆体以3 467,3 438,2 923,2 348,1 638,1 429,1 111,1 000,768,696和462 cm-1为特征峰,但其峰强有所不同,其活性也不同,参与二次水化反应的程度也不同,因此,水泥-钢渣浆体中Ca(OH)₂的含量明显高于水泥-粉煤灰浆体和水泥-锂渣浆体的现象;但不管是矿物掺合料替代水泥还是细集料,都在浆体中发挥着火山灰活性和填充作用。含三种100%矿物掺合料砂浆的抗折强度和抗压强度均高于纯水泥砂浆,分别(锂渣、粉煤灰和钢渣)约高37.77%/51.88%,14.71%/11.70%,91.95%/34.88%,但其达到峰值的掺量不同。因此,采用矿物掺合料替代细集料是可行的,能大幅度提高工业废渣在混凝土行业中的使用,且能达到节能减排的效果。     

Hydration properties and compressive strength development of Low Heat Cement

For Low Heat Cement (LHC), its hydration reaction properties and compressive strength development were studied by quasi-elastic neutron scattering (QENS) and compressive strength test. In the QENS experiments, the amount of hydration products was estimated and the hydration behavior of LHC was revealed in the early hydration times of 7 days. Simultaneously, we compared the compressive strength of a mortar specimen made from LHC with the amount of hydration products. It was found that the compressive strength is strictly proportional to the amount of hydration products in the range from 15 h to 7 days of hydration.     

The1HT 1 study of the influence of clay addition on Portland cement hydration

Effects of addition of three standard clay minerals, Na-montmorillonite, illite and kaolinite, on Portland cement hydration properties were studied. The¹H spin-lattice relaxation of exchangeable water was monitored during hydration time and the data were processed by spin-grouping analysis. The values and evolution dynamics of both resolvedT ₁ components and corresponding magnetization fractions show that each day mineral lowers the fluidity of Portland cement paste and accelerates its hydration in dormant. In advanced stages of hydration, the Na-montmorillonite provides the accelerating influence, while the kaolinite exhibits the retarding effect. The final values of gel pores to capillaries percentage fractions ratio indicate a slightly lower porosity of samples with Namontmorillonite and a higher porosity of pastes with the same percentage of illite or kaolinite, regarding to the pure hardened Portland cement.     

Proton NMR relaxation as a probe for setting cement pastes

We report a 20-MHz proton nuclear magnetic resonance T1 relaxation study of cement paste hydration in the early stages of setting, using different centimeter-sized samples of cements of various origins and different water-to-cement ratios. In every sample, during the first few minutes of hydration, it is found that inverse Laplace processing of inversion-recovery measurements systematically exhibits at least two T1 values: a long one, around 100 ms, whose value correlates well with water content and which may be attributed to bulk water surrounding cement grains; and a short one, around 2 ms, which is quite insensitive to water-to-cement ratio and which may be attributed to water embedded in floculated cement grains before setting occurs. The time evolution of the longest T1 value for several hours is also shown to exhibit a characteristic five-stage behavior that is well correlated with known stages of the hydration process: initial reaction, induction period, acceleration period, deceleration period and slow hydration reaction. These results are compared with calorimetric measurements and electrical conductivity literature.     

The Effect of Curing Temperature on Early Hydration of Gray Cement Via Fast Field Cycling-NMR Relaxometry

In the present work, we use fast field cycling (FFC) nuclear magnetic resonance relaxometry to evaluate the influence introduced by the curing temperature on the hydration process of gray cement. The main advantage of FFC relaxometry as compared with other relaxation studies performed at a specific frequency is that it is sensitive to a wider range of molecular motions and better separates the surface and bulk contributions from the global measured relaxation rate. In the case of cement hydration, the relaxation process is dominated by the interaction of water protons with the paramagnetic centers located on the surface of cement grains. This allows us in the frame of a two-phase exchange model to monitor the temperature dependence of the transverse diffusional correlation time at the surface of cement grains. An increase of the surface diffusion coefficient of water molecules with the temperature was revealed. Another outcome is that the surface-to-volume ratio of capillary pores continuously increases during the early hydration and this process is strongly enhanced by rising the temperature.     

The Boson peak interpretation and evolution in confined amorphous water

We study the evolution of the Boson peak for water confined in cement paste obtained by means of Inelastic Neutron Scattering from room temperature to the deep supercooled regime. We analyze the data in terms of a universal-like model, developed for glass forming liquids in the frame of the energy landscape. In such a way it is possible to discriminate between the fragile and strong glass forming character of supercooled liquids and the dynamical crossover, fragile to strong, between them. Hence, we confirm the link between the Boson peak and the water polymorphism. In particular, the main result is represented by a crossover from a local energetic minima configuration to that characteristic of saddle points on going towards the deep supercooled regime up to the dynamical arrest.     

The hardening of Portland cement observed by1H spin-lattice relaxation and single-point imaging

The nature of hydration in ordinary Portland cement samples from different Polish factories was investigated by the proton spin-lattice relaxation timeT ₁ and single-point imaging (SPI). The time dependence of the spin-lattice relaxation timeT ₁ shows the usual behavior:T ₁ is constant at the beginning of hydration (the dormant period), then, during the setting process, decreases to the constant value in the hardening period of hydration. However, there are some anomalies inT ₁ connected with the different amount of the mineral components in the cement sample. The unusual behavior of the stretched parameterα, after about 20 h of hydration, is observed and connected with the microcracking of the cement matrix. The 1-D and 2-D SPI images of the cement paste taken at various times of hydration show the distribution of the water throughout the sample and clearly demonstrate the usefulness of SPI in the study of the internal structure of the porous system.     

Micropore size analysis in hydrated cement paste by NMR.

We present a time evolution of 1H spin-lattice relaxation rates in the laboratory (1/T(1)) and in the rotating frame (1/T(1rho)) of a synthetic cement paste. The typical results found for both rates allows us to follow the main hydration stages of the cement paste and the refinement of its microporosity. In particular the texturation of the porosity and the structuration of the surface of the material is evidenced.     

Conductivity in hydrated proteins: no signs of the fragile-to-strong crossover

Dielectric spectroscopy studies of hydrated protein demonstrate smooth temperature variations of conductivity. This observation suggests no cusplike fragile-to-strong crossover (FSC) in the protein's hydration water. The FSC at T approximately 220 K was postulated recently on the basis of neutron scattering studies [Chen, Proc. Natl. Acad. Sci. U.S.A. 103, 9012 (2006)] and was proposed to be the main cause for the dynamic transition in hydrated proteins. Following Swenson et al. , we ascribe the neutron results to a secondary relaxation. We emphasize that no cusplike solvent behavior is required for the protein's dynamic transition.     

Translational diffusion of water in compacted clay systems

The translational diffusion of water in compacted clays at a high hydration level has been investigated by quasielastic neutron scattering at a time-of-flight spectrometer FOCUS (SINQ). Four compacted clays with systematic structural differences have been studied: Na-montmorillonite, Na-illite, kaolinite and pyrophyllite. The QENS experiments were performed using two different incident wavelengths in order to access a larger Q range and verify the data analysis. The translational diffusion coefficient for water in Na-montmorillonite and Na-illite are lower than those for bulk water, whereas the preliminary results for kaolinite and pyrophyllite show larger diffusion coefficient.