The rim zone of cement-based materials – Barrier or fast lane for chemical degradation?

Sophisticated evaluation models for the long-term stability of cement-based systems demand a precise knowledge of the mechanisms of deterioration reactions, particularly respecting a permanent exposure to aqueous environments. Commonly, insights into these mechanisms are deduced from long-term investigations. However, these chemical reactions start immediately after exposure to aggressive environments causing rapid changes of composition and structure. Consequently, properties of its rim zone change, which affects transport processes in aqueous solutions. In laboratory experiments, the influence of these surface processes on the stability of cement-based materials exposed to different chloride solutions was studied as a function of time and temperature. Analysis of compositional and structural changes beneath the surface reveal the role of crystalline covering layers for chemical resistance. Such layers are often described as protective barriers. However, these processes in the rim zone can accelerate chemical degradation and subsequently reduce the resilience of the cement-based materials to aggressive aqueous environments.     

Comparison of observed and simulated cement microstructure using spatial correlation functions

The microstructure of cement pastes, as revealed by SEM-BSE image analysis, was compared with a simulated structure generated by the University of Twente version of the CEMHYD3D hydration simulation model. The spatial array of unhydrated cement particles was simulated by the model. However, spatial features in capillary pore structure obtained by the simulation are different from the observed microstructure. This disagreement in the spatial structure is to be expected since there are fundamental differences in porosity as represented by the two methods. Only coarse pores are detected in the SEM examination while the total capillary porosity and its whole spatial distribution are virtually simulated in the model. A subset of the visible pores must be different in spatial statistics from the universal set of total porosity. Care must therefore be taken in interpreting agreement between simulation output and microscopically observed microstructure in images.     

Bleeding and sedimentation of cement paste measured by hydrostatic pressure and Turbiscan

Sedimentation and bleeding of cement pastes with lignosulfonate were studied by visual observation, HYdroStatic Pressure Test (HYSPT) and Turbiscan measurements showing two bleeding stages: a fast initial phase followed by a phase with diminishing sedimentation rate. A turbid bleeding zone establishes during the fast bleeding phase and the top layer gradually becomes transparent in the diminishing phase within minutes or hours depending on admixture and solid fraction. The bleeding rates measured visually and by HYSPT in the first 2 h are higher than the ones calculated by Kozeny-Carman Equation, whereas turbiscan shows lower rates. Both HYSPT and Turbiscan monitor the particle and fluid fluxes and thus describe bleeding from turbid to clear zone respectively by observing the density variation of bulk paste or the change in optical density of the surface region. Lignosulfonate reduces bleeding by improving particle dispersion to various degrees depending on types and dosages.     

A practical model for efficient anti-fatigue design and selection of metallic materials:Ⅱ.Parameter analysis and fatigue strength improvement

In the first paper,a Y-T-F model was proposed based on the restrictions of both strength and plasticity;the corresponding applications on the fatigue strength prediction have also been discussed.In this second paper,the emphasis will be put on the issues of fatigue strength improvement.Based on the primary form of the Y-T-F model,the parametersare further analyzed and quantified,to clarify the influences of various factors on fatigue strength.Firstly,the damage capacity C is proved to be sensitive to the elastic modulus E,which could change with the alloying components and nano-scaled grain boundaries;the increase of E would lead to the increasing C,thus increase the fatigue strength.Secondly,the microstructure charac-teristic coefficient a,as well as the yield strength σy and tensile strength σb in the crack initiation region could be influenced by the processing mode,grain size and microstructure uniformity of materials;the change of microstructure characteristics would affect the changing tendency of tensile strength-fatigue strength relation via varying the values of a,σy and σb.Thirdly,the damage weight coefficient ω is found to be a reflection of the fatigue strength declination induced by defects;the defect dimension D,the defect shape correlated stress concentration coefficient Kt,as well as the strengthening level of matrix materials σb are all corresponding factors.Quantified correlations between the above parameters and corresponding factors are comprehensively built up,hence obtaining the influences of either a single fac-tor or multiple factors on fatigue strength.This further developed Y-T-F model would be helpful to clarify the direction of fatigue strength improvement,and contribute to the anti-fatigue design optimization of metallic materials.     

Rheological characterization of a time dependent fresh cement paste

Combining equilibrium and non-equilibrium rheological tests, the yield stress and the plastic viscosity corresponding to the completely build-up structure of a fresh cement paste have been determined. This experimental method is proposed to characterize time-dependent fluids because the values of these parameters are not dependent on the particular structural state of the system and, consequently, are not affected by the particular way in which the rheological test has been designed.     

The relations between the shear modulus, the bulk modulus and Young's modulus for porous isotropic ceramic materials

A relation between the shear modulus and Young's modulus of isotropic porous ceramics has been derived based on the Mori–Tanaka mean-field approach. The applicability of the relation has been evaluated using the experimental values available in the literature for the shear modulus, the bulk modulus and Young's modulus of porous ceramics prepared using various processing techniques and powder sizes. It is also shown that the ratio of the shear to Young's modulus of porous ceramics can be approximated by a constant value of 0.391.     

Localized elastic modulus distribution of nanoclay/epoxy composites by using nanoindentation

The enhancement of mechanical properties by the use of nanoclay platelets in epoxy resin has been extensively investigated through numerous experimental techniques recently. Elastic modulus was obtained mainly from the tensile test of bone-like nanoclay/epoxy specimens. The results from the tensile test have only showed the globalized mechanical properties of composites and their localized elastic modulus distribution has been neglected. Despite the orientation and the degree of exfoliation of nanoclay platelets inside nanoclay/epoxy composites, the localized elastic modulus is important for the understanding of the distribution of agglomerations of nanoclay platelets. The elastic modulus of nanoclay/epoxy composite samples made under different sonication temperatures would be examined by nanoindentation to compare their localized mechanical behaviors. Scanning electron microscopy (SEM) would also be employed to study the distribution of the nanoclay clusters throughout the composites. The results showed that the elastic modulus varied throughout the composites and the nucleation theory of clusters was modified to explain the behavior of nanoclay agglomerations under different sonication temperatures in which the viscosity of the epoxy resin was varied. The gravitational effect was significant to cause the non-uniform distributions of nanoclay clusters at low sonication temperature.     

Investigation into the stabilization/solidification performance of Portland cement through cement clinker phases

This research studied the influence of individual heavy metal on the hydration reactions of major cement clinker phases in order to investigate the performance of cement based stabilization/solidification (S/S) system. Tricalcium silicate (C3S) and tricalcium aluminate (C3A) had been mixed with individual heavy metal hydroxide including Zn(OH)2, Pb(OH)2 and Cu(OH)2, respectively. The influences of these heavy metal hydroxides on the hydration of C3S and C3A have been characterized by X-ray diffraction (XRD) and differential scanning calorimetry-thermogravimetry (DSC-TG). A mixture of Zn(OH)2, Pb(OH)2 and Cu(OH)2 was blended with Portland cement (PC) and evaluated through compressive strength and dynamic leach test. XRD and DSC-TG data show that all the heavy metal hydroxides (Zn(OH)2, Pb(OH)2 and Cu(OH)2) have detrimental effects on the hydration of C3A, but only Zn(OH)2 does to the C3S at early curing ages which can completely inhibit the hydration of C3S due to the formation of CaO(Zn(OH)2).2H2O. Cu6Al2O8CO(3).12H2O, Pb2Al4O4(CO3)(4).7H2O and Zn6Al2O8CO(3).12H2O are formed in all the samples containing C3A in the presence of metal hydroxides. After adding CaSO4 into C3A, the detrimental effect of heavy metals increases due to the coating effect of both calcium aluminate sulphates and heavy metal aluminate carbonates. The influence of heavy metal hydroxide on the hydration of C3S and C3A can be used to predict the S/S performance of Portland cement.     

Transient plane source measurements of the thermal properties of hydrating cement pastes

A transient plane source measurement technique is applied to assessing the heat capacity and thermal conductivity of hydrating cement pastes in their fresh state and during the course of 28 d of hydration at 20°C. Variables investigated include water-to-cement mass ratio (w/c – 0.3 or 0.4) and curing conditions (sealed or saturated curing). The heat capacity data for the fresh cement pastes are compared to a simple law of mixtures, and analytical expressions are developed to estimate the heat capacity as a function of degree of hydration for the two curing conditions. The measured thermal conductivities of the fresh pastes along with the known thermal conductivity of water are used to estimate the thermal conductivity of the original cement powder via application of the Hashin-Shtrikman (H-S) bounds. Hydration is seen to have only a minor influence on the measured thermal conductivity. Extension of the law of mixtures for heat capacity and the H-S bounds for thermal conductivity to predicting the corresponding properties of concretes are discussed.     

Elastic Constants of Mantle Minerals at High Temperature

A simple theoretical model is developed to investigate the temperature dependence of elastic constants. The method is based on a new expression for the temperature dependence of the bulk modulus and the formulation derived from Tallon’s model. The proposed relationship is applied to study the elastic constants of the solids of significance to geophysics applications. The calculated values of elastic constants are found to show good agreement with available experimental data.     

水泥熟料固化重金属Cu2+和Zn2+及水化浸出行为分析

危废处理是当前的热点问题,水泥窑协同处置作为一种有效的处理方式,逐步为社会所接受。多数的危废中包含Cu²⁺和Zn²⁺,文章研究了危废中重金属Cu²⁺和Zn²⁺在水泥熟料中的固化性能和在熟料中的分布,并探讨了重金属在水泥净浆中的浸出行为和环境安全性。通过熟料易烧性X射线衍射(XRD),矿物相分离萃取,浸出实验得出:Cu²⁺和Zn²⁺均改善了熟料易烧性;Cu²⁺促进了C₄AF的生成,同时也促进了C₃S晶粒的生长,并固溶在其中;Zn²⁺与熟料形成新的矿物相Ca₁₄Al₁₀Zn₆O₃₅。通过相对分布系数(D)和分配系数(K_f)说明Cu²⁺主要分布在硅酸盐相中,硅酸盐相固化Cu²⁺的能力强于中间相;Zn²⁺主要分布在中间相中,中间相固化Zn²⁺的能力强于硅酸盐相。掺量为2.0%的Cu²⁺和Zn²⁺在水泥净浆7 d龄期的浸出浓度最大,分别为1.724和0.387 mg·L^-1。水泥熟料固化Cu²⁺和Zn²⁺在水泥使用过程中不会对环境造成二次污染。