Fatigue behavior of hardened cement paste

The fatigue behavior of cement hardened paste was studied in compression loading with an R ratio (σ min / σ max) equal to 10. The samples were immersed in water at room temperature. The fatigue life as a function of the stress amplitude, displayed a large scatter. The longitudinal and the radial deformations were recorded. Three stages were revealed. At the beginning of the tests the deformation per cycle increased. A steady, state was then reached, which occupied most of the life of the specimen ; during this stage the longitudinal deformation per cycle was constant whereas there was no axial deformation. The deformation rate increased again at the end of the test just before the specimen shattered to pieces. A very good correlation was found between the fatigue life and the deformation rate per cycle during steady state. The acoustic emission and the ultrasonic waves attenuation displayed the same behavior as the deformation rate.     

Study of interactions between sodium monofluorophosphate and hardened cement paste and their consequences on concrete transfer properties

Surface applied corrosion inhibitors are one of the methods used to protect embedded reinforcing bars from corrosion. This paper presents a study of interactions that can occur between sodium monofluorophosphate (MFP) and concrete during its transport from the applied surface to the steel. Interactions of MFP with portlandite have clearly been observed with X-ray diffraction diagrams. Tests with solutions have been realised, and show that an amorphous gel appears when MFP is in contact with a saturated calcium hydroxide solution. Percolation tests have highlighted the consequences of these reactions on concrete properties. In the last part of this study, quantitative chemical analysis of the percolate is presented: they show that when the concentration of the MFP solution increases, more MFP is found in the percolate even if a large part is kept in the concrete sample.     

Effect of the curing temperature on the creep of a hardened cement paste

Uniaxial creep tests have been performed at ambient temperature on a saturated hardened class G cement paste hydrated at 60 °C and 90 °C. The results have shown that creep is enhanced at higher curing temperature. The visco-damage model of Challamel et al. (2005) has been extended and used to analyze the experimental results. The model parameters have been calibrated on the results of the creep tests performed at different stress levels for both curing temperatures. The model correctly reproduces the effect of the curing temperature on time dependent properties of the material. The enhanced creep at higher curing temperature is attributed to the development of more damage in cement paste and to significant weakening of the mechanical properties.     

Observation of an electromechanical effect of hardened cement paste

A bending moment was created by an external load in a bar of hardened cement paste. Two electrodes had been attached to opposite surfaces. With this experimental set-up a stress dependent voltage could be determined between the tension and compression zone. This observation is called direct electromechanical effect. The generation of an electric field between the two electrodes results in a bending moment in the bar. The bending of the bar caused by this effect (converse effect) was recorded. Movement of adsorbed water layers, e.g. electrically charged double layers (electroosmosis), is supposed to be responsible for this behaviour.     

锂渣掺量对水泥-减水剂浆体流变特性和混凝土性能的影响

本文研究了锂渣掺量对水泥-减水剂浆体流变特性、新拌混凝土工作性及硬化混凝土强度的影响。从紧密堆积理论和固体颗粒体积分数角度解释了锂渣掺量对水泥-减水剂浆体流变性能的影响。结果表明：锂渣水化初始的屈服应力和塑性黏度均随锂渣掺量的增加而增大，流变性能劣化。新拌混凝土的工作性与锂渣掺量呈负相关；随锂渣掺量的增加，7 d抗压强度不断降低，28 d抗压强度呈现出先增后降的趋势。     

Fracture of hardened cement paste and concrete

The paper reviews literature on work in the field of fracture mechanics of hardened cement paste, cement mortar and concrete from the very beginning in 1929. Application of Griffith's theory for fracture of brittle solids to heterogeneous composite cement and concrete materials is discussed. Experimental data on fracture parameters from previous work by others is tabulated and compared. Macroscopic fracture mechanism and crack patterns of cement paste and concrete in tension and compression, as well as microscopic aspects of fracture mechanism in cement paste, are discussed.     

Effect of the variations of clinker composition on the poroelastic properties of hardened class G cement paste

The effect of the variations of clinker composition on the poroelastic properties of class G oil-well cement pastes is studied using a multiscale homogenization model. The model has been calibrated in a previous work based on the results of a laboratory study. Various compositions of class G cements from literature are used in a hydration model to evaluate the volume fractions of the microstructure constituents of hardened cement paste. The poroelastic parameters such as drained bulk modulus, Biot coefficient, and Skempton coefficient are evaluated using the homogenization model. The results show that the variations in chemical composition of class G cements have no important effect on the variations of the poroelastic properties.     

煅烧气氛对水泥浆体流变性的影响

用旋转黏度计和微型坍落度仪测定水泥浆体的流变性,又利用岩相分析、XRD、水化热等测试方法分析相关的作用机理,研究探讨煅烧气氛对水泥浆体流变性的影响。结果表明,在还原气氛下煅烧的熟料,其中的Fe2O3部分被还原为FeO、FeS,因而减少了C4AF含量,增大了C3A含量,使得水泥的水化速度加快,浆体的流变性降低。     

Study on the high‐temperature behavior and rehydration characteristics of hardened cement paste

The high‐temperature behavior and rehydration characteristics of the hardened cement paste and their mechanisms have been studied in this paper. X‐ray diffraction and thermogravimetry are used to establish the effect of elevated temperatures on the mineralogical changes that occurred in the hardened cement paste. The change of microstructure was characterized by scanning electron microscopy. The results showed that with the temperature increased, the compressive strength of hardened cement paste first increased and then decreased. According to micromeasurements, at 400°C, the porosity and average pore diameter of hardened cement paste increased slightly, while at 800°C, the porosity and average pore diameter of hardened cement paste increased sharply. When hardened cement paste was cured after exposing to 400°C, its pore structure and phase composition had no change, while when hardened cement paste was cured after exposing to 800°C, there are new hydration products, and its pore structure may be finer, but it cannot fully recover to the original state. Copyright © 2014 John Wiley & Sons, Ltd.     

The effect of undrained heating on a fluid-saturated hardened cement paste

The effect of undrained heating on volume change and induced pore pressure increase is an important point to properly understand the behaviour and evaluate the integrity of an oil well cement sheath submitted to rapid temperature changes. This thermal pressurization of the pore fluid is due to the discrepancy between the thermal expansion coefficients of the pore fluid and of the solid matrix. The equations governing the undrained thermo-hydro-mechanical response of a porous material are presented and the effect of undrained heating is studied experimentally for a saturated hardened cement paste. The measured value of the thermal pressurization coefficient is equal to 0.6 MPa/°C. The drained and undrained thermal expansion coefficients of the hardened cement paste are also measured in the heating tests. The anomalous thermal behaviour of cement paste pore fluid is back analysed from the results of the undrained heating test.     

Effect of curing conditions on the total water content of hardened portland cement paste

The effect of various curing conditions on the amount of water held by pastes with low water/cement ratios was investigated. It was concluded from the results that factors significantly influencing the total water content of a cement paste made with ordinary Portland cement and cured at room temperature are: water/cement ratio, curing medium, curing period and curing history.     

Experiments on the contact angle between mercury and hardened cement paste

Experiments on the contact angle of mercury with hardened cement paste (HCP) were performed. Circular holes of 150 to 1,050 micrometers in diameter were formed by casting HCP around small diameter wires. The pressure required to intrude the unevacuated, optically measured holes was used to determine a range of upper-bound values. For HCP, this was found to be from 139° to 150°. Furthermore, the contact angle was not significantly affected by the cleanliness of the mercury, the water-cement ratio of the paste, nor vacuum drying of the HCP.     

On the significance of electroosmosis in hardened cement paste

The observation of an electrochemical effect of hardened cement paste has recently been described in the literature. Further experiments underline the fact that electroosmosis is the origin of this effect. Water movement under the action of an electric field causes partial swelling and shrinkage. A bending moment is created in a similar way as in a bimetal strip. The electromechanical effect has been studied as a function of water/cement ratio, of age of specimen and of moisture content. Results are discussed on the basis of a tentative and simplified explanation.     

Immediate and delayed thermal expansion of hardened cement paste

Experiments are reported in which the thermal deformations were measured of thin tubular specimens of hcp conditioned to various relative humidities between 0 and 100%. Attention was concentrated on the first cycle of heating and cooling and on a second heating; the level of temperature was also an experimental variable. It was found that the thermal movement could be considered as the sum of two deformations, the immediate (taking place during the temperature change) and the delayed (taking place after temperature equilibrium was reached). The immediate deformation was approximately the same for all three temperature changes, was approximately linear with temperature, and exhibited a maximum value at an intermediate conditioning humidity towards 100%. The delayed deformation had a smaller reversible component, and a larger irreversible component which was associated only with the first heating. With only a few exceptions all the measurements of delayed deformation were of the same sense as the preceding immediate deformation. Measurements were made of two distinct types of evaporable water. There were no significant changes in the quantities of either as a result of temperature cycling.     

Water sorption properties of hardened cement paste cured or stored at elevated temperatures

Structural changes take place when mature hardened cement paste cured at room temperature is heated. BET V_m values were found to decrease as a function of the time the specimens had been stored in water either at 60°C or at 100°C. The decrease is greatest at 100°C. The complete water adsorption isotherms show that heating also makes the pore structure more coarse. The development of BET surface areas for pastes cured near 100°C depend strongly on the temperature history before the curing temperature is reached.     

Some properties of hardened cement paste and reinforcing bars upon cooling from elevated temperatures

The post-cooling properties of reinforced concrete constituents are of great relevance in the assessment of a structure for re-use after fire. The expansive rehydration of dissociated calcium hydroxide on post-cooling exposure to air can cause a total loss in strength if cement paste specimens are heated to and maintained at 400°C or above. The use of cement replacement agents can mitigate this effect. The strength and ductility of hot-rolled mild steel and cold twisted steel on gradual cooling or quenching from various temperatures can be correlated with microstructural phenomena. There is a significant increase in strength and loss of ductility when both types of steel are quenched from temperatures above 723°C. Some practical implications of these properties are described, using examples of fire-damaged structures.     

矿物外加剂及测试方法对硬化水泥浆体自收缩值的影响

分别采用两种不同试验方法对比研究了单掺粉煤灰、硅灰和矿渣微粉这三种常用的矿物外加剂(即水泥混合材)对水泥浆体早期自收缩的影响。研究结果表明,单掺粉煤灰、硅灰和矿渣微粉后,硬化水泥浆体的自收缩值分别随其掺量的增加而减小、增大和增大。本文分析研究了这三种常用的矿物外加剂对硬化水泥浆体自收缩产生影响的原因,并对比分析了本试验采用的两种试验方法,发现波纹管法更能准确地测量水泥浆体的早期自收缩,参照现行行业标准JC/T313-1982测试无法正确地反映硬化水泥浆体更早期的收缩现象。     

矿渣和熟料的比表面积及粉磨方式对水泥浆体流变性的影响

通过净浆流动度和Marsh筒实验,测试不同比表面积熟料、矿渣以及不同粉磨方式的水泥试样的流变性。试验发现,熟料、矿渣比表面积与水泥浆体的流变性呈负相关关系,且熟料远大于矿渣的影响程度;混合粉磨的水泥流变性优于分别粉磨,且它们的水化放热曲线与流变性有很好的相关性。另外,还对流变性较好的试样考察了其强度、凝结时间、标准稠度用水量等指标。     

Stresses and strains in the hardened cement paste — Water system

The length change theory proposed by Flood and Heyding for microporous solids is applied to cement paste in considering the effect of porosity and compressibility on the solid phase. The theory is also used to estimate the dependence of modulus of elasticity on humidity and is in general agreement with experiment. Some insight into microstructural change and its effect on length change is provided by estimates of a structure factor, κ. For humidities up to 56%, values of κ are approximately constant, but major changes in structure occur at higher humidities.     

水泥浆体的微结构及其与强度的关系

在亚微观尺度上描述了水泥石的微结构，简要回顾了其研究进程，详细介绍了主要水化产物以及掺加不同混合材可改变水化产物组分含量。归纳了水泥石的微观结构与强度的关系，并展望了其研究前景。