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.     

Association of macroscopic laboratory testing and micromechanics modelling for the evaluation of the poroelastic parameters of a hardened cement paste

The results of a macro-scale experimental study performed on a hardened class G cement paste [Ghabezloo et al. (2008) Cem. Con. Res. (38) 1424-1437] are used in association with the micromechanics modelling and homogenization technique for evaluation of the complete set of poroelastic parameters of the material. The experimental study consisted in drained, undrained and unjacketed isotropic compression tests. Analysis of the experimental results revealed that the active porosity of the studied cement paste is smaller than its total porosity. A multi-scale homogenization model, calibrated on the experimental results, is used to extrapolate the poroelastic parameters to cement pastes prepared with different water-to-cement ratio. The notion of cement paste active porosity is discussed and the poroelastic parameters of hardened cement paste for an ideal, perfectly drained condition are evaluated using the homogenization model.     

Time-dependent behaviour of hardened cement paste under isotropic loading

The experimental results of isotropic compression tests performed at 20 °C and 90 °C on a class G hardened cement paste hydrated at 90 °C (Ghabezloo et al., 2008, Cem. Conc. Res. 38, 1424–1437) have been revisited considering time-dependent response. Within the frame of a viscoplastic model, the non-linear responses of the volumetric strains as observed in drained and undrained tests and of the pore pressure in undrained tests are analysed. The calibration of model parameters based on experimental data allows to study the effect of the test temperature on the viscous response of hardened cement paste showing that the creep is more pronounced for a higher test temperature. The effect of the hydration temperature on the time dependent behaviour is also studied by evaluating the model parameters for a cement paste hydrated at 60 °C. The time-dependent deformations are more pronounced for hydration at a higher temperature.     

CoFeMgAl-LDHs/CNTs复合材料对水泥水化及微观结构的影响

通过进行水化热测试、非蒸发水含量测试、X射线衍射分析、扫描电镜分析、压汞测试及强度试验,研究了钴铁镁铝水滑石-碳纳米管复合材料(CoFeMgAl-LDHs/CNTs)对水泥水化过程、硬化水泥净浆孔结构及强度的影响。结果表明,CoFeMgAl- LDHs/CNTs复合材料通过成核作用显著提升了水泥3 d内的水化程度,从而显著提高了水泥净浆3 d内的抗压强度,3 d后强度提高效果逐渐减小,7 d后复合材料对硬化水泥浆体强度没有明显的提高作用。掺入CoFeMgAl-LDHs/CNTs复合材料的水泥净浆与空白净浆相比,胶凝孔和毛细孔含量明显增多,大孔含量有所降低,同时复合材料的桥联作用进一步优化了水泥净浆的微观结构,从而提高了水泥基体薄弱部位的应力承载能力。因此在同一龄期,复合材料掺入后硬化水泥净浆的强度有所增大。由于复合材料掺量的变化对孔径分布没有明显的影响,改变复合材料的掺量对同龄期硬化水泥净浆强度提升影响较小。     

Micromechanics analysis of thermal expansion and thermal pressurization of a hardened cement paste

The results of a macro-scale experimental study of the effect of heating on a fluid-saturated hardened cement paste are analysed using a multi-scale homogenization model. The analysis of the experimental results revealed that the thermal expansion coefficient of the cement paste pore fluid is anomalously higher than the one of pure bulk water. The micromechanics model is calibrated using the results of drained and undrained heating tests and permits the extrapolation of the experimentally evaluated thermal expansion and thermal pressurization parameters to cement pastes with different water-to-cement ratios. It permits also to calculate the pore volume thermal expansion coefficient α_? which is difficult to evaluate experimentally. The anomalous pore fluid thermal expansion is also analysed using the micromechanics model.     

Poromechanics of partially saturated COx argillite

Our experimental study aimed originally at: 1) measuring argillite Biot's coefficient; 2) identifying the main drained poro-elastic parameters in the stabilized partially-saturated state. In doing this, we showed that the current isotropic poro-elastic Biot's model used for COx argillite might be improved.Our work was based on two experimental set-ups: one for drained uniaxial compression, and the second for triaxial “change in pore pressure” poromechanical measurement. The latter was developed to measure poro-elastic parameters for saturated and partially-saturated material using gas as the fluid injected within the porous network.In terms of stiffness, COx argillite was observed to be transverse isotropic. This was not the case as regards Biot's tensor which was found isotropic. The apparent Biot's coefficient was measured equal to 1, whatever the mass water content w(%). Biot's coefficient was labeled apparent because it was measured at stabilized w(%) (no transient water saturation regime during the tests). This allowed the use of the Terzaghi's definition of effective stress tensor, as the sum of the external hydrostatic stress tensor and of the pore pressure (times the identity tensor). The impact of w(%) on argillite stiffness was also observed.     

硫酸盐侵蚀下硬化水泥浆体微结构演变及膨胀过程的数值模拟

根据硫酸盐侵蚀机理,利用改进的CEMHYD3D水化模型和随机概率方法,建立了硫酸盐侵蚀下硬化水泥浆体的微结构演变模型。在微观层次上,模拟了浆体孔溶液中硫酸根离子的自由扩散、随机碰撞和转化反应,分析了膨胀性侵蚀产物生长导致的微结构损伤和体积膨胀,计算了侵蚀过程中石膏和钙矾石的生成量及浆体的膨胀应变,并与已有试验结果对比分析验证了模型的合理性。在此基础上,数值模拟了硫酸盐侵蚀下不同水灰比水泥浆体的微结构演变及膨胀过程。结果表明:同一硫酸盐浓度下,硬化水泥浆体中氢氧化钙和含铝物相与孔隙的接触面积越小,浆体的膨胀应变越低;水灰比为0.25、0.30和0.35的硬化水泥浆体的孔隙填充程度分别达到9.09%、9.27%和9.41%时,浆体膨胀应变开始快速增大;硫酸盐侵蚀溶液浓度增大,浆体体积快速膨胀的时间提前。     

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.     

Fracture of porous viscoelastic materials under multiaxial state of stress

Crack growth in porous and viscoelastic materials has been studied. The decrease of strength of hardened cement paste and concrete under high sustained load can be described with this theoretical approach. Creep as well as time dependence of elastic modulus and strength enter the calculation. Stress relaxation in the material near cracktips can be taken into account by introducing a function m(t,τ). Results are the same in the case of uniaxial and biaxial state of stress. [It could be shown that the probability for shear cracks in a cylindrical sample increases rapidly with increasing confining pressure.] Good agreement with experimental data was found.     

Effects of porosity on leaching of Ca from hardened ordinary Portland cement paste

Aiming at evaluating the effects of porosity in hardened cement paste on dissolution phenomena, we prepared hardened ordinary Portland cement (OPC), with variation in pore volume, and then leached them in deionized water. It was found that the bulk density and pore volume were affected by the dissolution of portlandite. The larger the pore volume of the sample, the more rapidly portlandite is dissolved. An electron probe microanalysis (EPMA) performed on the cross-section of the solid phase showed the ‘portlandite (CH) dissolution front’. As the leaching period became longer, the CH dissolution front shifted towards the inner part. In addition, the movement of the CH dissolution front was described by the diffusion model, with consideration of the dissolution of portlandite. It was concluded that the transport of leached constituents is diffusion controlled, and the major leached constituents of hardened OPC are portlandite and C-S-H gel. Large pore, which was generated associated with the leaching of portlandite, was considered significantly to affect the diffusion of leached constituents.     

Assessing the efficiency of entrained air voids for freeze-thaw durability through modeling

This paper models concrete's resistance to cyclic freeze/thaw using the solution of the poromechanical problem, which describes the freezing of an individual air void surrounded by hydrated cement paste. This enables calculation of the pore pressures and the volume of water expelled into the air void upon freezing. The model was applied to concrete specimens with entrained air voids with polydispersity in size, and subjected to water absorption, thereby simulating a cyclic freeze/thaw laboratory test. The mean and maximum pore pressures obtained by the simulation were compared to a series of experimental tests per ASTM C666, and results suggest the model may be used to predict satisfactory durability in the laboratory test. This framework may be useful tool to study the effects of porespace and entrained air void size distribution on concrete's freeze/thaw durability. Furthermore, it allows for a theoretical basis for assessing the entrained air void system parameters.     

Pore structure of air-entrained hardened cement paste

The effect of air entrainment on the pore structure of hardened cement paste was investigated. Air-entrained and air-free samples of various water-cement ratios and ages were prepared by a well-defined procedure. The first and second-intrusion pore-size distribution curves of the samples were determined by mercury intrusion porosimetry. It was observed that sample preparation technique affects the pore-size distributions of hardened cement pastes. The second-intrusion curves indicated a decrease in the total volume and a reduction in the size of pores that are uniform in cross section with decreasing age and water: cement ratio. The second-intrusion curves of air-entrained and air-free pastes of equal water: cement ratio and age matched with each other. It was concluded that air entrainment introduces only large air voids observable by a naked eye and does not alter the characteristic fine pore structure of hardened cement paste appreciably.     

Inkbottle Pore-Method: Prediction of hygroscopic water content in hardened cement paste at variable climatic conditions

The aim of this work is the development of a practicable method for the reliable prediction of the equilibrium hygroscopic water content in hardened cement paste and cement mortars at changing climatic conditions. Sorption thermodynamics and multi-scale pore structure of hardened cement paste build the basis of the new computation procedure. Drying and chemical aging lead to a formation of inkbottle pores. Their influence on sorption behaviour will be considered in particular by including them into the pore model. Experimental data of adsorption, desorption and scanning-isotherms verify the new computation method, which has been called “IBP-Method” (inkbottle pores).     

水泥石结构强度数学模型

波特兰水泥石的结构是一个多相、多组分、多孔结构体系，且其结构随水化硬化的时间、空间和环境面演变。     

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.     

Strength development and pore size distribution of cement paste cured with high hydrostatic pressure

Compressive strength, pore size distribution and total pore volume of cement pastes cured with high hydrostatic pressure were examined. Compressive strength (S) of paste cured with high hydrostatic pressure (P) is shown in the following formula: S = a · logP + b. Compressive strength of paste having total pore volume (V) is calculated from the following:formula: S = c · V + S_o. When hardened paste cured at atmospheric pressure is placed in high hydrostatic pressure, the larger the strength is, the larger the lowering it becomes.     

Confocal imaging of porosity in hardened concrete

We present the results of an exploratory 3D study of fine pore structures in hardened cement paste. A laser scanning confocal microscope was used in reflected (epi) fluorescent imaging mode, allowing for the very high optical resolution of features well below 1 μm in size. Images of pore structures were captured using a ‘reverse imaging contrast’ technique where spaces in the material were impregnated with a fluorescent, dyed epoxy-resin, and scanned in the x-y plane by a laser probe. A vertical ‘stack’ of these optical sections was acquired by imaging sequentially through the z-axis. Using this technique it was possible to observe, partially hydrated cement grains, porous natural quartz aggregate interfaces, micro-cracking in the hydrated cement paste and aggregate particles, as well as fine capillary pores and very small air voids. Operating at the limit of its capability the microscope was able to perform high resolution imaging of the internal areas of partially reacted cement grains, and pore structures approximately 0.17 μm across were measured. 3D models were produced to help visualise the true morphology and distribution of porous features.     

低温下油井水泥石孔结构与抗压强度的关系

根据Powers理论模型,通过引入相对水化程度及选定油井水泥石的本征强度参数,结合Balshin方程和Schiller方程,建立了低温条件下油井水泥石孔结构和抗压强度的数学模型。比较结果表明:Balshin方程的计算值与实验值有着良好的线性关系,在水泥浆体的水化龄期小于90d时结果比较吻合,而Schiller方程则不适用。     

A device for direct observation of cracking of cement paste or mortar under compressive loading within a scanning electron microscope

A device is described which permits the compressive loading of small specimens of hardened cement paste or mortar within the specimen chamber of a scanning electron microscope. The crack patterns which develop may then be seen at fairly high magnifications while the specimens remain under load. The loading frame itself is about 80 mm long by about 50 mm in diameter, and is activated by a pressurized nitrogen cylinder. Some preliminary results of observations made with the device are described.     

Compressive strength of cement paste as a function of loading rate: Experiments and engineering mechanics analysis

Uniaxial compressive strength of cement paste increases with increasing loading rate. Two experimental campaigns are described: 2-day-old cement paste samples are tested by using quasi-static stress rates. 6-month-old and oven-dried cement paste samples are tested by using quasi-static and high-dynamic strain rates. As for the analysis of the former tests, we develop a nonlinear viscoelastic–brittle model. It explains our test data very well, and it suggests that strength decreases with decreasing loading rate, because the duration of the test increases and this provides the possibility for creep-related damage mechanisms to reduce the strength of the material. As for the analysis of the latter tests, we develop an elasto-brittle model considering crack propagation, at the Rayleigh wave speed, in loading direction. The model is free from fitting parameters and explains our measurements very well, indicating that high-dynamic strength increase is a structural effect.