Poromechanical behaviour of hardened cement paste under isotropic loading

The poromechanical behaviour of hardened cement paste under isotropic loading is studied on the basis of an experimental testing program of drained, undrained and unjacketed compression tests. The macroscopic behaviour of the material is described in the framework of the mechanics of porous media. The poroelastic parameters of the material are determined and the effect of stress and pore pressure on them is evaluated. Appropriate effective stress laws which control the evolution of total volume, pore volume, solid volume, porosity and drained bulk modulus are discussed. A phenomenon of degradation of elastic properties is observed in the test results. The microscopic observations showed that this degradation is caused by the microcracking of the material under isotropic loading. The good compatibility and the consistency of the obtained poromechanical parameters demonstrate that the behaviour of the hardened cement paste can be indeed described within the framework of the theory of porous media.     

Microwave absorption of hardened cement paste

The absorption of microwaves by hardened cement paste has been studied in the frequency range of 8.5 GHz to 12.3 GHz (x-band). Preliminary experiments have been carried out using the free wave method. Much smaller specimens can be used with a conventional experimental set-up with the help of rectangular wave-guides with a cross section of 10×22.8 mm². A new method to determine the complex permittivity from absorption data has been developed. ge′ and ?″ are determined as a function of moisture content, temperature and duration of hydration. Results and further possible application of this method are discussed.     

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.     

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.     

Internal friction in polymer-impregnated hardened cement paste

Hardened cement paste (HCP) specimens were prepared with hydration times of 1, 3, 7, 28, and 180 days. Specimens were oven dried and subsequently impregnated with methyl methacrylate, tert-butylstyrene, polyethylene glycol, and an epoxy resin system via a vacuum–pressure technique. Internal friction (Q^?1) of in situ polymerized specimens was measured over the temperature range from 100° to 500°K. Results indicate that the internal friction spectra of polymer-impregnated specimens closely parallels the internal friction spectra of the bulk polymers used for impregnation. Magnitudes of the internal friction peaks in the impregnated specimens were found to be less than that expected on the basis of volume fraction of polymer present. These results are attributed to hindered motion at the cement/polymer interface.     

Impedance spectroscopy study of hardened Portland cement paste

In this paper, the differential impedance analysis (DIA) has been applied to the study of the dielectric properties of hardened Portland cement paste. Two time constants are found in the impedance spectra obtained in the frequency region form 100 kHz to 15 MHz. One time constant has been attributed to the solid matrix and the other one to the liquid phase filling the pores. The effect of the cement paste-electrodes interface has been quantified using two different experimental set-ups. Measurements using direct contact between electrodes and cement paste have been compared with measurements using an air gap technique in which the specimen “floats” between the electrodes. The two referred time constants have been found in both types of measurements. The influence of drying on the dielectric parameters is also studied.     

The analysis of ettringite in hardened cement paste

The solubilities of pure hydrates and the mixture of hydrates existing in hardened cement paste to the ethylene glycol-methanol solution, the effects of co-existing Ca(OH)₂ upon the solubility and rate of dissolution of each hydrate, suitable concentration of ethylene glycol, and the limit of the amount of ettringite capable of being analysed in paste were investigated. From the results obtained, the reliable analytical method for the determination of ettringite in hardened cement paste was established.     

A method for description of mechanical behaviour of hardened cement paste by evaluating adsorption data

An improved method for the evaluation of sorption data is outlined. This “thermo-dynamic approach” takes the change of surface tension of adsorbed liquid films into consideration. No assumption on the pore shape has to be introduced. In addition to the pore size distribution changes of the surface free energy can be calculated with the help of this new method. There are distinct advantages when systems are to be studied in which capillary condensation can hardly be separated from pure sorption. The results obtained by the thermodynamic approach are discussed in comparison with mechanical properties of hardened cement paste. It is possible to describe shrinkage creep and change of strength on this basis.     

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.     

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.     

The mechanism of frost damage in hardened cement paste

The results of freezing experiments performed on samples of hardened portland cement-flyash pastes, saturated with benzene and water respectively, are presented. The specimens contained portland cement and flyash in the ratio of 55:45 by weight, and had a water-solids ratio of 0.50. A length change anomaly which occurred in the region of the bulk freezing point of benzene is explained by the hydraulic pressure theory. In discussing their own test results, as well as those of others, the authors deal at some length with thermodynamic considerations of moisture movement as it relates to possible mechanisms of frost action.     

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.     

The effect of rate of drying on the drying/wetting behaviour of hardened cement paste

Miniature hcp specimens were dried at various rates from saturation to equilibrium at 53% rh, and were then rewet. Strain and weight changes were measured. The results show: a) recoverable and irrecoverable shrinkage was not affected appreciably by rate of drying; b) after the first 15% of the pores had emptied, rate of drying did not influence shrinkage vs. moisture loss; and c) the rate at which specimens were dried had no effect on the subsequent swelling vs. time or swelling vs. moisture gain relationships. It is concluded that spurious effects caused by internal moisture gradients were not significant; for a wide range of drying rates, miniature specimens can be used to measure the unrestrained response of the material.     

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。     

Development of technique for observing pores in hardened cement paste

Mercury intrusion porosimetry is a widely used technique to determine the pore size distribution in porous materials. However, this technique does not provide information about the shapes and locations of pores. A new technique is developed, in which gallium (Ga) is used as an alternative intrusion liquid because of its property of being solid at normal room temperature (melting point: 29.8 °C). This permits the examination of pores using image analysis. The technique is applied to hardened cement paste. The distribution of solid Ga is observed through an electron probe microanalyzer (EPMA), and the shapes and locations of pores in cement paste are discussed.     

煤矸石对硬化水泥浆体结构形成的影响

通过对硬化水泥浆体物理力学性能的检测,结合XRD和SEM分析,研究了不同掺量的煤矸石对硬化水泥浆体水化性能的影响。结果表明:随着煤矸石掺量的增加,水泥的标准稠度用水量增加,凝结时间缩短,抗压强度降低,熟料矿物的水化速率提高,水泥-煤矸石体系的水化速率降低。煤矸石掺量不同,水化模式亦不同。     

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.     

Micro-spectroscopic investigation of Al and S speciation in hardened cement paste

Synchrotron-based micro X-ray fluorescence (micro-XRF) and micro X-ray absorption near edge spectroscopy (micro-XANES) have been used to determine the spatial distribution of Al and S and to identify the Al- and S-bearing species in compact hardened cement paste hydrated at 50 °C. The contribution of the S-bearing cement phases to the composed S K-edge XANES spectra collected in ten S-rich regions was determined using least-squares fitting. Ettringite and calcium monosulfoaluminate were identified as the main S-bearing species in the selected regions. Factor analysis was employed to determine the contribution of the various Al-bearing cement minerals to the composed Al K-edge XANES collected in different Al-rich regions of the cement matrix. Principal component analysis revealed that all spectra could be fitted using three components. Target transformation further suggested that the two Al-bearing clinker phases (aluminate, ferrite) and secondary phases of the hydrate assemblage (ettringite, AFm phases, hydrotalcite) contributed to the set of components that made up the experimental spectra. Least-squares fitting allowed the relative contribution of each reference compound to be determined. Aluminate and/or ferrite were detected in all Al-rich regions. AFm phases were identified in six out of the ten regions studied, while ettringite was detected in only two regions. The study confirmed that AFm phases are important cement minerals in hardened cement paste hydrated at 50 °C.     

Critical aspects of nano-indentation technique in application to hardened cement paste

Several open questions related to the experimental protocol and processing of data acquired by the nano-indentation (NI) technique are investigated. The volume fractions of mechanically different phases obtained from statistical NI (SNI) analysis are shown to be different from those obtained by back-scattered electron (BSE) image analysis and X-ray diffraction (XRD) method on the same paste. Judging from transmission electron microscope (TEM) images, the representative volume element of low-density calcium-silicate hydrates (C–S–H) can be considered to be around 500 nm, whereas for high-density C–S–H it is about 100 nm. This raises the question how the appropriate penetration depth for NI experiments should be selected. Changing the maximum load from 1 mN to 5 mN, the effect of penetration depth on the experimental results is studied. As an alternative to the SNI method, a “manual” indentation method is proposed, which combines information from BSE and atomic-force microscopy (AFM), coupled to the NI machine. The AFM allows to precisely indent a high-density C–S–H rim around unhydrated clinkers in cement paste. Yet the results from that technique still show a big scatter.     

Carbonation around near aggregate regions of old hardened concrete cement paste

Analogous with most modern cities, waste disposal is a pressing issue due to limited landfill and public filling (land reclamation) areas in Hong Kong in which construction and demolition (C&D) waste forms the major source. Concrete, apportioning the largest portion of C&D waste, has the greatest potential for recycling. However, the knowledge on micro-structural behavior of concrete waste is immature to give adequate details on the macro-behavior of concrete waste. This paper attempts to examine the problems of recycling old concrete by investigating the microstructure and phase transformation of the concrete samples collected from buildings with 46 and 37 years of services. From the results of Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM) examination, it is found that there are a lot of pores at the near layers of aggregate where carbonation of the hardened cement paste (HCP) is high. The pores may be generated as a result of poor workmanship such as insufficient concrete mixing time, trapping of air voids beneath coarse aggregate, inappropriate water to cement ratio, and the microclimate conditions such as humidity that affects the demand on water from the aggregate during mixing.