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。     

The shrinkage of hardening cement paste and mortar

This paper is an abstract from the report of the commission B10: “The influence of the shrinkage of cement on the shrinkage of concrete”, of the Netherlands Committee for Concrete Research.Measurements of pulse velocity, volume shrinkage and heat of hydration on hardening portland cement support the idea that the formation of ettringite is an important link in the mechanism of shrinkage in the plastic stage of cement paste and mortar.Mechanical tests on prisms of 4 × 4 × 16 cm³ gave some information about the difference in sensitivity to surface corrosion of different types of cement.     

A study on the relationship between porosity of the cement paste with mineral additives and compressive strength of mortar based on this paste

Mercury intrusion porosimetry study was carried out on ordinary Portland cement (OPC) pastes with 10% to 40% mineral additives, such as steel-making slag, granulated blast furnace slag and fly ash. For all samples, the porosity of paste and compressive strength of mortar based on this paste were determined at 3, 7, 28, 90 and 180 days. Relationship between the porosity and strength was investigated and some equations for the strength-porosity relationship were presented according to Balshin multiplicative model. Results show that mineral additives delayed process that micropore structure of OPC paste developed and strength development of sample with mineral additives was faster than that of OPC sample. Balshin equation fits the results of strength and porosity of all samples and there is a strongly quantitative relationship between strength and porosity. After being mixed with mineral additives, the intrinsic strength σ₀ and power n both increased and the sequence of σ₀ and n for different mineral additives was fly ash>steel-making slag>blast furnace slag.     

Rapid ice formation in hardened cement paste,mortar and concrete due to supercooling

The aim of these experiments was to study the extent of rapid ice formation and length change due to supercooling of water in cementstone and its influence on the destruction of mortar and concrete. It could be shown that the initial freezing temperature decreases with increasing water content. Smaller specimens show a lower initial freezing temperature than greater ones. The expansion accompaning the abrupt temperature rise shows a considerable increase with increasing temperature rise. Immediately after abrupt temperature rise one part of the observed expansion contracts rapidly probably due to displacement of liquid in the structure of the hardened cement paste. After the end of a freezing and thawing cycle the magnitude of the remaining expansion increases with increasing value of rapid temperature rise.     

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.     

Experimental study on the basic phenomena of shrinkage and cracking of fresh mortar

Evaporation, horizontal shrinkage, settlement, setting and capillary pressure of mortar mixes were measured during first hours with samples which were exposured to wind (velocity 4 m/s, T = 20 °C and RH 40%). The effects of different admixtures (super plasticizer, accelerating, retarding, air-entraining agent and one type of fibre) on the shrinking behaviour of the mortars were studied accordingly. The well-known fact that proper long-term wet curing is vital for the crack-free surfaces of concrete or mortar was confirmed experimentally. The beginning of setting can be regarded as a critical moment. After that the mortar has capacity to resist the capillary forces and thus shrinkage. The mix modified by super plasticizer behaved unexpectedly, and the surface of the sample cracked. This cracking was indicated by zig-zag behaviour in the measured horizontal shrinkage and capillary pressure. Air- entraining agent reduced horizontal shrinkage considerably. Use of fibre reduced shrinkage about 30% when compared to mix without fibre. Based on the results some conclusions are drawn concerning properly limed trowelling on horizontal shrinkage.     

Effect of maturation time on the fresh and hardened properties of an air lime mortar

The restoration and maintenance of old renders is one of the key aspects of correct rehabilitation practice. The ideal course of action is to replace the damaged material by a material with compatible characteristics. This work aims to analyze the effect of the maturation process on hardened state characteristics of hydrated powder lime mortars. The rheological characterization shows an air lime mortar thickening behaviour with the length of the test. The different mixes were subjected to a maturation process consisting on keeping them in the fresh state, covered with water, isolated from CO₂, during seven days. The specimens and applications were prepared both with the non-matured and the matured mortars. Maturation seems to influence the hardened state characteristics causing a decrease in the capillary values, and an increase on the mechanical strength, which are more evident for mortars with higher binder contents.     

Early longitudinal dimensional changes of fresh fly ash mortar exposed to drying conditions

The present work examines the effect of fly ash (ASTM Class F) on early longitudinal dimensional changes in fresh cement-fly ash (10, 25 and 40 percent cement replacement) mortar mixes exposed to drying conditions immediately after casting at elevated temperature (30 C, 86 F).

It was found that the fly ash affects expansion in the fresh mortar mix: the larger the cement replacement percentage (i.e. increased amount of fly ash in the mix), the higher the expansion values and the longer its duration. However, the effect of the fly ash on plastic shrinkage during drying has no clear trend, as it is governed both by the preceding expansion and by the duration of the shrinkage phase (both influenced in turn by the specific chemical composition and physical properties of the fly ash and the cement).     

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。     

On the mechanism of strength enhancement of cement paste and mortar with triisopropanolamine

Recent work on the strength-enhancing mechanism of triisopropanolamine (TIPA) suggested that TIPA enhances the mechanical properties of mortar and concrete by acting on the interfacial transition zone (ITZ) between paste and sand or aggregate rather than improving the properties of the hydrated binder. This paper presents compressive strength data for 10 Portland cements tested as cement paste as well as two different kinds of mortar after 28 days hydration, so that these two mechanisms could be compared directly. The average strength improvement with TIPA was 10% in the hydrated portland cement paste and 9% in the mortar, clearly showing that the strength enhancement is not dependent on an ITZ mechanism.     

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.     

Measuring the change in ultrasonic p-wave energy transmitted in fresh mortar with additives to monitor the setting

Research on ultrasonic methods to monitor the setting of concrete has mainly focussed on the wave velocity as a useful quantity. To investigate the application of also the wave energy as a parameter, an experimental program was set up to apply the ultrasonic wave transmission technique on several mortar samples containing air entrainer, blast-furnace slag or fly ash causing clearly different setting behaviour.The increase of the relative energy E/E_ref during setting is generally retarded if ordinary Portland cement is replaced by blast-furnace slag or fly ash. The mixtures with cement of a lower strength class or with large air content were difficult to test with the energy measurements since they were more sensitive to poor sensor contact due to shrinkage. For the other samples, the thresholds E/E_ref = 0.02 and 0.13 are proposed to easily determine respectively initial and final setting based on the ultrasonic energy measurements.     

熟料细度对粉煤灰水泥浆体强度和自收缩的影响

通过实验室球磨机制备出比表面积分别为280m2/kg、370m2/kg和670m2/kg的3种水泥熟料,与不同掺量的粉煤灰配制成不同颗粒级配的粉煤灰水泥,并测试了粉煤灰水泥浆体的抗压强度、自收缩、孔隙率和显微结构。结果表明：提高熟料细度能在很大程度上降低粉煤灰水泥浆体的孔隙率并提高复合水泥浆体早期抗压强度;粉煤灰的掺入降低了水泥体系的自收缩,提高了粉煤灰水泥浆体的体积稳定性;粉煤灰水泥浆体背散射图像表明,提高熟料细度可显著减少粉煤灰水泥浆体中未水化的水泥颗粒含量,并在一定程度上减少未水化粉煤灰颗粒含量。     

Upscaling quasi-brittle strength of cement paste and mortar: A multi-scale engineering mechanics model

It is well known from experiments that the uniaxial compressive strength of cementitious materials depends linearly on the degree of hydration, once a critical hydration degree has been surpassed. It is less known about the microstructural material characteristics which drive this dependence, nor about the nature of the hydration degree–strength relationship before the aforementioned critical hydration degree is reached. In order to elucidate the latter issues, we here present a micromechanical explanation for the hydration degree–strength relationships of cement pastes and mortars covering a large range of compositions: Therefore, we envision, at a scale of fifteen to twenty microns, a hydrate foam (comprising spherical water and air phases, as well as needle-shaped hydrate phases oriented isotropically in all space directions), which, at a higher scale of several hundred microns, acts as a contiguous matrix in which cement grains are embedded as spherical clinker inclusions. Mortar is represented as a contiguous cement paste matrix with spherical sand grain inclusions. Failure of the most unfavorably stressed hydrate phase is associated with overall (quasi-brittle) failure of cement paste or mortar. After careful experimental validation, our modeling approach strongly suggests that it is the mixture- and hydration degree-dependent load transfer of overall, material sample-related, uniaxial compressive stress states down to deviatoric stress peaks within the hydrate phases triggering local failure, which determines the first nonlinear, and then linear dependence of quasi-brittle strength of cementitious materials on the degree of hydration.     

The effect of admixtures on length change anomalies due to slow cooling and warming of hardened cement paste

Studies are reported on the effect of several admixtures (fly ash, water-reducing agents, mineral powder and an air-entraining agent) on the length-change anomalies present when hardened cement paste undergoes a slow cooling-warming cycle over the range 70°F to 0°F (20°C to ?18°C). Porosity, as expressed by the hydration parameters, appears to be the most important parameter influencing dimensional stability during freezing of saturated pastes, regardless of the presence of admixtures.     

Relationship between the surface free energy of hardened cement paste and chemical phase composition

Special emphasis is put on to correlating the changes in phase composition with wettability of hardened cement paste. The results were found in contradiction, because it was demonstrated by XRD analysis that by increment in curing ages, high energy products are developed in the matrix which increases surface free energy. In contrast, surface evaporation of the water and bulk hydration cause a decrease in hydrophilic properties of the surface free energy. It was concluded that the latter phenomenon has more important impact than surface free energy.     

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.     

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.     

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。     

直接光度法测定印染废水中铈(IV)

在硫酸溶液中,铈(IV)的强氧化性对间甲酚紫有褪色作用,其褪色程度与铈的浓度有关,据此建立了直接光度法测定痕量铈的新方法。考察了各种实验条件对灵敏度的影响。结果表明,最佳实验条件为:硫酸1.3 m L,间甲酚紫1.4 m L,反应温度80℃,加热反应时间10 min。线性范围为0⁶μg/m L,检出限为0.046μg/m L。可用于印染废水中铈的测定。