碳化与氯盐复合作用下水泥浆体的微结构演变

为了阐明碳化与氯盐复合作用下硬化水泥浆体的微结构,基于X射线计算机断层扫描成像(X-CT)和电子探针微区分析(EPMA)技术探明了氯盐对水泥浆体碳化速率的影响,测定了碳化作用下水泥浆体内Cl、S和Na元素的浓度分布。结果表明:氯盐可细化养护龄期为28d的水泥浆体孔结构,提高其密实度并减缓碳化速度;碳化作用下水泥浆体的碳化区易出现裂缝,二氧化碳气体通过这些裂缝扩散到水泥浆体内部进行碳化,致使水泥浆体碳化深度不均匀;碳化过程中Cl、S和Na元素向非碳化区迁移和浓缩,初始均匀分布的元素在碳化区含量减少,在非碳化区含量升高。所得结论为混凝土结构耐久性设计提供科学的理论依据。     

3D spatial distribution of the calcium carbonate caused by carbonation of cement paste

Carbonation of cement-based materials is one area of concern for the durability of concrete structures. The calcium carbonate caused by carbonation is an important indicator of carbonation degrees. The present paper, using 3D tomography data, proposes a nondestructive method to characterize the 3D spatial distributions of calcium carbonate. It allows monitoring of 3D carbonation evolutions. The evolution of the calcium carbonate distributions in a specimen of cement paste with different carbonation degrees is given using the current method. The results are compared with the average quantity of calcium carbonate determined by thermal analysis. From the sharp edge of the calcium carbonate distribution, we conclude that the accelerated carbonation in this experimental condition is a diffusion controlling process.     

Rheology of fresh cement paste with superplasticizer and nanosilica admixtures studied by response surface methodology

Thirteen cement pastes, keeping the water/cement ratio constant at w/c = 0.30, with different amounts of two admixtures were prepared. A superplasticizer (modified polycarboxylic ether polymer) was studied in the range of 0.14–1.00% (over cement weight), while a viscosity-modifying admixture (nanosilica aqueous dispersion) was tested at 0.50–3.00% range (over cement weight). Oscillatory and steady shear rheometry, as well as results of modified slump test and Marsh funnel flow time of 13 formulations were evaluated by response surface methodology, in order to find the optimum recipe that reduces the viscosity while keeping the paste workability. It was predicted that a formulation with 0.39% w/w of superplasticizer and 2.78% w/w of viscosity-modifying admixture would provide a paste with a fifth of the viscosity of control (cement paste without any admixture), but with an acceptable slump. This formulation was prepared and tested, and the plastic viscosity was 1.00 Pa-s, while the oscillatory yield stress was 330 Pa. The admixtures did not affect the compression strength (48 MPa average, after 28 days), compared with the control.     

氧化石墨烯对水泥净浆流动度及水泥石结构和性能的影响

采用Hummers法及超声破碎分散法制备了氧化石墨烯(GO)纳米相分散液,研究了GO对水泥净浆流动度和水泥石微观结构的影响,用FT-IR、AFM、XRD及SEM对GO及水泥石结构进行了表征。结果表明GO的掺入降低了净浆流动性,每增加0.01%的GO需要增加0.02%的聚羧酸系减水剂(PCs)以保持水泥净浆流动度在3 h内在200 mm以上,GO的掺入能够使水泥石的微观结构发生明显的变化,当GO/PCs掺量为0.01%/0.24%~0.03%/0.28%时,水化龄期7 d的水泥石出现了大量分散均匀的花状微晶体,当GO/PCs掺量为0.05%/0.32%~0.07%/0.36%时,同龄期水泥石中出现大量的片状晶体,在水化龄期延长到28 d时有转化为密实结构的趋势,结果说明GO具有调控水泥水化产物形貌的能力及增强增韧的作用,此研究结果对于提高水泥基材料的力学性能具有重要意义。     

Some results of the effect of carbonation on the porosity and pore size distribution of cement paste

Summary The effect of carbonation has gained in significance during the development of concrete technology because of the usage of thinner structures and the more exact utilization of the real properties of concrete. It is known that carbonation changes the microstructure of cement paste and affects the strength and deformation properties of concrete. However, a knowledge of the phenomenon and its effects on concrete is still relatively poor and even contradictions in opinions exist. The research reported briefly in this paper is an investigation to elucidate the basic alterations in the microstructure of cement paste produced by carbonation. The results, although of tentative nature, indicate 1) that the total porosity of the carbonated cement paste is essentially smaller than the total porosity of non-carbonated cement paste, and 2) that the carbonation affects more the greater pores than the smaller ones.

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Résumé Le développement de la technologie du béton a mis en plus grande évidence l'action de la carbonation, en raison de l'utilisation de structures plus minces et de l'exploitation plus juste des propriétés réelles du béton. On sait que la carbonatation modifie la microstructure de la pate de ciment et qu'elle affecte la résistance et la déformation du béton. Pourtant, on sait encore très peu de choses sur ce phénomène et son action sur le béton, sans compter les opinions contradictoires qu'il suscite. L'étude dont on rend brièvement compte ici tente de mettre en lumière les altérations fondamentales de la micro-structure de la pate de ciment produite par la carbonatation. Les résultats, quoique non définitifs, indiquent 1) que la porosité totale de la pate de ciment carbonatée est radicalement plus faible que la porosité totale de la pate de ciment non carbonatée et 2) que la carbonatation agit plus sur les pores les plus grands que sur les pores les plus petits.

     

Chloride maximum phenomenon near the surface of cement paste induced by moisture evaporation and carbonation

Abundant studies of cyclic wetting–drying of concrete have suggested a “maximum phenomenon” by which chloride concentrations first increase with depth to a local maximum and then decrease at greater depths. This phenomenon may arise primarily because of carbonation, but may also be exacerbated by cyclic capillary suction and moisture evaporation during wetting–drying cycles. This paper investigates the mechanism responsible for the maximum phenomenon under conditions that exclude cyclic capillary suction. The results show that different chloride maxima form near the surface of all specimens under conditions of either accelerated carbonation or carbonation in normal air environment. A large amount of bound chloride is released when carbonation causes decomposition of Friedel’s salt and C–S–H gel, and the decrease in water content significantly increases the chloride concentration in the pore solution near the specimen surface. The resulting chloride concentration gradient drives free chloride diffusion from the surface to the interior, thereby reducing the actual chloride content near the exposed surface. More severe carbonation conditions increase the severity of the maximum phenomenon.     

Effects of cement size distribution on capillary pore structure of the simulated cement paste

The evolution of tricalcium silicate (C₃S) microstructure during hydration is tri-dimensionally simulated based on an “Integrated Particle Kinetics Model”. The hydration degree, the contact surfaces between the hydrated particles, the hydraulic radius and the capillary pore size distribution of the simulated cement paste at various degrees of hydration are calculated. Three examples of the C₃S microstructure development with different size distributions are presented. The effects of the cement size distribution on pore structure of cement paste are demonstrated and the results are discussed. In these examples, the cement size distribution varies between 3–40, 5–40 and 10–40 μm, respectively.     

Fracture measurements on cement paste

This paper describes an investigation into the fracture behaviour of hardened cement paste. Notched specimens of the material were tested to failure in flexure and tension. In the initial flexural tests on beams of fixed overall depth, the stress intensity factor at failure as calculated from linear-elastic fracture mechanics appeared to be a material constant. However, further investigation showed that this factor varied with specimen size, and suggested that linear-elastic fracture mechanics and the concept of fracture toughness are not readily applicable to hardened cement paste, which would appear to be a relatively notch insensitive material whose strength is not greatly reduced by the presence of flaws. A “tied crack” model explains semi-quantiatively the observed behaviour.     

Heat deformations of cement paste phases and the microstructure of cement paste

The paper presents the results of an experimental investigation of the influence of thermal deformations of phases present in hardened cement paste on its microstructure. For the observation of the thermal deformation course in the range of temperature20–800°C, a method of complex thermal analysis (DTA, DTD, TD) was employed. The microstructure of heated cement pastes was observed by both optic and electron microscopes. From an analysis of the results of the investigation it has been found that chemical processes stimulated by temperature in particular phases of cement paste have a significant influence on the thermal deformation course. It is shown that micracracks caused by different thermal deformations, appear first in the aregas of Ca (OH)₂ concentration (ca.300°C), and next (ca.400°C) as well as in the areas of occurrence of unhydrated large clinker grains.

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Résumé On rend compte d'une étude expérimentale sur les causes de la destruction de la microstructure de la pate de ciment portland exposée à la chaleur dans une gamme de températures allant de20 à800°C. On a prêté une attention particulière aux déformations thermiques de chacune des phases reconnues de la pate de ciment durci. On a utilisé une méthode d'analyse thermique complexe (DTA, TD, DTD) pour l'étude de l'évolution de déformations thermiques. On a observé les microstructures de la pate de ciment chauffée au microscope optique et électronique. On a étudié les phases suivantes de la pate de ciment: clinkers (C₃S, β-C₂S, C₃A, C₄AF), minéraux hydratés de ciment (hydrates C₃S, β-C₂S, C₃A, C₄AF), et produits d'hydratation [Ca(OH)₂, ettringite]. Les résultats de l'étude ont permis à l'auteur de déterminer l'allure de la déformation thermique des phases que présente la pate de ciment durcie sous températures croissantes. On a constaté que les processus chimiques que l'élévation de température induit dans chacune des phases de la pate de ciment exercent une influence notable sur l'évolution des déformations thermiques. Les microfissures causées par différentes déformations thermiques des phases apparaissent dans les zones de concentralisation de Ca(OH)₂ à environ300 et400°C aussi bien que dans les zones où se rencontrent des grains de clinker non hydraté.

     

Effect of cement paste on drinking water

When water conduits are made of concrete or are lined with cement mortar, the cement paste can be leached by water. This occurs to a significant degree if the water remains in prolonged contact with the cement paste. The consequences of leaching are an increase in pH and in the content of CaCO₃. Recent rules of the European Union endeavour to deal with this problem. As the tolerable amount of leachate depends on the original properties of the water, tests are not easy to perform. The use of admixtures and of high-alumina cement are discussed. The actual levels of various chemical species in different waters in service and of tolerable quantities of these species need to be established so that a safe use of cement in conduits can be assured.     

Direct observation of the carbonation process on the surface of calcium hydroxide crystals in hardened cement paste using an Atomic Force Microscope

The surface carbonation process of calcium hydroxide crystals in samples of hardened Portland cement paste has been investigated at the nanometer scale with the aid of an Atomic Force Microscope (AFM). The AFM, encapsulated in a glove-box, was operated in contact mode at ambient temperature. Successive real-time measurements were performed in (i) a pure non-reacting N₂ atmosphere, (ii) a N₂ + H₂O, (iii) a N₂ + CO₂ and finally (iv) in a N₂ + CO₂ + H₂O atmosphere, respectively. In the N₂ + H₂O atmosphere, until 30–40% relative humidity, a minor change as surface smoothing with an occurring instability was detected. In the N₂ + CO₂ atmosphere, no change was detected, except for some very small grains becoming after two days even a little bit smaller. However, in the N₂ + CO₂ + H₂O atmosphere, i.e., in a simultaneous CO₂ + H₂O environment, with a stepwise increase of the relative humidity until 26%–30%, on the surface of the calcium hydroxide crystals several small scattered spots were found. These spots were weakly linked to the surface and could be pushed away with the scanning tip. Under constant conditions (temperature, humidity- and CO₂-content), the small spots start to grow, and after a long-term exposition of the crystals to the ambient humid air, develop a specific spherular structure. The latter is interpreted to be calcium carbonate, a result of the surface carbonation process of the initial calcium hydroxide crystals.     

Effects of transition zone densification on fiber/cement paste bond strength improvement

This paper investigates the effectiveness of transition zone densification on the fiber-cement paste system. By controlling the water: cement (w:c) ratio and the condensed silica fume content, environmental scanning electron microscopy studies confirm that transition zone densification can be achieved in all brass, steel, and polyethylene fiber-cement systems. However, single fiber pullout tests indicate that densification only enhances the brass-cement paste interface bond strength and not the other systems. Further microscopy investigation of the surface of fibers peeled off from a composite fracture surface and of the groove left by the fiber on the cement paste suggests that bond failure for the brass-cement system is of a cohesive type, whereas bond failure for the other two systems is of an adhesive type. It is concluded that the transition zone densification technique should be effective in fiber-cement systems in which bond strength is controlled by cohesive failure of the transition zone material.     

Effect of nanosilica and montmorillonite nanoclay particles on cement hydration and microstructure

The need to produce sustainable cements has driven research towards nanotechnology. The main cement hydration product, calcium silicate hydrate, is nanosized; hence, the addition of nanoparticles to blended Portland cement formulations can remarkably modify mechanical strength, porosity and durability. The present paper discusses the material aspects of two different nanoparticles, nanosilica and montmorillonite nanoclay, the complications that arise from their addition to cement pastes and ways to mitigate these limitations. It is deduced that nanosilica solids in blended cement pastes should be limited to 0.5%, whereas nanoclay solids to almost 1?mass-% binder. Competitive reactivity of nanoparticles with other constituents is expected, and the possible pozzolanic activity is critically addressed. Notwithstanding progress made, there are significant potentials related to inorganic nanoclays.     

Frost action in cement paste

An earlier theory of frost action is further developed and its implications considered, particularly with regard to testing. Factors determining durability of paste, dimensions of sample, degree of saturation and rate of cooling are discussed. It is shown that cumulative residual expansion is a good indicator for testing frost resistance: samples can be evaluated after 10 to 15 freeze-thaw cycles. The results are discussed in relation to existing tests.     

Water in hardened cement paste

Summary Some properties of adsorbed water in hardened cement paste and in the three completely hydrated main constituents of portland cement have been examined by means of differential thermoanalysis, thermo-gravimetry and nuclear magnetic resonance. When a dried specimen is re-wetted, the adsorbed water first contributes to the build-up of a mono-layer. Only in the higher humidity range is the sorbed water partly bound as inter-layer hydrate water and as hydrate water. According to the first results of the nuclear magnetic resonance measurements the water adsorbed in the monolayer behaves like a two-dimensiional Van-der-Waals-gas.

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Résumé Quelques propriétés de l'eau adsorbée dans la pate de ciment durcie et dans les trois constituants principaux complètement hydratés du ciment Portland ont été étudiées avec l'analyse thermique différentielle, la thermo-gravimétrie et la résonance magnétique nucléaire. Lorsqu'une éprouvette séchée est réhumidifiée, l'eau adsorbée contribe d'abord à la formation d'une mono-couche. L'eau adsorbée, en tant qu'eau d'hydratation, n'est partiellement liée dans la structure du sel que pour les humidités les plus élevées. Selon les tout premiers résultats fournis par les mesures de la résonance magnétique nucléaire, l'eau adsorbée dans la mono-couche se comporte comme un gaz bi-dimensionnel de Van-der-Waals.

     

Three-dimensional surface texture of Portland cement concrete pavements containing nanosilica

Pavement surface texture is critical to tire/pavement interaction. Texture characteristics of concrete containing nanosilica pavement surface has not yet been directly investigated, although researchers have found that nanosilica improved pavement friction values and durability. Specimens with various nanosilica content are tested for three-dimensional (3D) texture height maps which are decomposed using discrete wavelet transform for the calculation of 3D texture indices for macrotexture and microtexture. It is found that 3D texture indices increase correspondingly with the increment of nanosilica content. Significant relationship with R ² values between 0.80 and 0.99 is found between various texture indices and nanosilica content. The increased texture amplitude indicates enhanced pavement friction and then safety. The increment of core material volume implies more texture in the core region, which indicates better longevity of texture. The findings of this research agree with the results of other studies that nanosilica increased the abrasion resistance and frictional property of concrete surface.     

Lateral creep of maturing cement paste

This investigation was conducted to study systematically the lateral creep of maturing cement paste under constant stress-strength ratio. The age at loading ranged from 0.75 to 28.75 days. It has been found from these tests that lateral creep is much more for maturing cement paste than for mature cement paste. When hydration is stopped by cooling to ?11°C, the creep values are the same. The creep Poisson's ratio values tend to be higher for earlier age of loading.     

Electrical conductivity of drying cement paste

Previous research has shown that electrical measurements can be used to monitor moisture movement inside concrete. The interpretation of these measurements is frequently based on empirical relationships between moisture changes and electrical properties of concrete. As such, these empirical relationships can limit the application of the electrical measurements to a specific material or exposure history. To facilitate the development of a general method that is applicable to a concrete member in service, this paper characterizes the electrical conduction in cement paste subjected to drying (desorption) and moisture absorption. The paper quantifies how the electrical conductivity is dependent on the volume and connectivity of the moisture inside the pores and the conductivity of pore solution. This paper also presents a procedure to quantify the contribution of the surface (solid-pore) conduction on the overall conductivity of the cement paste. The results of this investigation contribute to the development of an embedded relative humidity sensor that can be used to monitor changes in the internal humidity of concrete during its service life.