Cement content determination through selective stain in hardened concrete

A cement quantification method has been developed in Ø 15 × 30 cm concrete specimens applying selective stain. A tannic acid–tartaric acid solution was used to stain the cement paste.This paper explains the procedure to determine the quantity of cement in the samples: the reagents utilised for the stain, preparation, cutting and stain of the specimens, the capture and treatment of images of the stained samples, and the preparation of the calibration curves.Next, we assess the accuracy of this method and include the resolution of a practical case in which a comparison is drawn between the values obtained through the application of this method and those obtained using a ‘reference’ method.     

Local porosity analysis of pore structure in cement paste

Three-dimensional (3-D) local porosity theory (LPT) was originally proposed by Hilfer and recently used for the analysis of pore space geometry in model sandstone. LPT pursues to define the probability density functions of porosity and porosity connectivity. In doing so, heterogeneity differences in various sandstone samples were assessed. However, fundamental issues as to the stochastic concept of geometric heterogeneity are ignored in Hilfer's LPT theory. This paper focuses on proper sampling procedures that should be based on stochastic approaches to multistage sampling and geometric heterogeneity. Standard LPT analysis provides a 3-D microscopic modeling approach to materials. Traditional experimental techniques yield two-dimensional (2-D) section images, however. Therefore, this paper replaces the method for assessing material data in standard LPT theory to a more practical one, based on stereological, 3-D interpretation of quantitative image analysis data. The developed methodology is used to characterize the pore structure in hardened cement paste with various water/cement ratios (w/c) at different hydration stages.     

Properties of plain and latex modified Portland cement pastes and concretes with and without superplasticizer

This paper deals with the effects of latex concentration on the workability and strength characteristics of Portland cement pastes with and without superplasticizer. Durability assessments are made by immersing these pastes in 5% Na₂SO₄ and 2.5% NaCl solutions. From the results obtained, it is found that the superplasticizer and superplasticizer-latex combinations may improve the workability of the Portland cement pastes. The Portland cement pastes with superplasticizer have much higher strengths than the latex modified Portland cement pastes with and without superplasticizer. In general, curing in lime-saturated water adversely affect the strength of the pastes containing latex from about 28 days onwards. In the durability test, the resistance of latex modified Portland cement pastes with and without superplasticizer to NaCl is decreased. Degradation mechanism depends on the characteristics of the corrosive medium as well as the resistance of the material itself to the resulting chemical action. The character of strain-stress data of latex modified concretes becomes more prominent as the latex concentration increases. These data are anomalous when compared with the data normally observed for concretes without admixture. The proposed equations are found adequate to describe the stress-strain behaviour of latex modified concretes in compression. These equations can also be applied in calculating the initial modulus of elasticity and proportional limit in the case of polymer modified concretes, which exhibit non-linear behaviour at high stress.     

Transport of a surface-applied corrosion inhibitor in cement paste and concrete

Surface-applied corrosion inhibitors are a kind of repair material and usually contain an aminoalcohol and a component forming a salt with the aminoalcohol. According to the manufacturers, this type of inhibitor penetrates very rapidly into concrete; however, the transport mechanisms have not been sufficiently investigated so far. The major part of the study therefore focused on the transport of the ingredients of an inhibitor in cement paste and concrete, which contained an aminoalcohol and a phosphorous compound. It has been shown that the latter forms an insoluble calcium salt in the environment of cement and precipitates quantitatively. It is thus unable to penetrate from the outside into the alkaline concrete zone and cannot develop its inhibiting effect there. The aminoalcohol, on the other hand, is not bound by cement, but remains largely dissolved in the pore liquid, thus providing optimal conditions for high mobility. The analysis of the transport mechanisms involved has revealed that diffusion in the dissolved state is by far the most efficient transport mechanism. While basically the transport of the aminoalcohol via the gaseous phase is possible, it does play an inferior role only. Surprisingly, the substance had hardly been absorbed by concrete by capillary suction, but at first remained close to the concrete surface.     

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.     

Estimation of the structure of air entrained concrete using a flatbed scanner

This paper describes the possibilities of estimating the structure of concrete with the help of automatic image analysis using a high-resolution flatbed scanner. Sample preparation techniques and components of the image analysis system are described. Concerning the structure of the entrained air voids system, it was found that the results obtained using a high-resolution flatbed scanner were comparable to those obtained by conventional methods, i.e. using a system equipped with stereomicroscope. The possibility of automatically measuring the paste content in hardened concrete by analysis of plane sections, taking into account information available in RGB histograms, is also presented. A new procedure is proposed for the identification of the distance from the cement paste to periphery of the nearest air void, which enables rapid evaluation of the quality of the air entrainment treatment The procedure also allows some characterization of the aggregate component.     

Experimental study of mechanical behaviour of cement paste under compressive stress and chemical degradation

This paper presents experimental investigations of mechanical behaviour of a pure cement paste subjected to compressive stresses and chemical degradation. Two series of laboratory tests have been performed: decoupled and coupled chemical-mechanical tests. Hydrostatic and triaxial compression tests have first been realized respectively on sound and chemically degraded samples. The obtained results allow the characterization of basic mechanical responses of the tested cement paste and the identification of chemical degradation effects on the mechanical behaviour. In the coupled tests, the samples are simultaneously subjected to deviatoric stresses and chemical leaching by aggressive solution flow. Variations of deformation of cement paste samples are measured during chemical degradation process. The results obtained in these tests can be used for the validation of chemo-mechanical constitutive modelling.     

Stress relaxation of foamed high-alumina cement paste

A series of stress relaxation tests on foamed high-alumina cement pastes with different relative densities under various temperatures and imposed fixed strains were conducted to study the effects of relative density and imposed strain on the stress relaxation rates of foamed high-alumina cement pastes. At the same time, the activation energy for stress relaxation of foamed high-alumina cement paste was determined from experimental results. Experimental results on the stress relaxation rates of foamed high-alumina cement pastes are also compared to a theoretical expression obtained from a cell-edge relaxation-bending model. Consequently, the microstructural coefficients included in the theoretical expression for describing the stress relaxation rates of foamed high-alumina cement pastes are found. Furthermore, the stress relaxation rates of foamed high-alumina cement pastes can be predicted from the theoretical expression once their relative density and the imposed strain are known.     

Young's modulus of cement paste at elevated temperatures

Calcium silicate hydrate is a porous hydrate that is sensitive to temperature and readily loses strength at elevated temperatures. Mechanical and chemical changes in the microstructure, due to escaping water, can significantly affect the mechanical properties, but these changes occur over different temperature ranges. By measuring Young's modulus as a function of temperature using the dynamic mechanical analyzer, the temperature range in which the greatest change in stiffness occurs can be identified. Additional mineralogy, pore size distribution, and composition analysis from high temperature X-ray diffraction, nitrogen sorption, and thermogravimetric analysis will demonstrate the changes in the microstructure. The results demonstrate that over 90% of the loss in stiffness occurs below 120 °C. Therefore, the damage is due to microcracking caused by pore water expansion and evaporation and not the change in mineralogy or composition. More damage, as indicated by greater loss in stiffness, occurs in stiffer and less permeable samples where higher stresses can develop.     

Three-dimensional visualization of pore structure in hardened cement paste by the gallium intrusion technique

It is widely known that the pore structure of concrete strongly influences its physical properties. Therefore, we developed a technique for the visualization of the pore structure because of clearing it correctly. However, this visualization is limited to two-dimensional imaging for sections of the specimen. As a result, in this study, we developed a technique for reconstructing the acquired 2D images of the pore structure into 3D form by stacking them. By using this image, the relationship between water permeability and pore connectivity was clarified, and it was shown clearly that the pore connectivity strongly affects the water permeability.     

水泥混凝土水化热的研究与进展

水泥的水化反应是一个放热反应。水泥水化放热的周期很长,但大部分热量是在3天内放出的,尤其是在水泥浆发生凝结、硬化的初期放出。大多数情况下,硬化水泥浆体和混凝土的早期体积变形,主要源于水泥的水化热温升,因此,降低水泥混凝土的水化热是防止其早期开裂的有效途径。本文综合分析了水泥混凝土水化热对其性能的影响,总结了前人在水泥混凝土水化热研究方面提出的一些理论计算公式,介绍了国内外关于水泥混凝土水化热的最新研究进展和水泥生产中降低水化热的技术措施。     

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.     

水化非常早期水泥浆体尺寸变化及其与凝结时间关系的研究

研发了一种基于LC振荡电路的微距测量仪,分辨率可以达到微米尺度,用于测量水化非常早期具有塑性的水泥浆体尺寸变化。研究中采用的水泥样品来自广西各地的水泥厂,均为P.O型32.5和42.5等级;采用自吸水方式制备水泥净浆。研究结果表明,在与水接触后的20min龄期内,不同样品的塑性浆体在初始几分钟内呈现了或膨胀、或收缩的不同的特点,但随着水化的持续,浆体均呈现膨胀的趋势。数据的相关性分析表明,水化非常早期的水泥浆体尺寸变化率与凝结时间呈现负相关的趋势,经过统计学分类后水泥净浆的尺寸变化率与水泥终凝时间存在着良好的线性负相关,相关系数均大于0.9。     

ESEM analysis of polymeric film in EVA-modified cement paste

Portland cement pastes modified by 20% weight (polymer/cement ratio) of poly(ethylene-co-vinyl acetate) (EVA) were prepared, cured, and immersed in water for 11 days. The effects of water saturation and drying on the EVA polymeric film formed in cement pastes were observed using environmental scanning electron microscopy (ESEM). This technique allowed the imaging of the EVA film even in saturated samples. The decrease of the relative humidity inside the ESEM chamber did not cause any visual modification of the polymeric film during its drying.     

X-ray absorption study of drying cement paste and mortar

X-ray absorption was used to observe water evaporation with hydration time in paste and mortar specimens, with the aim of studying the influence of water/cement (w/c) ratio, presence of aggregates, curing conditions on drying during early hydration. For the samples subjected to surface drying immediately after mixing, there exists a moisture gradient within the internal part of the specimen. However, obvious top-down drying only occurs within a small zone near the surface for early age cement pastes and mortars. The evaporation rate of water is very high in the first day after casting and is drastically reduced afterwards due to the formation of a microstructure that greatly improves specimens resistance to moisture loss. Mortars reveal a slightly lower evaporation rate since the aggregate increases the length of the transport route because of a larger tortuosity. However, the effect of sealed curing is much more important than the tortuosity effect of the aggregates.     

Modelling elasticity of a hydrating cement paste

Concrete is a complex multi-scale composite involving multi-physics processes. As it is the only evolving component of concrete, the cement paste has a major influence on the mechanical properties of concrete at early age. This paper focuses on the increase of the elastic properties of a cement paste during its hydration. The homogenization theory for disordered media is used in order to estimate the evolution of the effective elastic moduli of the hydrating paste. The morphological model refers to two types of C-S-H (calcium silicate hydrates, main hydration products of Portland cements) distinguished by many authors: inner products or high density C-S-H build up layers surrounding the anhydrous particles, while the outer products or low density C-S-H play the role of a porous matrix.The simulations of the effective Young's modulus at late age during hydration and at the end of hydration prove to be in excellent agreement with the experimental results available in the literature.     

Internal relative humidity distribution in high-performance cement paste due to moisture diffusion and self-desiccation

This investigation was carried out to study internal relative humidity (IRH) distribution of cement paste made with different water / cement ratios (w / c) and mineral admixtures in isothermal drying conditions. IRH changes in cement paste resulting from self-desiccation and moisture diffusion, respectively, at different ages were studied. The change laws of IRH in cement paste resulting from combining moisture diffusion with self-desiccation were discussed. The results indicate that IRH reduction of cement paste with w / c higher than 0.4 is mainly affected by moisture diffusion. However, IRH reduction of cement paste with w / c no higher than 0.4 is controlled by both moisture diffusion and self-desiccation. With the decrease of w / c, IRH reduction of cement paste resulting from self-desiccation increases, and IRH reduction resulting from moisture diffusion decreases at a given age. IRH decrement of cement paste incorporated with silica fume and ground blast-furnace slag is higher than that of control paste. w / c and the distance to the exposed surface play a significant role in IRH change resulting from moisture diffusion in isothermal drying condition. Change laws of IRH in cement paste with silica fume due to moisture diffusion considering self-desiccation are different from those in cement paste without silica fume.     

Autogenous shrinkage in high-performance cement paste: An evaluation of basic mechanisms

In this paper, various mechanisms suggested to cause autogenous shrinkage are presented. The mechanisms are evaluated from the point of view of their soundness and applicability to quantitative modeling of autogenous shrinkage. The capillary tension approach is advantageous, because it has a sound mechanical and thermodynamical basis. Furthermore, this mechanism is easily applicable in a numerical model when dealing with a continuously changing microstructure. In order to test the numerical model, autogenous deformation and internal relative humidity (RH) of a Portland cement paste were measured during the first week of hardening. The isothermal heat evolution was also recorded to monitor the progress of hydration and the elastic modulus in compression was measured. RH change, degree of hydration and elastic modulus were used as input data for the calculation of autogenous deformation based on the capillary tension approach. Because a part of the RH drop in the cement paste is due to dissolved salts in the pore solution, a method is suggested to separate this effect from self-desiccation and to calculate the actual stress in the pore fluid associated with menisci formation.     

Rheological properties of cement paste: Thixotropic behavior and structural breakdown

In this work, a new material model is presented to simulate rheological behavior of cement paste. This material model is among others based on combined concepts by Hattori and Izumi and by Tattersall and Banfill. More precisely, coagulation, dispersion and re-coagulation of the cement particles (giving a true thixotropic behavior) in combination with the breaking of certain chemically formed linkages between the particles (giving a so-called structural breakdown behavior) are assumed to play an important role in generating the overall time-dependent behavior of the cement paste. The model evaluation is done by comparing experimental data with model prediction.     

Experimental study of gas diffusion in cement paste

This paper presents an experimental study of gas diffusion in binary mixtures of hydrogen-nitrogen and xenon-nitrogen through cement pastes (CEM I and CEM V) of different water/cement ratios (0.35 and 0.45). First, the impact of water saturation on gas diffusion is investigated by performing tests on samples pre-conditioned in specific atmospheric conditions (dry, 55, 70, 82, 93 and 100% RH) by means of saline solutions. The comparison of the results obtained for the CEM I and the CEM V samples (w/c ratio of 0.45) demonstrate the importance of pore size distribution/connectivity on gas diffusion. Second, diffusion tests at different total pressures and using two different mixtures (hydrogen-nitrogen, xenon-nitrogen) are performed to study the nature of gas diffusion in cement paste. Results demonstrate that gas diffusion in cement paste is controlled by Knudsen and ordinary diffusion at pressures greater than 100 kPa and mainly by Knudsen diffusion at pressures less than 100 kPa.