Influence of surface charge on ingress of chloride ion in hardened pastes

The amount of free chloride content in concrete is one of major factors in initiating the corrosion process. The material and environmental factors play a key role in diffusing the chloride ion through the cover concrete to reinforcement. Thus, the electrochemical study is indispensable to understand the mechanism of chloride ingress into concrete. Determination of surface charge and its influence on diffusion of chloride ion into cement matrix of concrete are researched for Ordinary Portland Cement (OPC) paste and cement paste containing Ground Granulated Blastfurnace Slag (GGBS). Different kinds of experiments such as measurement of membrane potential, determination of porosity and pore size distribution, determination of pore solution concentration, and steady state diffusion coefficient of chloride and sodium ions are employed to understand the mechanism of chloride ingress. The obtained results show that the positive surface charge on the pore walls of hardened paste regardless of GGBS’s presents. The surface charge of hardened paste mainly depends on pore solution concentration and cement composition. The physiochemical characteristics of the pores are affecting on transporting ions through it. Hardened paste has greater resistance to diffusing sodium ions than chloride ions. Moreover, there is a strong interaction between transport of chloride ion and surface charge in matured hardened paste.     

Prediction of the diffusivity of cement-based materials using a three-dimensional spatial distribution model

A three-dimensional image of hardened cement paste was reconstructed using a backscattered electron image (BEI) and used to predict the diffusion properties of hardened cement paste. After the BEI observations, an autocorrelation function (ACF) was calculated for each phase of the hardened cement paste, including the unhydrated cement, portlandite, and large pores. A three-dimensional image was reconstructed on the basis of the ACF based on random distributions. The dynamic elastic modulus and diffusion coefficient were calculated using a finite-element or finite difference method with the reconstructed three-dimensional images. The elastic modulus of the C-S-H phase was determined by micro-indentation, and the diffusivity of C-S-H was calculated using this elastic modulus based on previous reports. The resulting predicted dynamic elastic moduli and diffusion coefficients were in good agreement with the experimental results. Although, it was observed that the predicted values of the diffusivity of the blended cement pastes is different from the measured values, a new relationship between diffusivity and porosity of C-S-H in blended cement pastes was developed in this study.     

Density mapping of hardened cement paste using ptychographic X-ray computed tomography

Ptychographic X-ray computed tomography (PXCT) allows for a non-destructive, three-dimensional mapping of the electron density. Its quantitativeness combined with a resolution in the 100 nm range makes it a suitable tool for the assessment of densities of the individual phases in complex materials, such as hardened cement pastes. Here we present results of an experiment performed on a cylindrical sample of epoxy-impregnated hardened cement paste of about 30 μm in diameter. Two-dimensional cross sections of the three-dimensional electron density map show a microstructure that bears distinct similarity to that observed by back-scattered scanning electron microscopy. Domains of various residues of cement grains, calcium hydroxide, calcium carbonate, epoxy-resin impregnated calcium silicate hydrates, epoxy-resin impregnated porosity, and unimpregnated porosity are revealed and are manifested as distinguishable peaks in the histogram of the three-dimensional electron density map. On assumptions of (i) a priori knowledge of the chemical composition and (ii) the purity of the analysed regions, the mass densities of the above mentioned individual material phases are estimated. The potential of PXCT for the science of cement and concrete is discussed.     

Investigation into engineering properties and strength mechanism of grouted macadam composite materials

To further understand engineering properties of grouted macadam composite materials (GMCM) used as a surfacing layer in pavement, the mechanical properties and durability characteristics of GMCM were evaluated, and the relevant strength mechanisms were investigated at the micro level. Results indicate that GMCM has better high-temperature stability, fatigue performance and moisture stability than that of conventional asphalt mix, while it shows an acceptable decrease in low-temperature crack resistance due to the relative brittleness of hardened cement paste. The hardened cement paste also generates a spatial network crystalline lattice in asphalt mix skeleton to form a three-dimensional integral coagulation-crystalloid structure. This facilitates the asphalt mix skeleton and hardened cement paste to bear loads in unison and increase durability of the GMCM. Further, the fibre-like hydrated products of fresh cement slurry on the bitumen film surface increase the interfacial strength between bitumen and hardened cement paste due to toughening and bridging effects, which plays an important role to enhance mechanical properties and durability of GMCM. Finally, GMCM strength is from the internal friction of asphalt mix skeleton, the network structure of hardened cement paste and the adhesion between porous asphalt mix and hardened cement paste. It is concluded that GMCM can better meet the requirements of mechanical properties and durability characteristics than the conventional asphalt mix.     

The microstructure of cement paste and concrete––a visual primer

This paper represents an attempt to provide an introduction to the microstructure of cement paste and concrete as seen in backscatter-mode scanning electron microscopy (SEM). Illustrations are provided of the ‘internal architecture' underlying the microstructure of hardened cement paste and paste in concrete, at the size scales accessible to this instrument. Concrete is a uniquely complex engineering material. The aim of this paper is to provide researchers and practitioners who deal with it an acquaintance with its internal structure.     

The viscoelastic response of cement paste to three-dimensional loading

The majority of the viscoelastic constitutive data for cement paste or concrete found in the literature deal exclusively with uniaxial loading. To predict the isotropic response of concrete or cement paste under multiaxial loading or multiaxial prescribed deformation, it is necessary to have knowledge of at least two viscoelastic constitutive properties. In the past, the typical treatment of three-dimensional modeling of concrete viscoelasticity has involved the assumption of a time-independent viscoelastic Poisson ratio. However, the experimental evidence supporting this simplification is inconclusive. In this study, experiments were performed on hardened cement paste that allowed the simultaneous measurement of both the dilatational and shear compliances, allowing the full three-dimensional characterization of the constitutive response. It was found that the dilatational compliance leveled off after several days for three of four mixtures tested. In these three materials, the Poisson’s ratio was found to be an increasing function of time. Prediction of the measured uniaxial compliance using the measured bulk and shear compliances indicated that the confined compressive test used in this research may cause changes in the material which affect the measured dilatational compliance, and therefore the calculated viscoelastic Poisson ratio.     

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.     

The effect of water sorption on the dynamic modulus of elasticity of desiccated concrete materials

The effects of desorption and water sorption on the dynamic modulus of concrete, mortars and hardened Portland cement paste are investigated. Desiccation of the latter results in a significant reduction in the dynamic modulus. Cyclic drying and wetting of the concrete system further decreases the modulus value. Shrinkage induced microcracking is proposed as the major mechanism of the modulus variation. It is concluded that the first drying of the concrete system results in some irreversible changes in the structure of the hardened cement paste.

     

辅助胶凝材料对硬化水泥浆体结合氯离子的影响综述

辅助胶凝材料富含SiO2和Al2O3,参与二次水化反应生成更多的C-S-H凝胶,降低硬化水泥浆体的碱度,对硬化水泥浆体结合氯离子具有显著的影响。本文重点综述了矿渣、粉煤灰、硅灰、偏高岭土和煤矸石对硬化水泥浆体结合氯离子的影响,并对物理吸附和化学结合氯离子机理进行了深入分析。掺辅助胶凝材料氯离子结合等温线均适合Freundlich非线性吸附关系,最后对辅助胶凝材料结合氯离子研究的不足进行探讨,为后续的研究工作提供理论依据。     

Nanosilica effects on composition and silicate polymerization in hardened cement paste cured under high temperature and pressure

Cement pastes of water to cement ratio (w/c) of 0.45 with and without nanosilica are hydrated under two conditions, room condition (20 °C with 0.1 MPa pressure) and an oil well condition (80 °C with 10 MPa pressure) for 7 days. For the cement pastes with nanosilica, 1% and 3% of cements weights were replaced by nanosilica. The composition of the hardened cement pastes is investigated using X-ray diffraction (XRD). Nuclear magnetic resonance (NMR) experiments are used to quantify the silicate polymerization in hydrated cement paste. Microstructural phases are identified according to the corresponding mechanical property using nanoindentation. The results showed that under room curing conditions, hardened cement paste with 1% nanosilica has the highest level of calcium silicate hydrate (C–S–H) polymerization. However, under high temperature and pressure curing conditions, hardened cement paste with 3% nanosilica has the highest level of C–S–H polymerization. A new relatively stiff microstructural phase is observed in cement pastes incorporating nanosilica and cured under elevated pressure and temperature conditions. The significance of curing conditions and nanosilica content on the polymerization and stiffness of hydrated cement pastes are discussed.