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An experimental was presented in this article to investigate the impact of the low relative humidity (low RH) on the pullout properties of the straight and hooked-end fibers embedded into cement matrices. It was shown that decrease in curing RH results in a reduction in the interfacial carrying capacity and the pullout energy absorption ability, owing to the microstructure of the fiber-matrix ITZ deteriorating as the humidity decreased for all types of fibers. Besides, a novel model was developed to simulate the bond-slip behaviors of the steel fiber-matrix interface under low RHs, and a theoretical method was carried out to estimate the parameters in the model. Furthermore, various analytical models were proposed based on the bond-slip model to simulate the pullout behaviors of the steel fibers embedded in matrix under low RHs, and the validation results revealed that the suggested model is consistent with the experimental data. [ABSTRACT FROM AUTHOR]

The structural build-up of cement-based materials is often characterized by the determination of the evolution of static yield stress with time. This property is crucial to many applications of concrete. However, the measurement of static yield stress may disturb the structure of cement-based material. As a consequence, the true structural build-up of the materials may not be detected. In this paper, the evolution of static yield stress and storage modulus (G′) determined by small amplitude oscillatory test was determined to characterize the evolution of the structural build-up of cement pastes. Results showed that G′ and static yield stress developed similarly with time. When the tests were carried out on the same sample, the measurement of static yield stress disturbed the structure of cement paste and had a significant effect on the following measurement of G′, but a slight effect on the following measurement of static yield stress. [ABSTRACT FROM AUTHOR]

It has been long recognized that low air pressure (AP) affects the moisture transfer in concrete, and thus its microstructure and bulk property, but less is known about the thermodynamic mechanism. This research investigated the microstructure evolution of young white Portland cement paste during wetting or drying at different APs (i.e. 101 kPa, 60 kPa, 20 kPa), the experimental and numerical analyses reveal the moisture transfer changes at low APs, which matters for the microstructural features of hardened cement paste. It was found that low APs increase the water vapor diffusion and convective mass transfer coefficients, and the intrinsic permeability of cement paste sample, which results in higher water contents within cement paste during wetting at 98 % RH but more rapid water losses during drying at 43 % RH. Owing to the distinct moisture transfer behaviors, the C-S-H structures were linked with water contents within confined pores. [ABSTRACT FROM AUTHOR]

Gypsum has been known to regulate cement setting since more than 100 years, whereas the understanding of the physio-chemical mechanism remains incomplete. Here, we investigated the influence of gypsum on elasticity evolution of fresh tricalcium silicate (C 3 S) paste through small amplitude oscillation shear method. Isothermal calorimetry, X-ray diffraction, 1H nuclear magnetic resonance, and zeta potential measurements were performed to explore the corresponding changes in hydration, microstructure, and colloidal interaction. It was found that incorporating gypsum is beneficial for inter-particle agglomeration by significantly weakening electrostatic repulsion force between particles, which accelerates elasticity development in the early period of setting. Although the retardation effect of gypsum on C 3 S hydration hinders the further development of elasticity in the later period, the more divergent needle-like calcium silicate hydrate (C-S-H) and the stronger cohesion of the C 3 S particle network in presence of gypsum lead to a higher elasticity at the same hydration degree. [ABSTRACT FROM AUTHOR]

AC impedance spectroscopy (ACIS) is a promising technique for monitoring the microstructure evolution of fresh cement paste in real-time. This paper compared the change of bulk electric resistivity (ρ bulk) obtained from ACIS with the developments of storage modulus (G′) and the mean transverse relaxation times (T 2) of fresh Portland cement pastes within 5 h. It was found that the three different phases (Phase I, II, and III) on the microstructural build-up process of fresh paste can be accurately distinguished by analyzing ρ bulk , as well as G′ and T 2. The use of ρ bulk fails to characterize the microstructural changes in Phase I due to the great sensitivity on the electrical conductivity of the interstitial solution. However, it can successfully reflect the developing features of microstructure in Phases II and III, and reliably evaluate the impacts of water to cement ratio, superplasticizer, and supplementary cementitious materials on the microstructural development. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the effect of hydroxyl ions on chloride penetration depth measurement using the colorimetric method was studied. Equivalent silver nitrate solution (i.e. Ag+ =Cl−) was added to the NaCl+NaOH solution with different concentrations, then the amount of precipitated silver chloride and silver oxide were determined by chemical methods, and the color of the precipitated products was examined. Results show that the amount of silver chloride formed decreases linearly as OH− to Cl− ratio (r) increases. Thus, the chloride concentration at color change boundary changes with the pH value of the concrete. AgCl has a white color, while Ag2O has a dark brown color. When the value of r exceeds 4, the color of the mixture looks brown, and color change boundary cannot be easily distinguished. [Copyright &y& Elsevier]

This paper monitored the viscoelasticity development of fresh C 3 A-gypsum paste in situ by small amplitude oscillation shear (SAOS) within 5 h. The development of viscoelasticity of paste with time can be characterized by two different stages. Isothermal microcalorimetry, X-ray diffraction (XRD), scanning electronic microscopy (SEM) and low field 1H nuclear magnetic resonance (NMR) measurements were used to investigate the underlying mechanisms behind these two stages. It was found that the first stage is governed by the increase of solid volume fraction while the second stage is controlled by the rigidification of ettringite networks. The growth of loss modulus is restricted since the formation of ettringite linkages between particles. Moreover, the considerable elasticity development of fresh C 3 A-gypsum paste is stemmed directly from the increasing ettringite precipitation. [ABSTRACT FROM AUTHOR]

This paper measured the zeta potential and chloride concentration index of hardened cement paste, which was defined as the ratio of chloride concentration in the expressed pore solution to that in soaking solution. A model was proposed to explain the relationship of zeta potential and concentration index of hardened cement pastes immersed in chloride solution. The results showed that with the increase of chloride concentration in soaking solution, both the chloride concentration index and zeta potential of hardened cement paste gradually decreased. The addition of slag influenced the chloride concentration index and zeta potential. The relationship between chloride concentration index, chloride concentration in soaking solution and slag replacement level, as studied by the Gouy-Chapman model was well correlated with the measured results. [ABSTRACT FROM AUTHOR]

Abstract: AgNO3 colorimetric measurement of chloride in cement-based materials can be implemented owing to chemical reactions between Ag+ and Cl− or OH− ions, which result in the precipitation of AgCl and Ag2O. When the mass ratio of AgCl to precipitated AgCl+Ag2O reaches a critical value (Pcrit-AgCl), a color change boundary appears. In this paper, chloride concentrations required for the formation of the critical mass ratio of AgCl to precipitated AgCl+Ag2O derived from the AgNO3–NaCl–NaOH system were calculated based on the solubility products of silver hydroxide and silver chloride, and mass conservation of the chemical reactions. Pcrit-AgCl was determined as 0.65 through a series of AgNO3 colorimetric measurements on cement-based materials. Calculated results indicated that if Ag+ could react with all OH− and Cl−, there existed a linear relationship between the critical chloride concentration for color change (Ccrit-Cl– ) and OH− concentration (COH– ), namely Ccrit-Cl– =1.6COH– . If Ag+ could react with only partial OH− and Cl−, there existed an exponential relationship among Ccrit-Cl– and COH– , Ag+ concentration (CAg+ ), volume of AgNO3 solution (VAg+ ) as well as the volume of NaCl+NaOH solution (VOH– +Cl– ), namely Ccrit-Cl– =0.00695COH– +0.608CAg+ VAg+ /VOH– +Cl– . It can be concluded that the alkalinity of cement-based materials, concentration and volume of sprayed AgNO3 solution and volume of pore solution have effects on chloride concentration at the color change boundary (Cd). Calculated Cd values ranged between 0.03 and 0.96mol/L (in pore solution) or 0.011% to 2.27% (by the mass of binder), respectively. This explains why reported range of chloride concentration at the color change boundary in publications varied in a broad range. [Copyright &y& Elsevier]

The pumping pressure loss of fresh concrete has traditionally been modeled as a linear function of the pumping distance. However, previous studies have suggested that this relationship is nonlinear in nature. This research has identified that the nonlinearity is due to changes in the rheological properties of fresh concrete during pumping. In this study, a new prediction equation has been proposed for the loss of pumping pressure for concrete mixtures with w/c ratios between 0.17 and 0.46, and an equivalent horizontal pipe length between 328 and 912 m. The equation provides a linear approximation of the nonlinear relationship of pumping pressure loss. To account for the effect of rheological changes during pumping, an empirical coefficient related to the initial slump flow of the concrete has been incorporated. The new function can improve the prediction accuracy of pressure loss by 25 % and 4.2 % for horizontal and vertical pumping, respectively. [ABSTRACT FROM AUTHOR]

This study aims to explore the impacts of alkali ions and aqueous Al species in pore solution on alkali-silica reaction (ASR). Wrapped accelerated mortar bar test (100% RH, 38 °C) was carried out on mortars prepared with binders containing various metakaolin (0% and 5%) and alkali content (Na 2 O eq = 0.65%, 1.25%, 2.25%, and 2.75%); solid and aqueous compositions of the corresponding hydrated pastes were analyzed using XRD and ICP-OES. A combined effect of alkali ions and aqueous Al species on ASR is identified, which can be characterized by a ratio of alkali (Na + K) to aluminum concentration in pore solution, and ASR expansion increases with this ratio. This finding provides new insights in understanding the relationship between pore solution composition and ASR, and indicates alternative approaches in controlling ASR, other than limiting alkalis content. [ABSTRACT FROM AUTHOR]