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Horst-michael Ludwig - One of the best experts on this subject based on the ideXlab platform.
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Early hydration of SCM-blended Portland cements: A Pore Solution and isothermal calorimetry study
Cement and Concrete Research, 2017Co-Authors: Axel Schöler, Barbara Lothenbach, Frank Winnefeld, Mohsen Ben Haha, Maciej Zajac, Horst-michael LudwigAbstract:Abstract In this study the hydration kinetics and the development of concentrations in the Pore Solution of cement pastes containing different supplementary cementitious materials (blast-furnace slag, Si-rich fly ash, limestone, quartz) at a cement replacement of 50 wt.% were investigated during the first 6 h of hydration. The results indicate that the degree of undersaturation with respect to alite is the primary factor driving the early hydration kinetics. The accelerating effect of the limestone is related to a higher undersaturation. The data reveal also that high aluminium and sulfate concentrations, which depend on the type and chemical composition of the used mineral addition, retard the reaction. High calcium concentrations had no adverse effect on hydration kinetics. The investigations underpin that the mechanisms controlling the early hydration are a complex combination of the filler effect, the Pore Solution chemistry as well as intrinsic reactivity of the mineral additions and their surface characteristics.
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Analysis of aluminum concentrations in the Pore Solution during hydration of tricalcium silicate
Cement and Concrete Research, 2017Co-Authors: Frank Bellmann, Horst-michael LudwigAbstract:Abstract The hydration of two batches of tricalcium silicate was followed by isothermal heat conduction calorimetry and analysis of Pore Solution composition. These two samples were different in Al 2 O 3 -content and free lime concentration. It was observed that the material containing aluminum reacts slower than the pure one. The reactivity was also analyzed in the presence of ettringite, gypsum and tricalcium aluminate, respectively. The aluminum concentration in the Pore Solution was always below the limit of detection during hydration of the samples independent of the presence of aluminum in tricalcium silicate or the addition of most supplementary phases. Elevated aluminum concentrations were only observed in the presence of tricalcium aluminate. The experimental investigations confirm earlier results that aluminum can reduce the reactivity of tricalcium silicate. It is shown in this study that this mechanism is not based on elevated aluminum concentrations in the Pore Solution.
Amir Poursaee - One of the best experts on this subject based on the ideXlab platform.
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Study of the passivation of carbon steel in simulated concrete Pore Solution using scanning electrochemical microscope (SECM)
Materialia, 2018Co-Authors: H. Torbati-sarraf, Amir PoursaeeAbstract:Abstract A protective passive layer is formed on the surface of embedded reinforcing carbon steel bar in the alkaline environment of concrete. Corrosion of steel in concrete environment is a function of formation, strengthen and stability of this layer. In this study, Scanning Electrochemical Microscopy (SECM) was exploited to study the kinetic and the formation of the passive layer in a simulated concrete Pore Solution. The specimen was cut, epoxy mounted and polished from a carbon steel reinforcing bar. Then, it was immersed in a concrete simulated Pore Solution containing 0.5 mM potassium ferrocyanide as a mediator for 48 h and amperometry SECM approach curves were recorded over the unbiased surface of the freshly polished surface. A platinum microelectrode tip with the diameter of 25 µm was used in the experiment. The effective heterogeneous electron rate coefficient (keff) variation for the mediator regenerating surface reaction was estimated during the time of exposure. Decreasing values of keff, throughout passivation period, indicates formation and increase in the strength of the passive layer formed on the surface of the steel in the alkaline environment in concrete Pore Solution.
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The impact of sandblasting as a surface modification method on the corrosion behavior of steels in simulated concrete Pore Solution
Construction and Building Materials, 2017Co-Authors: Ling Ding, Amir PoursaeeAbstract:Abstract This investigation aimed to study the passivation and corrosion performance of sandblasted steel in a concrete environment. The surfaces of the steel specimens were modified using sandblasting method for three durations: 5, 10, and 15 min. The specimens were immersed in the chloride-free concrete Pore Solution for 14 days and then 3% by weight of chloride ions were added to the Solution. The specimens were then kept in the chloride-contaminated Pore Solution for 60 days. Results from the electrochemical tests indicated that the passive layer formed on the surface on all specimens exposed to a simulated concrete Pore Solution were highly disordered n-type semi-conductors. In all specimens, except the 15 min sandblasted ones, the presence of chloride ions decreased the slope of the Mott-Schottky plots and increased the donor density which indicated formation of a thinner passive layer and corrosion. The results of electrochemical experiments on steel rebar exposed to chloride-contaminated Pore Solution showed significant improvement in corrosion resistance of the sandblasted specimens. This improvement was proportional to the increase in the sandblasting time. Microscopic analysis of the steel specimens at the end of the experiment showed the formation of a dense calcium-rich crystalline structure on the surface of sandblasted specimens. The density and the uniformity of the calcium-rich layer increased by increasing the sandblasting time. It was hypothesized that the formation of this layer combined with the enhanced passive layer in sandblasted specimens were the reasons for the improvement in corrosion resistance of the sandblasted steel specimens.
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Corrosion of steel bars in saturated Ca(OH)2 and concrete Pore Solution
Concrete Research Letters, 2010Co-Authors: Amir PoursaeeAbstract:Testing steel in Solution has the advantage of avoiding the long time necessary for chlorides to penetrate the concrete cover. It is well known that steel in high alkaline environments is passive and the protective capability of the passive film increases with pH. The pH of saturated calcium hydroxide Solution is lower than concrete Pore Solution which does induce passivation but not to the degree encountered by steel in good quality concrete. Nevertheless, saturated calcium hydroxide has been used in many studies of rebar corrosion as a substitute for Pore Solution. This paper discusses the electrochemical behavior of low carbon steel bars is chloride free and chloride contaminated Pore Solution and saturated calcium hydroxide Solution. Results show that the passive film on the steel immersed in Pore Solution and saturated Ca(OH) 2 have similar composition. However, as a result of lower pH in saturated Ca(OH) 2 Solution, the passive layer formed in this Solution is less protective and does not offer enough passivity to steel to simulate a realistic concrete environment.
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Reinforcing steel passivation in mortar and Pore Solution
Cement and Concrete Research, 2007Co-Authors: Amir Poursaee, Carolyn M. HanssonAbstract:Under field conditions, steel is embedded in concrete for a long period of time before chlorides penetrate. In studying the corrosion behaviour of steel in concrete, mortar or in simulated Pore Solution, it is essential to allow enough time for the steel to create a passive layer which is the subject of this study. This time is given to steel in chloride free concrete, naturally; while it should be provided to steel in synthetic Pore Solution, before adding chloride to the Solution. For determining this time, samples were made with steel with different surface conditions: as-received with mill scales and sand-blasted. One set of steel bars (as-received and sand-blasted) were embedded in mortar and one set were immersed in synthetic Pore Solution. Corrosion of each steel bar was monitored every hour by LPR technique for total time of 300 h. Also, half-cell potential of steel bars was measured during that time. Results show that steel needs to be kept at least three days in synthetic Pore Solution and seven days in mortar to be passivated.
Katsunobu Takeuchi - One of the best experts on this subject based on the ideXlab platform.
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Alkali-silica reaction and Pore Solution composition in mortars in sea water
Cement and Concrete Research, 1996Co-Authors: Mitsunori Kawamura, Katsunobu TakeuchiAbstract:Abstract The promotion of expansion of mortars containing a reactive aggregate in 1N NaCl Solution at 38 °C was attributed to a rise of OH − ion concentration in the Pore Solution in the mortars. However, it is ambiguous whether the promotion of expansion of mortars in sea water at a room temperature can be explained in the same way as in NaCl Solution at an elevated temperature. This study aims at pursuing the expansion behavior of mortars containing a reactive aggregate relating it to their Pore Solution composition and the extent of alkalisilica reaction occurring within reactive grains. The alkali-silica reaction in mortars in sea water and 0.51N NaCl Solution at 20 °C appears to progress differently from that in mortars in 1N NaCl Solution at an elevated temperature of 38 °C. The promotion of expansion of mortars in sea water at 20 °C was found to be responsible for an effect of Cl − ions on the alkali-silica reaction at early stages of immersion. Only when OH − ion concentration in the Pore Solution was relatively high, NaCl and sea water could accelerate the alkali-silica reaction in mortars at 20 °C.
Jan Olek - One of the best experts on this subject based on the ideXlab platform.
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alkali silica reaction kinetics of chemistry of Pore Solution and calcium hydroxide content in cementitious system
Cement and Concrete Research, 2015Co-Authors: Jan Olek, Hyungu JeongAbstract:Abstract This paper presents the results of the investigations on the chemistry of Pore Solutions, the contents of calcium hydroxide, and the expansions in mortars containing both reactive and non-reactive aggregates. In order to examine the effect of the temperature, experiments were performed at three different temperatures (23 °C, 38 °C and 55 °C). The compositions of the Pore Solution were measured at short time intervals for a period of up to 130 days in order to capture the kinetics of the chemistry of Pore Solution. The results showed that the changes in the concentrations of alkali ions can be best explained by the first order reaction. In addition, the proposed rate equation could reasonably simulate the changes in the actual concentrations of alkalis. Finally, the results in this paper suggest that the rate of the alkali–silica reaction in cementitious system containing highly reactive aggregate can be also expressed as the first order reaction.
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Alkali–silica reaction: Kinetics of chemistry of Pore Solution and calcium hydroxide content in cementitious system
Cement and Concrete Research, 2015Co-Authors: Jan Olek, Hyungu JeongAbstract:Abstract This paper presents the results of the investigations on the chemistry of Pore Solutions, the contents of calcium hydroxide, and the expansions in mortars containing both reactive and non-reactive aggregates. In order to examine the effect of the temperature, experiments were performed at three different temperatures (23 °C, 38 °C and 55 °C). The compositions of the Pore Solution were measured at short time intervals for a period of up to 130 days in order to capture the kinetics of the chemistry of Pore Solution. The results showed that the changes in the concentrations of alkali ions can be best explained by the first order reaction. In addition, the proposed rate equation could reasonably simulate the changes in the actual concentrations of alkalis. Finally, the results in this paper suggest that the rate of the alkali–silica reaction in cementitious system containing highly reactive aggregate can be also expressed as the first order reaction.
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Modeling of early age loss of lithium ions from Pore Solution of cementitious systems treated with lithium nitrate
Cement and Concrete Research, 2014Co-Authors: Jan OlekAbstract:Addition of lithium nitrate admixture to the fresh concrete mixture helps to minimize potential problems related to alkali-silica reaction. For this admixture to function as an effective ASR control measure, it is imperative that the lithium ions remain in the Pore Solution. However, it was found that about 50% of the originally added lithium ions are removed from the Pore Solution during early stages of hydration. This paper revealed that the magnitude of the Li{sup +} ion loss is highly dependent on the concentration of Li{sup +} ions in the Pore Solution and the hydration rate of the cementitious systems. Using these findings, an empirical model has been developed which can predict the loss of Li{sup +} ions from the Pore Solution during the hydration period. The proposed model can be used to investigate the effects of mixture parameters on the loss of Li{sup +} ions from the Pore Solution of cementitious system.
Hjh Jos Brouwers - One of the best experts on this subject based on the ideXlab platform.
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a method for predicting the alkali concentrations in Pore Solution of hydrated slag cement paste
Journal of Materials Science, 2011Co-Authors: Wei Chen, Hjh Jos BrouwersAbstract:The alkalinity of the Pore liquid in hardened cement paste or concrete is important for the long-term evaluation of alkali-silica reaction (ASR) expansion and corrosion prevention of steel bar in steel reinforced structures among others. It influences the reactivity of supplementary cementitious materials as well. This paper focuses on the alkali binding in hydrated slag cement paste and a method for predicting the alkali concentrations in the Pore Solution is developed. The hydration of slag cement is simulated with a computer-based model CEMHYD3D. The amount of alkalis released by the cement hydration, quantities of hydration products, and volume of the Pore Solution are calculated from the model outputs. A large set of experimental results reported in different literatures are used to derive the alkali-binding capacities of the hydration products and practical models are proposed based on the computation results. It was found that the hydrotalcite-like phase is a major binder of alkalis in hydrated slag cement paste, and the C–S–H has weaker alkali-binding capacity than the C–S–H in hydrated Portland cement paste. The method for predicting the alkali concentrations in the Pore Solution of hydrated slag cement paste is used to investigate the effects of different factors on the alkalinity of Pore Solution in hydrated slag cement paste.
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A computed-based model for the alkali concentrations in Pore Solution of hydrating Portland cement paste
2008Co-Authors: Wei Chen, Zhonghe Shui, Hjh Jos BrouwersAbstract:A computed-based model for the alkali concentrations in Pore Solution of hydrating Portland cement paste is proposed. Experimental data reported in different literatures with thirteen different recipes are analyzed. A 3-D computer-based cement hydration model CEMHYD3D is used to simulate the hydration of these pastes. The models predictions are used as inputs for the alkali partition theory, which is used to derive the alkali binding capacity of C-S-H in hydrating Portland cement paste. A linear relation between the amount of bound-alkali Na+ in C-S-H and its concentration in the Pore Solution is found, whilst a non-linear relation should be employed for the amount of bound-alkali K+ in C-S-H. New methods for predicting the alkali concentrations in the Pore Solution of hydrating Portland cement pastes are proposed based on the computer model CEMHYD3D, which is also validated with experimental results.
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Alkali concentrations of Pore Solution in hydrating OPC
Cement and Concrete Research, 2003Co-Authors: Hjh Jos Brouwers, Rj Van EijkAbstract:In order to study the Pore Solution, the release and binding of alkalis in a hydrating cement system have been studied. First, the binding factors for sodium and potassium as determined by Taylor [Adv. Cem. Res. 1 (1987) 5] and the corresponding distribution ratios as determined by Hong and Glasser [Cem. Concr. Res. 29 (1999) 1893; Cem. Concr. Res. in press] are related to each other. It follows that the sorption of sodium is practically identical, whereas for potassium Taylor [Adv. Cem. Res. 1 (1987) 5] predicts a substantial lower degree of sorption. The concept of alkali release, Pore Solution decrease and sorption by formed calcium-silicate hydrate (C-S-H), is incorporated in the NIST hydration model (CEMHYD3D). Subsequently, the model is compared with ordinary Portland cement (OPC) hydration experiments reported by Larbi et al. [Cem. Concr. Res. 20 (1990) 506]. Good agreement is obtained when the distribution ratios of Hong and Glasser [Cem. Concr. Res. 29 (1999) 1893] are applied. The results suggest that C-S-H is the only binder of alkalis in hydrating OPC.
