A low-temperature DSC investigation of hardened cement paste subjected to chloride action
TL;DR: Differential scanning calorimetry (dsc) was used to investigate the freezing behaviour of hardened cement paste (hcp) subjected to nacl and cacl2 solutions containing up to 5.4 mol cl/l as mentioned in this paper.
Abstract: Differential scanning calorimetry (dsc) was used to investigate the freezing behaviour (-175 deg c to 20 deg C) of hardened cement paste (hcp) subjected to nacl and cacl2 solutions containing up to 5.4 mol cl/l. Thermograms were also recorded for samples dried to relative humidities (rh) between 61% and 96%. The low-temperature phase transition at -38 deg c exhibited the freezing behaviour of an aqueous solution in gel pores of approximately equal to 4 nm radius. Chloride ions strongly reduce the coupling forces between gel particles and structure the gel pore water. On desorption critical behaviour of the low-temperature transition energy was observed at 0.52 mol cl/l which is the concentration most damaging to hcp. Above 1.3 mol cl/l freezing of supercooled bulk water in macropores was observed at more than one temperature. (Author/TRRL)
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TL;DR: In this article, a review of the state of the art in this field is presented, and proposed mechanisms are discussed, and their adequacy is judged based on their ability to account for the phenomenology.
Abstract: Salt scaling is a major durability issue for concrete. Despite this, and an extensive research effort, the cause of this damage is unknown. Therefore, no means for preventing salt scaling can be identified. One of the primary reasons for this shortcoming is the lack of a critical review on the state of the research in this field. Such a compilation is presented in this series of articles. In Part I, the characteristics of salt scaling were outlined. In this article, proposed mechanisms are discussed, and their adequacy is judged based on their ability to account for the phenomenology.
210 citations
TL;DR: A comprehensive review of the literature on salt scaling can be found in this article, where the authors reviewed the experimental studies that have revealed the phenomenology of salt scaling and proposed mechanisms for scaling.
Abstract: Salt scaling is a major durability issue for concrete, so the phenomenon has been the subject of an extensive research effort. Nevertheless, there is no agreement regarding the cause of this damage, so no means for preventing salt scaling can be identified. One of the primary reasons for this shortcoming is the lack of a critical review of the research in this field. Such a compilation is presented in the present series of articles. In Part I, we review the experimental studies that have revealed the phenomenology of salt scaling. In Part II, proposed mechanisms for scaling are discussed, and the adequacy of these mechanisms is judged based on their ability to account for the characteristics outlined here.
196 citations
Cites background from "A low-temperature DSC investigation..."
...However, the presence of salt reduces the amount of ice formed [14,20,40]....
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TL;DR: In this paper, the authors show that salt scaling is a consequence of the fracture behavior of ice and that the stress arises from thermal expansion mismatch between ice and concrete, which puts the ice in tension as the temperature drops.
Abstract: Over the past 60 years, concrete infrastructure in cold climates has deteriorated by “salt scaling,” which is superficial damage that occurs during freezing in the presence of saline water. It reduces mechanical integrity and necessitates expensive repair or replacement. The phenomenon can be demonstrated by pooling a solution on a block of concrete and subjecting it to freeze/thaw cycles. The most remarkable feature of salt scaling is that the damage is absent if the pool contains pure water, it becomes serious at concentrations of a few weight percent, and then stops at concentrations above about 6 wt%. In spite of a wealth of research, the mechanism responsible for this damage has only recently been identified. In this article, we show that salt scaling is a consequence of the fracture behavior of ice. The stress arises from thermal expansion mismatch between ice and concrete, which puts the ice in tension as the temperature drops. Considering the mechanical and viscoelastic properties of ice, it is shown that this mismatch will not cause pure ice to crack, but moderately concentrated solutions are expected to crack. Cracks in the brine ice penetrate into the substrate, resulting in superficial damage. At high concentrations, the ice does not form a rigid enough structure to result in significant stress, so no damage occurs. The morphology of cracking is predicted by fracture mechanics.
136 citations
TL;DR: In this article, electrochemical measurements of open circuit potential, linear polarization and electrochemical impedance spectroscopy (EIS) were utilized to investigate the corrosion behavior and chloride threshold value of reinforcing steels submitted to chloride and sulphate attack in simulated concrete pore solution.
Abstract: Electrochemical measurements of open circuit potential, linear polarization and electrochemical impedance spectroscopy (EIS) were utilized to investigate the corrosion behavior and chloride threshold value (CTV) of reinforcing steels submitted to chloride and sulphate attack in simulated concrete pore solution in this study. Determination of corrosion initiation was made by combining half-cell potential (Ecorr) with corrosion current density (Icorr) as well as EIS curves. Results showed that electrochemical measurements were effective in detecting corrosion behavior of steels. CTV of steels was 0.5–0.6 mol/L in simulated concrete pore solution contaminated by chloride ions while threshold value of steels submitted to sulphate ions was 0.2–0.3 mol/L. The concomitant presence of chloride and sulphate ions led to higher corrosion current density which indicated sulphate ions accelerated the corrosion of reinforcing steels in simulated concrete pore solution.
89 citations
TL;DR: In this paper, a low-temperature longitudinal guarded comparative calorimeter (LGCC) was developed to investigate the influence of deicing salt on freezing and thawing temperatures of pore solution and corresponding damage of mortar specimens.
Abstract: Deicing salts are often applied to the surface of pavements and bridge decks in the winter to melt ice, thereby improving safety for the traveling public. In this paper, the influence of NaCl deicing salt on freezing and thawing temperatures of pore solution and corresponding damage of mortar specimens were investigated. A low-temperature longitudinal guarded comparative calorimeter (LGCC) was developed to cool down a mortar sample at a rate of 2°C/h and to re-heat the mortar at a rate of 4°C/h. Heat flux during freezing and thawing cycles was monitored, and the temperatures at which freezing and thawing events occurred were detected. During cooling and heating, acoustic emission (AE) activity was measured to quantify the damage (cracking) caused by aggregate/paste thermal mismatch and/or phase changes. The results show that NaCl solution in a mortar sample freezes at a lower temperature than the value expected from its bulk phase diagram because of under-cooling. Conversely, the frozen solution in mortar melts at the same melting temperature as the bulk frozen NaCl solution. As the salt concentration increases, the freezing temperature is lowered. For samples containing more highly concentrated solutions, an additional exothermic event is observed whose corresponding temperature is greater than the aqueous NaCl liquidus line in the phase diagram. Damage also begins to occur at this temperature. For mortar samples saturated by solutions with 5 % and 15 % NaCl by mass, greater freeze/thaw damage is observed. The AE calorimeter developed herein is applicable for investigating damage behavior during freezing and thawing of different phases in pore solution (in mortars).
87 citations
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TL;DR: In this paper, a thermodynamic study of the liquid-solid phase transformations in porous materials provides the relationships between the size of the pores in which solidification takes place and the temperature of the triple point of the divided liquid, on the one hand, and between this temperature and the apparent solidification energy on the other hand.
Abstract: A thermodynamic study of the liquid—solid phase transformations in porous materials provides the relationships between the size of the pores in which solidification takes place and the temperature of the triple point of the divided liquid, on the one hand, and between this temperature and the apparent solidification energy on the other hand.
The experimental study of the phase transformations, carried out by means of a microcalorimeter, gives the values of the parameters necessary to calculate the free solid = liquid interphase extension energy γls at different temperatures. A formula γls f(T) is given for water and benzene. Once this factor is known, it is possible to study the numerical relationship between pore-radius and freezing energy at the equilibrium temperature.
By using these relations together with the solidification thermogram (the recording of the power evolved by the solidification of a capillary condensate during a linear decrease of temperature) the authors have been able to determine pore distribution curves. An emphasis is put on the comparison between this method, thermoporometry, and the B.J.H. method.
Last of all the comparison of the experimental data for solidification and melting provide information concerning pore shape by means of the evaluation of a thermodynamic shape factor or by a method of simulation of porous material.
703 citations
TL;DR: In this article, the freezing temperature of room temperature cured mature hardened Portland cement paste with different moisture contents has been measured, and the measurements were carried out continuoslly in the temperature range from +20° C to −60° C.
Abstract: Ice formation in room temperature cured mature hardened Portland cement paste with different moisture contents have been measured. The measurements were carried out continuoslly in the temperature range from +20° C to −60° C. For moisture contents higher than that corresponding to approximately three equivalent BET-monolayers, the initial freezing temperature increases with increasing moisture contents. Moisture contents below approximately three monolayers is not freezable. For specimens containing freezable water, there is an increase in the amount of non-frozen water content with increasing moisture contents. Further, there is a slight increase in the amount of non-frozen water with increasing water/cement ratios for specimens in equilibrium with the same relative water vapor pressure. It is postulated that frost problems only can be expected in specimens containing more water than corresponding to a relative water vapor pressure of about 0.9.
93 citations
TL;DR: In this article, changes in the dimensions and heat content of hydrated cement specimens were determined as a function of temperature and concentration of deicing agent in cooling-warming cycles between plus 15 and minus 70 C.
Abstract: Changes in the dimensions and heat content of hydrated cement specimens were determined as a function of temperature and concentration of deicing agent in cooling-warming cycles between plus 15 and minus 70 C. The concentration of the polar deicer (NaCl) solution varied from 0 to 26% and that of the nonpolar (urea) solution from 0 to 40%. The w/c ratios were 0.4, 0.6, and 0.8 plain and 0.5 air-entrained. Experiments were also conducted to clarify the effect of cooling rate and sample size. The observations can be explained by the mechanism previously proposed for phase transitions of absorbates. In the presence of salts, freezing and melting of liquid exuded from the pores on cooling proceed according to the bulk phase diagram, producing double peaks in the thermograms except at extreme concentrations. The detrimental effect of deices is attributed mainly to the high degree of saturation, a consequence of the low vapor pressure of the solutions. A beneficial aspect is the widening of the temperature range in which transitions occur. These opposing effects result in the worst conditions at a low deicer concentration (5% NaCl) and optimum conditions at a moderately high concentration (13% NaCl). Since the effect of deicers is physical, it should be common to all chemicals. Air entrainment, although beneficial in most circumstances, can be detrimental. The best protection against "salt scaling" appears to be reduction of porosity./AUTHOR/
67 citations
TL;DR: In this article, the specific surface of hardened cement paste at relative humidities (R.H.) between 0 and 100% has been investigated by small-angle X-ray scattering (SAXS).
Abstract: The specific surface of hardened cement paste (hcp) at relative humidities (R.H.) between 0 and 100% has been investigated by small-angle X-ray scattering (SAXS). On desorption from 100 to 40% R.H., a reversible 35% decrease in measured specific surface occurs. This is interpreted as the action of the disjoining pressure in small gel pores. Measurements on samples subjected to chloride action in aqueous NaCl solutions showed little change below 60% R.H.. At higher humidities the specific surface increases uniformly with NaCl concentration.
55 citations
TL;DR: In this paper, the freezing phenomena in hardened cement paste (hcp) were investigated by DTA and at least two phase transitions were observed in the temperature range of -10°C and -25°C, and near -43°C respectively.
Abstract: Freezing phenomena in hardened cement paste (hcp) were investigated by DTA. On cooling at least two phase transitions have been observed in the temperature range betwenn -10°C and -25°C and near -43°C respectively. Also a hysteresis between cooling and heating has been found. The phase transitions are correlated to the radius of the water-filled pores and so to relative humidity and water content. Comparable results were obtained in different kinds of hcp. These findings can be explained by the specific properties of water in porous systems induced by surface interaction.
25 citations