• Fire Science and Engineering
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• v.23 no.3
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• pp.103-109
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• 2009

The thermal degradation of concrete results mainly from two mechanisms. The first one is related to phase transformations of constituents at different temperatures. The initial constituents transform to other phases due to elevated temperature. The second mechanism is related to the temperature sensitivity of the mechanical properties of the constituents in concrete. Therefore, the degradation of concrete under high temperatures must be studied from both mechanical and chemical points of view. This study was performed as a basic study to propose the material models of concrete exposed to high temperatures considering above two mechanisms. This study presents a prediction model on the porosity of hardened cement paste considering phase changes according to temperature increase.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2019.05a
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• pp.120-121
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• 2019

In this study, FT-IR analysis was performed by exposing cement paste to high temperature in order to characterize the change of hydration behavior of concrete structure damaged by fire accident. As the holding time increased, the Al-O vibration region increased due to the increase of Si-O symmetric Ca₂(SiO₄) and Brownilerite, and the OH stretching region tended to increase due to thermal decomposition of Ca(OH)₂.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2018.11a
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• pp.169-170
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• 2018

In this study, TGA analysis of hardened cement paste with fire damage was performed. The mass reduction rate of 600 ℃ specimens was about 22 ~ 25%, and the sample of 800 ℃ showed the mass loss rate of 9 ~ 13%. As the target temperature and hold time increased, the mass reduction rate decreased. As the depth increased, the mass reduction rate decreased.

• Journal of the Korea Concrete Institute
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• v.17 no.6 s.90
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• pp.1019-1024
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• 2005

To research effects of various chemical superplasticizers(Lignosulfonic acid, Naphthalene sulfonated formaldehyde condensate, melamine sulfonated formaldehyde condensate, and Polycarboxylate) on the hydration of cement, experiments involving XRD, SEM, and DSC have been analysed with cement paste specimens. Regardless of types and dosages of superplasticizers, hydration reaction of specimen applied superplasticizer was delayed to 3 day, but then it showed similarity to plain which don't add superplasticizer. Moreover, the hydrating rate of cement paste was retarded as dosage of superplasticizer was increased. Also, kinetics related with hydrate of cement paste was slow in order of lignosulfonic acid, polycarboxylate, melamine and naphthalene sulfonated formaldehyde condensate. Nevertheless, when all kinds of chemical admixtures were used, morphologies of these hydrates were denser and more uniform than those of plain.

• Magazine of the Korea Concrete Institute
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• v.7 no.6
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• pp.209-215
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• 1995

Corrosion of steel reinforcment is the most significant factor of deterioration in reinforced concrete structures. Chloride ion is considered one of the most common culprits in the corrosion of steel in concrete. It breaks down the passive film and allows the steel to corrode actively at a high rate. The main objective of this study is to determine the critical chloride ion concentrations in the pore solutions and chloride binding effect of cement pastes made with and without fly ash. Cement pastes with water-binder ratio of 0.5, allowed to hydrate in sealed containers for 28 days and to express pore solution. The expressed pore fluids were analyzed for chloride and hydroxyl ion concentrations. Evaporable water on paralled specimens was determined a.s the loss of weight per 100g of unhydreded cement when the specimens were heated to constant weight at 105'C. It was found that the replaced cement with fly ash has negligible influnce on the chloride binding and chloride binding capacity and rises the $Cl^-$ /$OH^-$ ratio in pore solution.

• Resources Recycling
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• v.3 no.3
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• pp.27-31
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• 1994

We tested the limestone sludge produced in Pohang Iron & Steel Co., Ltd. as a filler powder for the effective use of portland cement. Hydration process was investigated by measuring the hydration rate, the amounts of non-evaporable water and compressive strength of cement-limestone sludge paste prepared by mixing limes-tone sludge with cement. The results obtained in this study can be summarized as follows: 1. There is no significant difference between the cases of adding up to 10% limestone sludge and those of unmixed cement system. However the reaction rate increases in the 5% limestone sludge system(due to the effects of fine). 2. The compressive strength increases proportionally with increasing the measured amount of non-evaporable water, Adding 5% limestone sludge also increases the strength a little higher, and the compressive strength and calcium silicate hydrates. In the case of the mixed limestone sludge, $2\theta$=$11.7^{\circ}$ peak appears in the samples of 28 days hydration. This peak indicted the presence of calcium carboaluminate hydrate. Although limestone sludge is generally regarded as a inert materials, some kinds of cement can produce a calcium carboaluminate by reacting with aluminate in cement pastes.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.36 no.2
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• pp.207-217
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• 2016

Permeability can not be expressed as a function of porosity alone, it depends on the porosity, pore size and distribution, and tortuosity of pore channels in concrete. There has been considerable interest in the relationship between microstructure and transport in cementitious materials, however, it is very rare to deal with the theoretical study on gas permeability coefficient in connection with carbonation of concrete and the effect of volumetric fraction of cement paste or aggregate on the permeability coefficient. The majority of these researches have not dealt with this issue combined with carbonation of concrete, although carbonation can significantly impact on the permeability coefficient of concrete. In this study, fundamental approach to compute gas permeability of (non)carbonated concrete is suggested. For several compositions of cement pastes, the gas permeability coefficient was calculated with the analytical formulation, followed by a microstructure-based model. For carbonated concrete, reduced porosity was calculated and this was used for calculating the gas permeability coefficeint. As the result of calculation of gas permeability for carbonated concrete, carbonation leaded to the significant reduction of gas permeability coefficient and this was obvious for concrete with high w/c ratio. Meanwhile, the relationship between gas permeability and water permeability has a linear function for cement paste based on Klinkenberg effect, however, which is not effective for concrete. For the evidence of the modeling, YOON's test was accomplished and these results were compared to each other.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2021.11a
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• pp.168-169
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• 2021

A hydrophobic surface increases the contact angle between water and cement paste. There are two methods to increase water contact angle, i.e. lowering the surface energy and adjusting the surface roughness of concrete. The hydrophobicity of concrete can be quantitatively evaluated according to the chemical and physical properties of the solid surface. So far, researches have shown the chemical properties of hydrophobic concrete, however it has not covered how to control surface. This study demonstrated the hydrophobic cement paste prepared by low-resolution molds printed with a 3D printer that exhibit rough surface. Thus, we presented the most hydrophobic characteristics of mold.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2023.11a
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• pp.125-126
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• 2023

This work explores the effectiveness of graphene nanoribbons (GNRs) in modifying the fire resistance of cement paste. The GNRs are added to the ordinary Portland cement at 0.10 wt% of the cement, and the sample is heated to target temperatures after curing for 28 days. Subsequently, the variations of compressive strength and pore structure are inquired by compared to the control sample without nano reinforcing and the sample with the same amount of carbon nanotubes (CNTs).

• Journal of the Korea Concrete Institute
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• v.20 no.1
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• pp.109-116
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• 2008

This study was performed to get information of self-healing mechanism in cement system and the influence of waterproofing admixtures for concrete on self-healing property. For testing, cement paste specimens cured for 7, 14 and 28 days were prepared and crushed into plate-shape pieces. Screened specimens with thickness not more than 1mm were covered with wet rags and cured in a plastic container for 7 and 28 days. After stopping hydration process of the specimens by treatment with acetone, the surfaces of specimens have been contacted with wet rags were analyzed by XRD, DSC, SEM and EDX. The analysis results showed that cement paste has self-healing property and this property is mainly affected by water. Self-healing in cement system is more effective and faster at an early stage of hydration as there is enough content of unreacted cement to make an additional hydration in this period. The results of this study also showed that waterproofing admixtures for concrete have a considerable effect on self-healing of cement pastes; i.e., they improved self-healing effect of cement and, especially, the specimen using admixture C has shown a lot of needle-like or fibrous hydration products which are estimated as ettringite. It is supposed that these ettringite products are effective to enhance self-healing in cement system.