• Computers and Concrete
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• v.13 no.3
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• pp.309-323
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• 2014

Based on the compositions and structures of cement-based materials, the geometrical models of the tortuosity of transport paths in hardened cement pastes, mortar and concrete, which are associated with the capillary porosity, cement hydration degree, mixture particle shape, aggregate volume fraction and water-cement ratio, are established by using a geometric approach. Numerical simulations are carried out to investigate the effects of material parameters such as water-cement ratio, volume fraction of the mixtures, shape and size of aggregates and cement hydration degree, on the tortuosity of transport paths in hardened cement pastes, mortar and concrete. Results indicate that the transport tortuosity in cement-based materials decreases with the increasing of water-cement ratio, and increases with the cement hydration degree, the volume fraction of cement and aggregate, the shape factor and diameter of aggregates, and the material parameters related to cement pastes, such as the water-cement ratio, cement hydration degree and cement volume fraction, are the primary factors that influence the transport tortuosity of cement-based materials.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2023.05a
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• pp.151-152
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• 2023

This study investigates the effect of nano-silica and nano-calcite on the hydration properties and mechanical performance of limestone calcined clay cement (LC³) paste. The pastes were synthesized by replacing limestone with nano-silica and nano-calcite in order to enhance the mechanical properties in both early and late stages of hydration. The nano-calcite enhanced the strength of LC³ pastes at 1 day of hydration, however, the strength decreased compared to the ordinary LC³ pastes afterwards due to excessive amount of carboaluminate produced in the pastes. On the other hand, nano-silica improved the mechanical properties of LC³ pastes at all ages of hydration. This is mainly due to the nucleation effect and pozzolanic reaction of nano-silica, affecting the early age and late ages of hydration, respectively. The nucleation effect of both nanomaterials were confirmed by the analysis of hydration heat, supporting the enhanced early age strength of nanomaterial incorporated LC³ pastes. Furthermore, the dense matrix was shown in the pore size distribution, and the increased C-S-H due to the pozzolanic reaction evidence the improved compressive and splitting tensile strength of nano-silica incorporated LC³ pastes.

• Journal of the Korean Ceramic Society
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• v.42 no.11 s.282
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• pp.737-742
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• 2005

Rheological properties of ordinary portland cement (OPC) containing metakaoline (MK), granulated blast furnace slag (GBS) and polycarboxylate type superplasticizer (PCA) were investigated using a mini-slump test, sedimentation test and viscometer. Fluidity of cement pastes containing MK (OPC-MK, OPC-MK-GBS systems) with PCA were higher than those of the cement pastes without MK(OPC, OPC-GBS systems). Colloid suspensions with $0.1\%$ PCA were changed from stable sedimentation behaviors to flocculation behaviors in the OPC-MK, OPC-GBS and OPC-MK-GBS systems. The colloid suspensions showed stable sedimentation behaviors with PCA greater than $0.2\%$. The OPC system showed shear thinning behavior. However, the other systems showed weak shear thinning behaviors with PCA. Rheological properties of cement pastes were improved when MK and GBS were contained together. The rheological properties of OPC-MK, OPC-GBS and OPC-MK-GBS systems were improved by PCA added greater than $0.2\%$.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.40 no.2
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• pp.177-182
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• 2020

In this study, the effect of nanofibers in cement pastes on the compressive and tensile strength of hardened cement pastes was studied. Two types of nanofibers, nylon 66 nanofibers and carbon nanotube-nylon 66 hybrid nanofibers, were manufactured by electrospinning methodology and mixed in cement powder respectively. The specimens for experiments were prepared by water to cement ratio of 0.5 and cured in water for 28 days. The effect of nanofibers on the increase of the compressive and tensile strength were confirmed by the experimental results. The well-linking effect of nanofibers in the microstructure of the hardened cement pastes has been found by scanning electron microscope (SEM) analysis and well-explained for the increase in mechanical strength. Besides, field emission transmission electron microscope (FE-TEM) analysis and thermal gravimetric analysis (TGA) have also been conducted to analyze the properties of nanofibers as well as the microstructure of the hardened modified cement pastes.

• International Journal of Concrete Structures and Materials
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• v.11 no.1
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• pp.59-68
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• 2017

Mercury intrusion and nitrogen sorption porosimetry were employed to investigate the pore structure of calcium sulfoaluminate ($C{\bar{S}}A$) and portland cement pastes with cement-to-water ratio (w/c) of 0.40, 0.50, and 0.60. A unimodal distribution of pore size was drawn for $C{\bar{S}}A$ cement pastes, whereas a bimodal distribution was established for the portland cement pastes through analysis of mercury intrusion porosimetry. For the experimental results generated by nitrogen sorption porosimetry, the $C{\bar{S}}A$ cement pastes have a smaller and coarser pore volume than cement paste samples under the same w/c condition. The relative dynamic modulus and percentage weight loss were used for investigation of the concrete durability in freeze-thaw condition. When coarse aggregate with good freeze-thaw durability was mixed, air entrained portland cement concrete has the same durability in terms of relative dynamic modulus as $C{\bar{S}}A$ cement concrete in a freeze-thaw environment. The $C{\bar{S}}A$ cement concrete with poor performance of durability in a freeze-thaw environment demonstrates the improved durability by 300 % over portland cement concrete. The $C{\bar{S}}A$ concrete with good performance aggregate also exhibits less surface scaling in a freeze-thaw environment, losing 11 % less mass after 297 cycles.

• Proceedings of the Korea Concrete Institute Conference
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• 2003.05a
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• pp.727-730
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• 2003

The objective of this work is to understand the deterioration mode of ordinary portland cement pastes and mortars immersed in 5% sodium sulfate solution for 510 days. In order to achieve the goal, x-ray diffraction (XRD) and scanning electron microscopy (SEM) are presented in this experimental work. Strength deterioration (SDF) and length change of the mortars were also measured to evaluate resistances to the attacking solution. The mortars were prepared by using water-cement ratio of 35%, 45% and 55%, respectively, and the water-cement ratio of pastes was fixed at 45%. Conclusively, the deterioration by sodium sulfate attack was primarily due to the formation of ettringite and thaumasite. This process of deterioration may submit the reasonable understanding on the sulfate attack mechanism of hardened cement pastes, mortars, and concretes.

• Journal of the Korean Ceramic Society
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• v.40 no.3
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• pp.219-224
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• 2003

Fly ashes containing 6.1~16.5 wt% of unburned carbon were treated thermally at 500$^{\circ}C$ for 3 h and thus, the content of unburned carbon was decreased below 2.1 wt%, the range of particle size distribution became narrower and the mean particle size became smaller. Besides, the properties of particles in fly ashes were improved, particularly the particle shape became close to a spherical type. The fluidity of cement pastes containing fly ashes treated previously at 500$^{\circ}C$ for 3 h was increased much than that of cement pastes containing original fly ashes. When the added amount of superplasticizer was over the saturation amount, there was no correlation between the amount of unburned carbon in fly ashes and the apparent viscosity of cement pastes actually. On the contrary, when the added amount of superplasticizer was below the saturation amount, there was a correlation.

• Journal of the Korea Institute of Building Construction
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• v.18 no.1
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• pp.51-57
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• 2018

The objective of the present study is to examine the feasibility on the development of high-insulation concrete using aerogels with hydrophilic surface treatment. To prevent the segregation and enhance the dispersibility of agerogels in the cement pastes, the substrate of aerogels was modified to be hydrophobic property using surfactant. The modified aerogels were added from 0% to 100% of the cement volume at the interval of 25% under the constant cement content. Some cement pastes showed segregation phenomenon and flocculation of aerogels during mixing phase. The addition of aerogels decreased the compressive strength of cement pastes but enhanced the thermal conductivity. The thermal conductivity of pastes with 100% aerogels was lower by 43% when compared with that measured in the conventional paste. To improve the compressive strength and insulation capacity of concrete containing aerogels, a reliable surface treatment method of aerogels needs to be further investigated.

• Journal of the Korean Recycled Construction Resources Institute
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• v.10 no.4
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• pp.489-497
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• 2022

In this study, the fluidity, mechanical properties and microstructure of cement pastes with carbon nanotube (CNT) were experimentally investigated. The 6 types of cement paste mixes with different PCE:CNT and w/b had been manufactured, and several tests including flow, compressive strength, absorption and water porosity were performed on cement pastes with or without CNT.Additionally, microstructural observations such as x-ray diffraction (XRD) and scanning electron microscopy (SEM) were carried out to examine hydrates formed in cement paste with CNT. As a result, it was found that the performance of cement pastes with CNT was better compared to that of control cement paste (OPC) due to both of hydration acceleration effect and filling effect. Furthermore, the SEM images clearly showed that CNT can bridge cracks formed in cement matrix. Conclusively, it is believed that the CNT, if mixed appropriately, could be an option as nono-materials to improve performance of concrete structures.

• Proceedings of the Korea Concrete Institute Conference
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• 1995.04a
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• pp.106-110
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• 1995

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-ratio of 0.5 allowed to hydrate in sealed containers for 28 days and to express poresolution. T도 expressed pore fluids were analyzed for chloride and hydroxyl ion concentrations. It was found that the replaced cement with fly ash have little effect on Chloride binding capacity ratio.