• Magazine of the Korea Concrete Institute
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• v.11 no.2
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• pp.85-94
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• 1999

It was investigated to evaluate the characteristics of cement-flyash paste which was affected the replacement level, curing method and chemical admixtures. The strength of cement-flyash paste was lower than that of cement paste only and the differences increased with increasing the replacement level. However, in steam curing, the strength of cement-flyash pastes was improved and specially, the early strength was effectively increased. The inclusion of $Na_2SO_4$ increased the early strength of cement-flyash paste. In addition, the strength of concrete including 30% of fly ash and $Na_2SO_4$ has improved and obtained the highest strength compared to other concrete mixes.

• 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.

• Journal of the Korea Concrete Institute
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• v.13 no.2
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• pp.175-183
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• 2001

Immersion tests with artificial seawater were carried out to investigate the resistance to seawater attack of 5 types of cement matrices. From the results of compressive strength and length change, it was found that blended cement mortars due to mineral admixtures, were superior to portland cement mortars with respect to the resistance to seawater attack. Moreover, XRD analysis indicated that the peak intensity ratio of low heat portland cement(LHC) paste, in portland cement pastes, had better results, and so did that of blended cement Paste. Pore volume of pastes by mercury intrusion porosimetry method demonstrated that total pore volume of ordinary portland cement(OPC) paste had a remarkable increase comparing with that of other pastes. In case of immersion of artificial seawater, the use of ground granulated blast-furnace slag and fly ash, however, showed the beneficial effects of 56% and 32% in reduction of total pore volume, respectively.

• 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 Korea Institute of Building Construction
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• v.22 no.5
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• pp.431-439
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• 2022

In this study, Neutralized red mud(LRM+S) at a pH of 6-8 was prepared by adding sulfuric acid to liquefied red mud(LRM) at a pH of 10~12. After adding LRM and LRM+S to the cement paste, the hydration heat, compressive strength, and hydration products were examined. The observed accumulated hydration heat revealed that the calorific value of the cement paste with LRM was low and its and peak was delayed when compared with that of plain cement paste(referred to as Plain), whereas the calorific value of the cement paste with LRM+S was similar to that of Plain. At the age of 28 days, the compressive strength of the cement paste with 20% LRM was 55% of the strength of Plain. Using X-ray diffraction, it was determined that the cement paste with 20% LRM exhibited a Ca(OH)₂ peak after 3 days, whereas the cement paste with 20% LRM+S and Plain exhibited a Ca(OH)₂ peak after an hour. Thus, the strength degradation of cementitious materials was improved by adding neutralized red mud prepared by adjusting the pH of highly alkaline LRM with sulfuric acid.

• Journal of the Korea institute for structural maintenance and inspection
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• v.21 no.5
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• pp.141-147
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• 2017

Recently, lots of researches on concrete with high volume mineral admixtures such as ground granulated blast-furnace slag(GGBS) have been carried out to reduce greenhouse gas. The high volume GGBS concrete has advantages such as low heat, high durability, but it has a limitation in practical field application, especially low strength development in early ages. This study investigated the compressive strength and hydration characteristics of high performanc and volume GGBS cement pastes with low water to binder ratio. The effects of fineness($4,330cm^2/g$, $5,320cm^2/g$, $6,450cm^2/g$, $7650cm^2/g$) and replacement(35%, 50%, 65%, 80%) of GGBS on the compressive strength, setting and heat of hydration were analyzed. Experimental results show that the combination of high volume slag cement paste with low water to binder ratio and high fineness GGBS powder can improve the compressive strength at early ages.

• Journal of the Korea institute for structural maintenance and inspection
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• v.21 no.5
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• pp.156-162
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• 2017

Recently, researches on High-Volume Fly ash Cement(HVFC), which is replacing high portion of cement to fly ash, have been actively conducted to reduce $CO_2$ formation. Though HVFC has various advantages, low strength development in early ages is pointed out as the biggest problem in the application of fly ash. In order to overcome such limitations, this study investigated the hydration and compressive strength characteristics of HVFC paste depending on the fly ash content with the mixing ratio varying from 0 to 80 %. Experimental results show that the HVFC paste with low water-binder ratio can overcome the limitation of low compressive strength at early ages. Also, from the result of heat flow delay, 50 % of fly ash weight ratio was the critical point of the filler effect.

• Journal of the Korea Institute of Building Construction
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• v.21 no.1
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• pp.31-39
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• 2021

The aim of this paper is to improve durability of cement paste by imparting hydrophobicity to the surface and sphere of cement-based materials. A cement paste mixed with a silane/siloxane-based water repellent, and the initial hydration performance, flow performance, and age-specific compressive strength were measured. In addition, the water contact angle, SEM, and XRD before and after surface polishing were measured. When 0.5% of the silane/siloxane-based water repellent was mixed into the cement paste, the compressive strength increased, but the compressive strength decreased as the mixing amount increased by 1.5% and 3.0%. When a silane/siloxane water repellent was incorporated into the cement paste, the hydrophilicity was improved and the contact angle was increased due to hydrophobicity. In addition, the contact angle after surface polishing was found to be larger than the contact angle before surface polishing.

• Journal of the Korean Recycled Construction Resources Institute
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• v.12 no.1
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• pp.33-39
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• 2024

In this study, the basic properties of cement paste with varying replacement ratio of micro-silica and fumed silica were analyzed to determine the suitability of nanomaterials for use as concrete admixtures. Referring to the ultra-high strength mix, the fluidity of cement paste was evaluated according to the nanomaterial replacement rate and the compressive strength characteristics were compared and analyzed. The related properties of the reactive nanomaterials to the cement hydrate were analyzed using SEM and EDS to observe the microstructure and identify the components of the hydration product. The reactive nanomaterials used in this study had tap densities between 0.061 and 0.264 g/cm³, which were lower than SF. Micro silica exhibited excellent compressive strength properties with increasing replacement ratio, but fumed silica, unlike micro white, obtained excellent compressive strength at replacement ratio of 0.01~0.1 %. The same trend was observed in the hydration characterization.

• Journal of the Korea Institute of Building Construction
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• v.21 no.4
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• pp.269-279
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• 2021

Palm Oil Fuel Ash(POFA) has been widely used to replace Portland cement to enhance the mechanical properties and durability of concrete. However, it reduces the workability of concrete due to the high content of unburnt carbon and its angular shape requiring the usage of superplasticizer to ensure a proper flowability. In this study, effects of different types and dosage of superplasticizer on the early mechanical and hydration properties of cement paste containing micro-POFA were evaluated using mini-slump test, early compressive strength, TGA, XRD, and SEM. The results indicated that the flowability of cement paste containing micro-POFA reduced as the replacement ratio of micro-POFA increased. As the dosage of superplasticizer increased, the flowability was also increased. In addition, the usage of superplasticizer reduced the early compressive strength, and the strength decreased with an increase in the dosage of superplasticizer. It was confirmed that superplasticizer hindered the formation of C-S-H leading to a relative increase in the formation of Ca(OH)₂.