• Proceedings of the Korean Institute of Building Construction Conference
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• 2020.11a
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• pp.151-152
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• 2020

When concrete is exposed to fire, the thermal decomposition of hydrates of Portland cement paste results in critical damage to the concrete structure of a building. Recently, nanosilica arose as the effective nano-additive which can enhance the thermal resistance of the cementitious materials. However, the mechanism of the enhancement was not elucidated specifically. In this study, we investigated the properties of calcium silicate hydrates(C-S-H)of the nanosilica incorporated cement paste after heating to different heating temperatures (200℃, 500℃, and 800℃) by 29Si nuclear magnetic resonance. The results showed that the polymerization of C-S-H of nanosilica incorporated samples was larger than ordinary cement paste after heating to 200℃, and C-S-H formed during heating process to 500℃ due to the pozzolanic reaction during heating process.

• Journal of the korean academy of Pediatric Dentistry
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• v.49 no.4
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• pp.379-391
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• 2022

The current study aimed to compare the pH, solubility value, and ion release capability of premixed mineral trioxide aggregates (MTAs) versus conventional pulp capping materials before and after setting. The following materials were used: resin-modified calcium silicate cement (TheraCal LC^®, TLC), resin-modified calcium hydroxide cement (Ultra-Blend^TM plus, UBP), and 2 kinds of premixed MTA (Endocem MTA^® premixed regular [EMPR] and Well-Root^TM PT [WRP]). The specimens of each material were prepared before and after setting and were immersed in distilled water. The materials' pH and solubility value were assessed. Next, three kinds of ion (calcium, sulfide, and strontium) released by pulp capping materials were evaluated via inductively coupled plasma atomic emission spectrometry. In the after-setting group, the pH of TLC and UBP decreased. However, the pH of the premixed MTAs increased with time. TLC released a higher concentration of strontium ion compared with the other materials. Meanwhile, EMPR released a significantly high concentration of sulfide ion (p < 0.05). In the after-setting group, the 2 kinds of premixed MTAs released a significantly higher concentration of calcium ion compared with the other materials (p < 0.05). In the after-setting group, EMPR had a significantly low solubility value (p < 0.05). The Kruskal-Wallis test, followed by the Mann-Whitney U test with Bonferroni correction, was used in statistical analysis. In conclusion, resin-modified calcium silicate cement, modified calcium hydroxide cement, and the 2 kinds of premixed MTAs had an alkaline pH and low solubility value and they released various concentrations of ions after setting.

• Journal of the korean academy of Pediatric Dentistry
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• v.50 no.3
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• pp.277-291
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• 2023

This study aimed to compare the effects of applying different types of dentin bonding agents (DBAs) on tooth and on the sealing ability of CSC in regenerative endodontic procedures (REPs). Specimens were made from 120 human first premolars and categorized into the following groups based on the presence and the type of DBA: Group I, no bonding; Group II, One-Step; Group III, Clearfil SE Bond; and Group IV, Single Bond Universal. Subsequently, the groups were categorized into Subgroup a and Subgroup b, in which Endocem MTA^® Premixed Regular and Biodentine were used, respectively. The CIE L^*a^*b^* on the photos taken prior to TAP placement (S0), 1 week after TAP placement (S1), 2 weeks after TAP placement (S2), 4 weeks after TAP placement (S3) and 2 weeks after CSC placement (S4) was used to assess discoloration. The samples were submerged in a 2% methylene blue solution and magnified by 30 times under a stereomicroscope to assess microleakage. The application of DBA during REP reduced crown discoloration, while Biodentine showed minimal discoloration with or without DBA and regardless of the DBA type. Additionally, the application of DBA did not increase microleakage.

• Journal of the Korean Recycled Construction Resources Institute
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• v.9 no.4
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• pp.443-450
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• 2021

The effects of blast furnace slag(BFS) and desulfurized gypsum(FDG) on the compressive strength of CFBA, and self-hydration of CFBA were studied. CFBA has self-hydrating and hardening properties, and it can be seen that the compressive strength of CFBA can be improved by using appropriate amounts of BFS and FDG. In addition, the self-hardening properties of CFBA are similar to the hydration reaction of 4CaO·Al₂O₃·Fe₂O₃ (C₄AF), a cement clinker mineral, and when free-CaO, CaSO₄ and CaCO₃ coexist, Compressive strength of CFBA is expressed by the formation of calcium carbo compounds and hydrates of ettringite, calcium silicate, and calcium aluminate.

• Journal of the Korean Recycled Construction Resources Institute
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• v.11 no.2
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• pp.153-160
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• 2023

In this study, nano-sized C-S-H crystals were synthesized using the liquid phase reaction method and their properties were investigated. The synthesized C-S-H crystals were added to the cement composite in suspension form to determine their effect on the hydration properties of the cement. The amount of chemical admixture was varied to obtain nano-sized C-S-H crystals with optimal agglomerated morphology, and SEM photographs were analyzed. A cleaning process was added to remove harmful substances other than the synthesiz ed C-S-H crystals. It was found that the concentration of harmful substances was reduced in the case of C-S-H crystals subjected to the cleaning process. The synthesized C-S-H suspensions were prepared with and without the cleaning process, and cement composites were prepared with the cement weight content as the main variable. The effect of C-S-H crystals on the initial hydration properties of the cement was confirmed by microhydration heat analysis. In addition, mortar specimens were prepared to measure the compressive strength over time. The test results showed that the nano-sized C-S-H crystals act as nucleation sites in the cement paste to promote the early hydration of the cement and increase the early compressive strength.

• Resources Recycling
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• v.31 no.6
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• pp.52-59
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• 2022

Calcium silicate based cement (CSC) is a low-carbon cement that emits less CO₂ by up to 70% compared to ordinary Portland cement during its manufacture. Most developed countries have commercialized CSC, whereas Korea is still investigating the manufacturing characteristics and basic properties of CSC. This paper provides a review of methods for manufacturing CSC using domestic raw materials and discusses the possibility of CSC localization based on an evaluation of the basic physical properties of manufactured CSC. The experimental results of this study indicate that the primary mineral components of CSC were CS, C₃S₂ C₂S, and unreacted SiO₂. This suggests the possibility of manufacturing CSC using domestic raw materials that exhibit mineral compositions similar to that of theoretical CSC. The compressive strength of CSC mortar is less than 1MPa at the age of 7 d under wet curing. This implies that hydration does not affect the property development of CSC mortar. Meanwhile, during carbonation curing, the compressive strength is 56 MPa or higher after 7 d, which indicates excellent early strength development. Furthermore, results of Thermogravimetric Analysis Differential scanning calorimetry (TG/DSC) show that a significant amount of CaCO₃ is formed, which is consistent with the results of previous studies. This implies that carbonation is associated significantly with the properties of CSC.

• Polymer(Korea)
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• v.36 no.2
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• pp.208-215
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• 2012

To study the effect of surface crosslinked layer on the crosslinked poly(sodium acrylate) (cPSA) absorbent, we synthesized several surface crosslinked cPSAs with 5, 10 and 20 g of ethylene glycol dimethacrylate (EGDMA) by an inverse emulsion polymerization method to delay the absorption of excess water in concrete. We measured the compressive and flexural strength of mortars having 0.5, 1.0 and 1.5 wt% cPSA-EGDMA. We observed the increase of compressive and flexural strength of the cPSA-EGDMA added cement mortars except for the 0.5 wt% cPSA-EGDMA (20 g) added cement mortar. 1.0 wt% cPSA-EGDMA (5 g) added cement mortar showed about 16% and 10% increased compressive and flexural strength than those of plain cement mortar. To study the effect of porosity on compressive and flexural strength, we used FE-SEM and porosimeter. FE-SEM analysis showed swollen cPSMAEGDMA (5 g) filled between calcium silicate hydrate (C-S-H) crystals. We observed the decreased porosity of the cPSA-EGDMA added cement mortars than that of plain cement mortar. 1.0 wt% cPSA-EGDMA (5 g) cement mortar showed the lowest porosity of 16.5%.

• Journal of the Korea Concrete Institute
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• v.18 no.4 s.94
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• pp.449-458
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• 2006

The discharge of fly ash that is produced by coal-fired electric power plants is rapidly increasing in Korea. The utilization of fly ash in the raw materials would contribute to the elimination of an environmental problem and to the development of new high-performance materials. So it is needed to study the binder obtained by chemically activation of pozzolanic materials by means of a substitute for the cement. Fly ash consists of a glass phase. As it is produced from high temperature, it is a chemically stable material. Fly ash mostly consists of $SiO_2\;and\;Al_2O_3$, and it assumes the form of an oxide in the inside of fly ash. Because this reaction has not broken out by itself, it is need to supply it with additional $OH^-$ through alkali activators. Alkali activators were used for supplying it with additional $OH^-$. This paper concentrated on the strength development according to the kind of chemical activators, the curing temperature, the heat curing time. Also, according to scanning electron microscopy and X-Ray diffraction, the main reaction product in the alkali activated fly ash mortar is Zeolite of $Na_6-(AlO_2)_6-(SiO_2)_{10}-12H_2O$ type.

• Resources Recycling
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• v.18 no.3
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• pp.42-48
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• 2009

Since there are $OH^-,\;[SiO_4]^{4-}$ ion of high concentration at early hydration in the system added with activator (NaOH+$Na_2OSiO_2$) in the blast furnace slag, different from cement hydration, hydration progresses fast without induction period and forms reaction rim around the blast furnace slag grain. $0.6{\mu}m$ reaction rim was formed around the blast furnace slag grain from the 1 day of reaction period, and the thickness of reaction rim increases over the reaction time, growing to $1{\mu}m$ on the 28 days. Unreacted blast furnace slag grain deformed from angular shape to the spherical shape. Mole ratio of Ca/Si tends to decrease from inside of blast furnace slag grain to reaction rim. Difference of Ca/Si mole ratio between reaction rim and inside the blast furnace slag grain decreased and generated hydrate was a poor crystalline CSH(I) with Ca/Si mole ratio less than 1.5.

• Journal of the Korea institute for structural maintenance and inspection
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• v.19 no.5
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• pp.104-111
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• 2015

This study shows the mechanical properties of alkali-activated slag cement (AASC) synthesized using sulfate with NaOH solution. The used sulfates were calcium sulfate ($CaSO_4$, denoted CS) and sodium sulfate ($Na_2SO_4$, denoted SS). The replacement ratio of sulfates was 2.5, 5.0, 7.5 and 10.0% by weight of slag. NaOH solution of 2M and 4M concentration was used. A sample was activated with sulfate and activated with blended activator (blending NaOH solution with sulfate) respectively. 24 mix ratios were used and the water-binder weight ratio for the test was set 0.5. This research carried out the compressive strength, flexural strength, ultrasonic pulse velocity (UPV), absorption and X-ray diffraction (XRD). In the case of samples with CS, sample with 7.5% CS, sample with 2M NaOH+5.0% CS and sample with 4M NaOH+5.0% CS showed the good performance in the strength development. In the case of samples with SS, sample with 10.0% SS, sample with 2M NaOH+7.5% SS and sample with 4M NaOH+2.5% SS obtained good performance in strength. The results of UPV and water absorption showed a similar tendency to the strength properties. The XRD analysis of samples indicated that the hydration products formed in samples were ettringite, CSH and silicate phases. In this study, it is indicated that when compared to the use of sulfate only, the use of both sulfate and NaOH solution makes mechanical properties of AASC better.