• Coupled systems mechanics
• /
• v.6 no.3
• /
• pp.335-349
• /
• 2017

To cope with the demand on giant and durable buildings, reinforcement of concrete is a practical problem being extensively investigated in the civil engineering field. Among various reinforcing techniques, fiber-reinforced concrete (FRC) has been proven to be an effective approach. In practice, such fibers include steel fibers, polyvinyl alcohol (PVA) fibers, polyacrylonitrile (PAN) carbon fibers and asbestos fibers, with the length scale ranging from centimeters to micrometers. When advancing such technique down to the nanoscale, it is noticed that carbon nanotubes (CNTs) are stronger than other fibers and can provide a better reinforcement to concrete. In the last decade, CNT-reinforced concrete attracts a lot of attentions in research. Despite high cost of CNTs at present, the growing availability of carbon materials might push the usage of CNTs into practice in the near future, making the reinforcement technique of great potential. A review of existing research works may constitute a conclusive reference and facilitate further developments. In reference to the recent experimental works, this paper reports some key evaluations on CNT-reinforced cementitious materials, covering FRC mechanism, CNT dispersion, CNT-cement structures, mechanical properties and fire safety. Emphasis is placed on the interplay between CNTs and calcium silicate hydrate (C-S-H) at the nanoscale. The relationship between the CNTs-cement structures and the mechanical enhancement, especially at a high-temperature condition, is discussed based on molecular dynamics simulations. After concluding remarks, challenges to improve the CNTs reinforcement technique are proposed.

• Proceedings of the Korean Institute of Building Construction Conference
• /
• 2023.05a
• /
• pp.115-116
• /
• 2023

This study aims to investigate and improve the carbon dioxide sequestration capability and the mechanical properties of non-hydraulic low calcium silicate cement especially designed for CO₂ reaction and ordinary Portland cement subjected to the carbonation curing facilitating pH swing method. Nitric acid (HNO₃) was utilized as an liquid for the mixing of cement paste to enhance the initial dissolution of Ca ions from the cements by promoting low pH environment and prevent the direct precipitation of Ca with the anion, owing to the high solubility of Ca(NO₃)₂ in water. The results presented that the higher the concentration of HNO₃, the higher the compressive strength and CO₂ sequestration (until 0.1 M). Ca dissolution caused by the harsh acid attack onto the anhydrous cement particle lead to the higher carbonation reaction degree, forming abundant CaCO₃ crystals after the reaction. However, cement paste mixed with excessively high concentration of HNO₃ presented deterioration due to the too harsh pH environment and abundant NO₃- ions which are known to retard the reaction of cement.

• The Journal of the Korean dental association
• /
• v.50 no.3
• /
• pp.148-154
• /
• 2012

Mineral trioxide aggregate (MTA) is mainly composed of lime and silica. Its four major phases are tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracaclcium aluminoferrite. MTA has relatively long initial setting time (2h 45m) and various additives can be added to reduce setting time. Compressive strength of MTA increases with time and reaches 100 MPa after 28 days. MTA has high pH of 9-12.5 because of the formation of calcium hydroxide during its hydration reaction. MTA has superior sealing ability to amalgam and IRM when it is used in perforation repair or root end filling. MTA is safe in cytotoxicity and genotoxicity and have potential to promote pulpal and periapical hard tissue formation.

• Journal of the korean academy of Pediatric Dentistry
• /
• v.49 no.4
• /
• pp.379-391
• /
• 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 Ceramic Society
• /
• v.47 no.3
• /
• pp.223-231
• /
• 2010

The serious issue of tall building is to ensure the fire-resistance of high strength concrete. The fire resistant finishing method is necessarily essential in order to satisfy the fire resistance time of 3 h required by the law. The fire resistant finishing method is installed by applying a fire resistant material as a method of shotcrete or a fire resistant board to high strength concrete surface. This method can reduce the temperature increase of the reinforcement embedded in high strength concrete at high temperature due to the installation thickness control. This study is interested in identifying the effectiveness of inorganic alumino-silicate compounds including the inorganic admixture such as fly ash and meta-kaolin as the fire resistant finishing materials through the analysis of fire resistance and components properties at high temperature. The study results show that the fire resistant finishing material composed of fly ash and meta-kaolin has the thermal stability of the slight decrease of compressive strength at high temperature. These thermal stability is caused by the ceramic binding capacity induced by alkali activation reaction by the reason of the thermal analysis result not showing the decomposition of calcium hydrate. Inorganic compounds composed of fly ash and meta-kaolin is evaluated to be very effective as the fire resistance material for finishing to protect the concrete substrate by the reason of those simplicity in both application and manufacture. The additional study about the adhesion in the interface with concrete substrate is necessary for the purpose of the practical application.

• Korean Journal of Materials Research
• /
• v.15 no.2
• /
• pp.97-105
• /
• 2005

[ $11\AA$ ] tobermorite$(5CaO{\cdot}6SiO_2{\cdot}5H_2O)$ is synthesized from the mixtures of calcium hydroride and quartz using alumina in a molar ratio $Ca(OH)_2/SiO_2$ of 0.8 at $180^{\circ}C$ for 8 and 24 hrs under saturated steam pressure. The influence of alumina on the formation of $11\AA$ tobermorite was investigated by X-ray diffraction, differential thermal analysis and infrared spectroscopy. $11\AA$ tobermorite containing increasingly larger amounts of aluminum showed a shift of the basal spacing from 11.3 to $11.6\AA$. In general, there was a direct linear relation between the basal spacing and added content of alumina. The differential thermal analysis curves showed that $11\AA$ tobermorite with increasing alumina contents exhibited the exothermic peak at high temperature, namely $11\AA$ tobermorite containing aluminum gave a sharp exothermic peak at temperature around $850\~860^{\circ}C$ in the case of $S_3\~S_5$. The absorption band at $1607\~1620cm^{-1}$ is attributed to the bending vibration of water, and the position of the main O-H stretching and Si-O lattice vibration of $11\AA$ tobermorite at 3500 and $965cm^{-1}$ respectively is not altered. Consequently the existence of alumina accelerates the crystallization of $11\AA$ tobermorite, and that the aluminum ion appears to substitute for the silicon ion in $11\AA$ tobermorite structure. Al-containing tobermorite is distinguished from Al-free tobermorite.

• Journal of the Korean Recycled Construction Resources Institute
• /
• v.12 no.1
• /
• pp.72-81
• /
• 2024

In this study, DEIPA was used for enhancing cementitious performance of bottom ash replaced cement. By applying the partial or no-known crystal structure method to X-ray diffraction data, the amounts of amorphous bottom ash and calcium silicate hydrate(C-S-H) could be separated and quantified. In the sample without DEIPA, the bottom ash hardly reacted, resulting in low compressive strength. However, the addition of DEIPA not only altered the hydration behavior of the cement but also enhanced the pozzolanic reaction between bottom ash and calcium hydroxide, leading to the generation of additional C-S-H. This resulted in high compressive strength not only in the early stages but also in the later stages. Therefore, with the addition of DEIPA during the pulverization of the bottom ash, the reactivity of the bottom ash was significantly improved. Hence, there is potential in the development of bottom ash replacement cement.

• Advances in Computational Design
• /
• v.3 no.2
• /
• pp.147-163
• /
• 2018

Cementitious composites are multiphase heterogeneous materials with distinct dissimilarity in strength under compression and tension (high under compression and very low under tension). At macro scale, the phenomenon can be well-explained as the material contains physical heterogeneity and pores. But, it is interesting to note that this dissimilarity initiates at molecular level where there is no heterogeneity. In this regard, molecular dynamics based computational investigations are carried out on cement clinkers and calcium silicate hydrate (C-S-H) under tension and compression to trace out the origin of dissimilarity. In the study, effect of strain rate, size of computational volume and presence of un-structured atoms on the obtained response is also investigated. It is identified that certain type of molecular interactions and the molecular structural parameters are responsible for causing the dissimilarity in behavior. Hence, the judiciously modified or tailored molecular structure would not only be able to reduce the extent of dissimilarity, it would also be capable of incorporating the desired properties in heterogeneous composites. The findings of this study would facilitate to take step to scientifically alter the structure of cementitious composites to attain the desired mechanical properties.

• Advances in concrete construction
• /
• v.8 no.1
• /
• pp.21-31
• /
• 2019

The rich recipe of ultra high performance concrete (UHPC) offers the higher mechanical, durability and dense microstructure property. The variable like cement/sand ratio, amount of supplementary cementitious material, water/binder ratio, amount of fiber etc. alters the UHPC hardened properties to any extent. Therefore, to understand the effects of these variables on the performance of UHPC, inevitably a stage-wise development is required. In the present experimental study, the effect of sand/cement ratio, the addition of finer material (fly ash and quartz powder) and, hybrid fiber on the fresh, compressive and microstructural property of UHPC is evaluated. The experiment is conducted in three phases; the first phase evaluates the flow value and strength attainment of ingredients, the second phase evaluates the efficiency of finer materials (fly ash and quartz powder) to develop the UHPC and the third phase evaluate the effect of hybrid fiber on the flow value and strength of ultra high performance hybrid fiber reinforced concrete (UHP-HFRC). It has been seen that the addition of fly ash improves the flow value and compressive strength of UHPC as compared to quartz powder. Further, the usage of hybrid fiber in fly ash contained matrix decreases the flow value and improves the strength of the UHP-HFRC matrix. The dense interface between matrix and fiber and, a higher amount of calcium silicate hydrate (CSH) in fly ash contained UHP-HFRC is revealed by SEM and XRD respectively. The dense interface (bond between the fiber and the UHPC matrix) and the higher CSH formation are the reason for the improvement in the compressive strength of fly ash based UHP-HFRC. The differential thermal analysis (DTA/TGA) shows the similar type of mass loss pattern, however, the amount of mass loss differs in fly ash and quartz powder contained UHP-HFRC.

• Proceedings of the Korean Society of Soil and Groundwater Environment Conference
• /
• 2004.09a
• /
• pp.121-121
• /
• 2004

The groundwater in the Pocheon area occurs from both a fractured bedrock aquifer in igneous and metamorphic rocks and an alluvial aquifer with a thickness of <50 m, and forms a major source of domestic and agricultural water supply. In this study, we performed a hydrogeochemical study in order to identify the control of geochemical processes on groundwater quality. For this study, groundwater level and physicochemical parameters (EC, Eh, pH, alkalinity) were monitored once a month from a total of 150 groundwater wells between June 2003 to August 2004. A total of 153 water samples (13 surface water, 66 alluvial groundwater, 74 bedrock groundwater) were also collected and analyzed in February 2004. Groundwater chemistry in the study area is very complex, depending on a number of major factors such as geology, degree of chemical weathering, and quality of recharge water. Hydrochemical reactions such as the leaching of surficial and near-solace soil salts, dissolution of calcite, cation exchange, and weathering of silicate minerals are proposed to explain the chemistry of natural groundwater. Alluvial groundwaters locally have very high TDS concentrations, which are characterized by their chloride(nitrate)-sulfate-bicabonate facies and low Na/Cl ratio. Their grondwater levels are highly fluctuated according to rainfall event. We suggest that high nitrate content and salinity in such alluvial groundwaters originates from the local recharge of sewage effluents and/or fertilizers. Likewise, high concentrations of nitrate were also locally observed in some bedrock groundwaters, suggesting their effect of anthropogenic contamination. This is possibly due to the bypass flow taking place through macropores. Tile degree of the weathering of silicate minerals seems to be a major control of the distribution of major cations (sodium, calcium, magnesium, potassium) in bedrock groundwaters, which show a general increase with increasing depth of wells. Thermodynamic interpretation of groundwater chemistry shows that the groundwater in the study area is in chemical equilibrium with kaolinite and Na-montmorillonite, which indicates that weathering of plagioclase to those minerals is a major control of hydrochemistry of bedrock groundwater. The interpretation of the molar ratios among major ions, as well as the mass balance calculation, also indicates the role of both dissolution/precipitation of calcite and Ca-Na cationic exchange as bedrock groundwaters evolves progressively.