Papers by Author: Kyu Hong Hwang

Abstract: Refractory castable specimens were prepared using industrial white fused alumina as aggregates, chromium oxide green and ultrafine alumina powder as matrix materials, solid aluminum dihydrogen phosphate (Al(H₂PO₄)₃) or liquid AlPO4 as binder materials. Effect of different binder additions on the properti_{Subscript text}es of high chromium oxide IGCC refractory castables, such as bulk density, apparent porosity, strength was evaluated and their microstructures were observed by SEM and mercury porosimetry.

Abstract: As a relatively new material, geopolymer concrete offers benefits as a construction material for sustainable development. It utilizes waste materials such as recycled concrete sludge, fly ash,etc. To reduce CO2 emission, geopolymer concrete is expected to replace the traditional Portland cement based concrete. It’s reported that geopolymer based concrete releases only 1/6 CO2 compared to those of based Ordinary Portland Cement (OPC). In this study, to obtain a new type of construction material with lower CO2 emission and energy consumption, fly ash, waste concrete sludge and metakaolin were used as the main components. Solution of NaOH/KOH and water glass were applied as alkaline activator. The amount of water glass and the ratio of alkaline/water glass were varied to reach an optimal value. Also, silica fume was added as a bonding matrix to enhance the strength, and the specimens were cured in air and oven, then their mechanical properties such as compressive strength and bending strength were measured and their microstructures were investigated.

Abstract: As a relatively new material, geopolymer concrete offers the benefits as a construction material for sustainable development. It utilized waste materials such as recycled concrete sludge, fly ash and etc. It has a very low rate of green house gas emission when compared to ordinary Portland cement. In this study, the component of geopolymer is concrete sludge, metakaolin and water glass, NaOH was used as alkalin activator. To improve the mechanical properties, the amount of NaOH and water glass were varied to obtain higher strength and the specimens were cured both in air and water, then their mechanical properties like compressive strength and bending strength were measured the microstructures were investigated.

Abstract: Porous concretes with continuous voids have been gaining more interest as an ecological material because of their useful functions such as water permeability and adsorption ability. Especially, to make porous concrete much more environmentalized, micropores play a role of nest for microorganism and germs to live in. So micropore distribution and the size of micropores especially open pores are the key point . In this study, the size and distribution of micropores of porous concrete were effected by the AE agents to the cement pastes and then by the treating types, treating times, treating temperatures, and the vacuum pressure during the treating. And another group specimens were added sodium bicarbonate and alum. And then physical properties were examined, digital microscope was also used to observe the micropores sizes and distributions. Cracks were observed on the cement-aggregate joint when the amount and vacuum pressure increased. And high treating temperature shows no good on the bonding of cement paste and aggregate.

Abstract: Pure Titanium alloys are superiorities of biocompatibility, mechanical properties and chemical stability. The biocompatibility of Ti alloy is related to the surface effect. In this study, Ti Alloys were treated by alkali and acid activation process. And through the sol coating layer, biocompatibility were investigated. Consequently, it appeared that the porous layer was generated on the surface of alloy by surface treatment and sol coating process. It was found that with surface treatment on Ti alloy, the formation speed of porous was much quicker compared with those ones without treatment. Therefore, the biocompatibility was improved.

Abstract: The reaction-sintered zirconia-alumina and zirconia-spinel ceramics having low firing shrinkage were prepared from ZrO2(Ca-PSZ)/Al and ZrO2(Baddellyite)/MgAl powder mixtures via the attrition milling and the effect of the characteristics of used raw powders was investigated. Flaky Al powders mixed with coarse Ca-PSZ powders was not effectively comminuted due to lower hardness of zirconia powders. So by using the alumina ball media or coarse Al2O3 powders rather than Al, the milling efficiency could be much more increased. When fused Ca-PSZ powder was reaction-sintered with Al at 1550
for 3 hours, the reaction-sintering and densification were somewhat difficult because the Ca-PSZ/Al powder mixture was not effectively comminuted. And the Ca ion in Ca-PSZ grains diffused into alumina grains during sintering so that the unstabilization of Ca-PSZ body was occurred which gave the cracks in the specimens. But when MgAl alloy powder was added to monoclinic zirconia, Mg and Al became to oxide at first and subsequently converted to spinel(MgAl2O4) during heating and finally unreacted MgO seemed to stabilize the zirconia. The oxides which formed at the oxidation process would have very fine grain size so that the reaction sintering was more effective to densification and homogeneous microstructure. The mechanical properties of reaction-sintered stabilized zirconia/spinel composite were better than only MgO stabilized zirconia. Sintering behavior in reaction and mechanical properties of sintered body were examined, with emphasis on the relations between spinel formation and mechanical properties.

Abstract: The innovative method based on prefabricated fiber laminate composites bonded to lightweight precast panel was applied for quality control of RC strengthening members. The experimental investigations for this method were presented for flexural member. Three different groups such as control group without strengthening, conventional fiber bonding group and innovated PFLC group bonded to precast panel. Tested results showed good or better capacities comparing to conventional strengthening method for flexural members. This investigation indicate that good enhancement with respect to quality control and reliability can be possible by the installation and epoxy injection with prefabricated panel using fiber laminate composites.

Abstract: Calcium phosphate ceramics (CPC) have been attractively used in different areas of biomedical applications, such as substances of artificial hard tissue replacement implants, drug delivery system due to their biocompatibility and bioactivity. In this work, three calcium phosphate powders between Ca/P molar ratio 1.50-1.67 were synthesized by aqueous precipitation method, using the mixture of Ca(NO3)2·4H2O and H3PO4 solutions to ammonia solution. During the precipitation reaction, Ca/P molar ratio was adjusted by controlling pH of the solution between 8.0 and 10.5 by the addition of ammonium hydroxide solution. All powders were treated at 800 oC for 2 hrs. The calcined powders were immersed in pH 7.4 distilled water at 37°C for 3 and 7 days. Decomposition and related dissolution with the various Ca/P ratios were investigated by XRD, FTIR, and TEM observation.

Abstract: Particle separations occurred frequently in sintered hydroxyapatite when immersed in distilled water or simulated body fluid. This dissolution initiated at grain boundary creating nano-size defects such as small pores and grew up to micro scale by increasing immersion time. The dissolution, probably due to the appearance of secondary phases in grain boundary, resulted in grain separation at the surfaces and finally in degradation and fracture. And the dissolution concentrated on those grains adjacent to pores rather than those in the dense region. Hydroxyapatite ceramics incorporated with calcium silicate glass were prepared by slip casting to enhance the sinterability as well as to reduce dissolution. Glass phase was incorporated into hydroxyapatite to act as sintering aids followed by crystallization in order to improve the mechanical properties without reducing biocompatibility. From dissolution test, significant damage was reduced even more than 7 days and the dissolution pattern somewhat changed than pure hydroxyapatite. X-ray diffraction and SEM showed no decomposition of secondary phases in grain boundary and fracture toughness somewhat increased.