Sujeong Lee

Imaging of the Inflammatory Response in Reperfusion Injury after Transient Cerebral Ischemia in Rats : Correlation of Superparamagnetic Iron Oxide-Enhanced Magnetic Resonance Imaging with Histopathology

Background: Acute inflammatory responses have been thought to play a central role in ischemia-reperfusion injury after acute ischemic stroke. Superparamagnetic iron oxide (SPIO) particles have been known to enable in-vivo monitoring of macrophage infiltration by magnetic resonance imaging (MRI) in the experimental ischemic rat brain. Purpose: To determine whether the accumulation of macrophages could be seen in vivo in a reperfusion animal model after focal cerebral ischemia using SPIO-enhanced MRI. Material and Methods: Thirty-four adult male rats were enrolled in this study. SPIO particles were injected into the rats at different time points after 1-hour transient occlusion of the middle cerebral artery, and three-dimensional (3D) T2*-weighted magnetic resonance (MR) images with a gradient-echo sequence were performed 24 hours later. Histochemical iron staining was compared with T2* signal abnormalities. Results: At days 3 and 4 post-reperfusion, focal areas of signal loss indicating local accumulation of SPIO particles appeared in a part of the damaged brain. Areas of signal loss corresponded to local accumulation of iron-laden macrophages in histologic sections, and SPIO-induced signal loss indicated active macrophage transmigration into the reperfused brain. Conclusion: SPIO-enhanced MRI demonstrated through in-vivo monitoring that macrophages participate in reperfusion injury at early stages of injury development. SPIO-enhanced MRI could be a useful tool to examine the inflammatory mechanisms involved in reperfusion brain injury.

Benefits of Sealed-Curing on Compressive Strength of Fly Ash-Based Geopolymers

There is no standardized procedure for producing geopolymers; therefore, many researchers develop their own procedures for mixing and curing to achieve good workability and strength development. The curing scheme adopted is important in achieving maximum performance of resultant geopolymers. In this study, we evaluated the impact of sealed and unsealed curing on mechanical strength of geopolymers. Fly ash-based geopolymers cured in sealed and unsealed moulds clearly revealed that retention of water during curing resulted in superior strength development. The average compressive strength of sealed-cured geopolymers measured after 1 day of curing was a modest 50 MPa, while after 7 day curing the average compressive strength increased to 120~135 MPa. In the unsealed specimens the average compressive strength of geopolymers was lower; ranging from 60 to 90 MPa with a slight increase as the curing period increased. Microcracking caused by dehydration is postulated to cause the strength decrease in the unsealed cured samples. These results show that water is a crucial component for the evolution of high strength three-dimensional cross-linked networks in geopolymers.

Influence of Fine Aggregate Properties on Unhardened Geopolymer Concrete

지오폴리머(geopolymer)는 메타카올린 혹은 석탄재와 같은 알 루미노실리케이트 물질을 알칼리 활성화제와 반응시켜 40-80°C 의 저온에서 양생하는 무기 결합재이다(Duxson et al. 2007; Shi et al. 2011). 포틀랜드 시멘트와 비교하였을 때 높은 내열화성, 내 화학성, 높은 초기 강도, 동결 융해 저항성, 균열 감소, 부식 및 풍화 저항성, 이산화탄소의 배출량 감소 등이 지오폴리머의 장점 이라고 할 수 있다(Lee et al. 2013; Lizcano et al. 2012; Suber 2004). 석탄재와 알칼리 활성화제의 혼합으로 원료의 실리케이트 와 알루미네이트는 용해되어 중합반응이 진행되고 지오폴리머의 Si-O-Al의 3차원 구조가 형성된다(Chindaprasirt et al. 2014). ASTM C 618에 따르면, 비산재는 화학조성에 따라 Class F, Class C, Class N으로 나뉘며, 비정질 실리카와 알루미나의 함량이 높기 때문에 지오폴리머의 제조에 중요한 원료가 된다(Guo et al. 2010). 뿐만 아니라, 메타카올린(metakaolin), 소다석회 유리질 시 멘트, 천연 포졸란(부석(pumice)이나 화산재), 바닥재(bottom ash) 와 연탄재 등도 지오폴리머의 원료가 될 수 있다(Lee et al. 2010). 잔골재 특성이 굳지 않은 지오폴리머 콘크리트에 미치는 영향 Influence of Fine Aggregate Properties on Unhardened Geopolymer Concrete

Improvement of Alignment Accuracy in Electron Tomography

We developed an improved method for tilt series alignment with fiducial markers in electron tomography. Based on previous works regarding alignment, we adapted the Levenberg-Marquardt method to solve the nonlinear least squares problem by incorporating a new formula for the alignment model. We also suggested a new method to estimate the initial value for inversion with higher accuracy. The proposed approach was applied to geopolymers. A better alignment of the tilt series was achieved than that by IMOD S/W. The initial value estimation provided both stability and a good rate of convergence since the new method uses all marker positions, including those partly covering the tilt images.

Effect of Particle Size and Unburned Carbon Content of Fly Ash from Hadong Power Plant on Compressive Strength of Geopolymers

Fly ash is one of the aluminosilicate sources used for the synthesis of geopolymers. The particle size distribution of fly ash and the content of unburned carbon residue are known to affect the compressive strength of geopolymers. In this study, the effects of particle size and unburned carbon content of fly ash on the compressive strength of geopolymers have been studied over a compositional range in geopolymer gels. Unburned carbon was effectively separated in the -46μm fraction using an air classifier and the fixed carbon content declined from 3.04 wt% to 0.06 wt%. The mean particle size (d50) decreased from 22.17μm to 10.79μm. Size separation of fly ash by air classification resulted in reduced particle size and carbon residue content with a collateral increase in reactivity with alkali activators. Geopolymers produced from carbon-free ash, which was separated by air classification, developed up to 50 % higher compressive strength compared to geopolymers synthesized from raw ash. It was presumed that porous carbon particles hinder geopolymerization by trapping vitreous spheres in the pores of carbon particles and allowing them to remain intact in spite of alkaline attack. The microstructure of the geopolymers did not vary considerably with compressive strength, but the highest connectivity of the geopolymer gel network was achieved when the Si/Al ratio of the geopolymer gel was 5.0.

Effect of Foaming Agent Content on the Apparent Density and Compressive Strength of Lightweight Geopolymers

콘크리트를 가볍게 만들면 각 부재의 부담이 경감되고 고정하 중이 감소될 뿐만 아니라 공사비를 낮출 수 있어 경제적으로 유리 하다. 또한 콘크리트의 열전도율이 낮아지기 때문에 단열효과를 기대할 수 있다(Han 2005; Rickard et al. 2012). 이는 재료 내부에 형성된 기공 내 공기층이 가열 혹은 냉각 시 공기의 대류를 막기 때문이다. 국내에서 경량콘크리트가 공동 주택의 온돌구조용 단열 재로 사용되는 이유이다. 경량 콘크리트는 콘크리트 부피의 70~80%를 차지하는 골재의 무게를 가볍게 한 경량골재 콘크리트와, 시멘트 페이스트에 기포 를 생성, 분산시켜 페이스트의 무게를 가볍게 한 경량기포 콘크리 트, 잔골재를 사용하지 않는 무세골재 콘크리트로 분류된다. 경량 콘크리트라고 하면 대부분 경량골재 콘크리트를 의미한다. 경량골 재를 사용하여 경량 콘크리트를 시공하는 경우 골재의 최대크기를 줄임으로서 시멘트 페이스트와 경량골재의 부착력을 증가시켜 압 축강도를 증가시키는 것이 유리하며 경량골재의 강도에 따라 단위 시멘트량이나 혼합재 혼입량도 달라지게 된다(Oh et al. 1987). 경량골재에는 부석이나 혈암, 화산자갈 같은 천연 경량골재와 다공구조를 갖는 인공적인 산업부산물 혹은 고온소성 과정을 통해 다공구조를 갖도록 제조한 인공 경량골재가 있다. 인공 경량골재 는 대부분 고로슬래그, 점토, 화력발전소 비산재 등을 분쇄, 성형한 후 1,000~1,300°C에서 소성, 발포시켜 제조된다(Yoon et al. 2010; Kwon et al. 2001; Choi et al. 2003; Kim et al. 2009; Kang et al. 2009). 한편 경량기포 콘크리트는 굳지 않은 콘크리트에 알루미늄 분 발포제 함량에 따른 경량 다공성 지오폴리머의 밀도와 강도 특성 Effect of Foaming Agent Content on the Apparent Density and Compressive Strength of Lightweight Geopolymers