Young Cheol Choi

Hydration Properties of STS-Refining Slag-Blended Blast Furnace Slag Cement

The refining process using an aluminum deoxidizer for fabricating stainless steel generated steel slag (STS-refining slag (SRS)) that contains a large amount of Al2O3 and is a compound of mainly 12CaO·7Al2O3 and 3CaO·Al2O3. When SRS was mixed with slag cement, rapid exothermic reaction occurred in the initial phase of hydration. During the hydration of slag cement and SRS, a large quantity of xCaO-yAl2O3-zH2O hydrate was observed. Until 10% weight replacement ratio of SRS to slag cement, the compressive strength was in the same level as in the existing slag cement. However, 20% replacement was accompanied by much strength degradation and high drying shrinkage. When a mixture of SRS and gypsum was added to slag cement, ettringite (3CaO·Al2O3·3CaSO4·32H2O) was actively created in the initial hydration phase. The compressive strength of the OPC-BFS-SRS-gypsum binder at 91 days was 91% of that of slag cement (B50) and was similar to that of OPC (O100). Besides, drying shrinkage was almost half that of slag cement, which indicates excellent performance for shrinkage. In case SRS is adequately used, which is an industrial by-product of the steel-making process, high shrinkage, a basic problem of slag cement, will be mitigated.

Basic Analysis on Fractal Characteristics of Cement Paste Incorporating Ground Granulated Blast Furnace Slag

This study aimed to conduct the basic analysis on the fractal characteristics of cementitious materials. The pore structure of cement paste incorporating ground granulated blast furnace slag (GGBFS) was measured using mercury intrusion porosimetry (MIP) and the fractal characteristics were investigated using different models. Because the pore structure of GGBFS-blended cement paste is an irregular system in the various range from nanometer to millimeter, the characteristics of pore region in the different scale may not be adequately described when the fractal dimension was calculated over the whole scale range. While Zhang and Li model enabled analyzing the fraction dimension of pore structure over the three divided scale ranges of micro, small capillary and macro regions, Ji el al. model refined analysis on the fractal characteristics of micro pore region consisting of micro I region corresponding to gel pores and micro II region corresponding to small capillary pores. As the pore size decreased, both models suggested that the pore surface of micro region became more irregular than macro region and the complexity of pores increased.

Compression Behavior of Confined Columns with High-Volume Fly Ash Concrete

The use of fly ash in ordinary concrete provides practical benefits to concrete structures, such as a gain in long-term strength, reduced hydration heat, improved resistance to chloride, and enhanced workability. However, few studies with high-volume fly ash (HVFA) concrete have been conducted that focus on the structural applications such as a column. Thus, there is a need to promote field applications of HVFA concrete as a sustainable construction material. To this end, this study investigated the compressive behavior of reinforced concrete columns that contain HVFA with a 50 percent replacement rate. Six columns were fabricated for this study. The study variables were the HVFA replacement rate, tied steel ratio, and tie steel spacing. The computed ultimate strength by the American Concrete Institute (ACI) code conservatively predicted the measured values, and, thus, the existing equation in the ACI code is feasible for confined RC columns that contain HVFA. In addition, an analysis model was calibrated based on the experimental results and is recommended for predicting the stress-strain relationship of confined reinforced concrete columns that contain HVFA.

Shrinkage Characteristics of Alkali-Activated Slag Mortar with SAP

The advantages of alkali activated slags (AAS) with adequate types and dosages of activators are high strength development, rapid setting, lower permeability, excellent durability, low hydration heat, high early-age strength and high resistance to chemical attack. The hydration products found in AAS are C-S-H with a low Ca/Si ratio related to GGBFS and the components of activators used. However, high autogenous and drying shrinkages of alkali-activated slag concrete are frequently reported. Therefore, researchers have continued their efforts to mitigate shrinkage strains and stresses. Two types of alkali activators and superabsorbent polymers were used to investigate autogenous shrinkage, and porosity characteristics of AAS. To mitigate shrinkage of AAS mortars, superabsorbent polymers (SAP) were applied to create an internal curing effect on the mixtures. As a result, autogenous shrinkage strains and AAS mortars that SAP were applied was significantly decreased, therefore, it is concluded that SAP played an important role on shrinkage mitigation of all mixtures.