Chul-Woo Chung

On the Evaluation of Setting Time of Cement Paste Based on ASTM C403 Penetration Resistance Test

The standard setting test for setting of cement paste, ASTM C191, uses a very stiff dough with a low water content and does not give information about the gradual microstructural development over time due to hydration; but the standard test for concrete, ASTM C403, can be used on cement paste, is not limited to paste with low water content, and does give information about microstructural development. In this study, the C403 test procedure was modified to provide more reproducible data for cement paste, with the most important modification being to not remove bleed water. Since the purpose of ASTM C403 is to measure time of setting of mortar extracted from concrete, the values of penetration resistance in paste that correspond to initial and final set in concrete were determined: These values were 2 MPa for initial set and 14 MPa for final set.

Formation of Hydroxyapatite in Portland Cement Paste

In order to increase the integrity of the wellbore which is used to prevent the leakage of supercritical CO₂, it is necessary to develop a concrete that is strongly resistant to carbonation. In an environment where the concentration of CO₂ is exceptionally high, Ca 2+ ion concentration in pore solution of Portland cement concrete will drop significantly due to the rapid consumption of calcium hydroxide, which decreases the stability of the calcium silicate hydrate. In this research, calcium phosphates were used to modify Portland cement system in order to produce hydroxyapatite, a hydration product that is strongly resistant to carbonation under such an environment. According to the experimental results, calcium phosphates reacted with Portland cement to form hydroxyapatite. The formation of hydroxyapatite was verified using X-ray diffraction analyses with selective extraction techniques. When using dicalcium phosphate dihydrate and tricalcium phosphate, the 28-day compressive strength was lower than that of plain cement paste. However, the specimen with monocalcium phosphate monohydrate showed equivalent strength to that of plain cement paste.

Effects of Sugar and Hydrated Cement Powder on the Reduction in Heat of Hydration

The heat of hydration can be reduced through the use of retarding agents. Typical retarding agents include sugar and glucose. However, these significantly delay the setting of cement paste. For the efficient use of sugar and glucose for mass concrete construction, it is necessary to develop a technique that can provide a setting behavior equivalent to that of plain concrete. In this work, the temperature rise of cement paste was monitored with the addition of various retarders including sugar and glucose. Hydrated cement powder was made with a water to cement ratio of 5 in order to accelerate the retarded cement pastes. It was found that the addition of hydrated cement powder in retarded pastes reduced the maximum temperature of cement paste. The use of hydrated cement powder could also successfully reduce the time to reach the maximum temperature.

Effects of Crushed Shells on the Physical Properties of Cement Mortar

Approximately 240,000 tons of waste shells are produced annually in the south and west coast of South Korea. Some of these waste shells (oyster, cockle) are recycled as seeding collector and fertilizer, but most are dumped illegally near the coast. One of the alternative solutions that can economically utilize a large amount of these waste shells is to apply them to the production of construction materials. In this research, the basic physical properties of waste shells such as oyster, cockle, clam, manila clam were investigated, and were used to prepare cement mortar with a 25% replacement ratio of sand. According to the results, the 28 day compressive strength of cement mortar with cockle and manila clam shells was similar to that of plain cement mortar. The compressive strength decreased by about 18% when clam was used. However, the cement mortar with oyster shell showed about a 35% reduction in 28-day compressive strength, and two times the absorption capacity of plain cement mortar. The reduction in compressive strength and the increase in absorption capacity were mostly associated with the porous nature of the oyster shell.

The Application of Various Indicators for the Estimation of Carbonation and pH of Cement Based Materials

Carbonation decreases the pH of the concrete and breaks the passivity of reinforcing steel. Therefore, carbonation is related to potential rebar corrosion even though carbonation itself does not directly damage the integrity of the concrete. The phenolphthalein method, which is the most often used method of determining carbonation, however, has a reliability problem. Therefore, this research mainly focuses on the use of various indicators to find the possible substitutes for phenolphthalein. The early carbonation and pH of the carbonated specimen was also investigated using various indicators. It was found from the results that thymolphthalein was the best substitute for phenolphthalein, and a combination of phenolphthalein and thymolphthalein, tropaeolin O, and indigo carmine enabled us to estimate the early carbonation. This research also reported the successful application of various indicators which can be extended for the pH estimation of solid mortar specimen.

A Preliminary Investigation on Pozzolanic Activity of Dredged Sea Soil

Recently, the amounts of dredge sea soil in south Korea have been increasing because of various maintenance works at harbors and rivers. Dredged sea soil contains various contaminants. Hence, prior to recycling the dredged sea soil, the various contaminants should be removed to prevent a secondary contamination due to the leaching of hazardous chemicals. Pretreated dredged sea soil can be buried under the ground or used for land reclamation. In this study, however, pretreated dredged sea soil was used to investigate the level of pozzolanic activity. The properties of pretreated dredged sea soil were investigated, the method for heat treatment was determined, and the compressive strength of mortar using dredged sea soil was examined. According to the XRF result, the main components of dredged sea soil were of over 55%, and and of some amounts. Results from XRD and TG/DTA showed that pretreated dredged sea soil can be used as a pozzolanic material. When dredged sea soil was thermally treated for 90 min at , a compressive strength result was similar to that of control mortar.

Evaluation on the Basic Properties of Phosphate Modified Portland Cement Paste for Potential Application of Geologic CO2 Sequestration

As global warming became a worldwide issue, a significant effort has been made on the development of technology related to CO2 capture and storage. Geologic sequestration of CO2 is one of those technologies for safe disposal of CO2. Geologic sequestration stores CO2 in the form of supercritical fluid into the underground site surrounded by solid rock, and concrete is used for prevention of CO2 leakage into the atmosphere. In such case, concrete may experience severe damage by attack of supercritical CO2, and especially in contact with underground water, very aggressive form of carbonation can occur. In this work, to prevent such deterioration in concrete, calcium phosphates were added to the portland cement to produce hydroxyapatite, one of the most stable mineral in the world. Temperature rise, viscosity, set and stiffening, and strength development of cement paste incorporating three different types of calcium phosphates were investigated. According to the results, it was found that the addition of calcium phosphate increased apparent viscosity, but decreased maximum temperature rise and 28 day compressive strength. It was found that monocalcium phosphate was found to be inappropriate for portland cement based material. Applicability of dicalcium and tricalcium phosphates for portland cement needs to be evaluated with further investigation, including the long term compressive strength development.

Effect of Waste Glass Wool on Mechanical Properties of Concrete

Glass wool is a material that has been used as a heat insulator in various fields including construction industry. Since it is a nonflammable material, it does not generate toxic gases on fire, and thus public agencies recommend using glass wool as a heat insulator instead of other organic materials. However, repeated drying and wetting cycles can deteriorate thermal property of glass wool due to the shrinkage and reduction in pore size. For this reason, it needs to be replaced periodically, and waste materials are generated. This research aims to utilize waste glass wool as additives for increasing mechanical properties of concrete. According to the experimental results, it was found that glass wool has weak pozzolanic activity, and beneficial effect on both compressive and flexural strength. The optimum amount found in this experimental work was 0.5% volumetric addition to the concrete.

Investigation on Properties of Cement Mortar Using Heat Treated Flue Gas Desulfurization Gypsum

Flue gas desulfurization gypsum is produced from emission process of fossil fuel power plant to remove sulfur dioxide (SO2) from exhaust gas. Production of flue gas desulfurization gypsum in Republic of Korea has been increasing due to the enforced regulations by government agency. Since flue gas desulfurization gypsum has characteristic that is similar to that of natural gypsum, there is a strong possibility for flue gas desulfurization gypsum to replace the role of natural gypsum. However, consumption of such material is still limited, only used for agricultural purposes or to make gypsum boards, it is necessary to expand the use of this material more aggressively. In this research, the chemical and mineralogical properties of flue gas desulfurization gypsum were investigated, and flue gas desulfurization gypsum with heat treatment was used to make cement paste. According to the results, it was found that flue gas desulfurization gypsum used in this experiment was a very high purity gypsum, and shown to have similar property to that of natural gypsum. Heat treating flue gas desulfurization gypsum above 100°C was shown to bring beneficial effect on both compressive strength and drying shrinkage.

Influences of Slag Replacement on the Properties of Shotcrete Using a Slurry-Type Set Accelerator

The set accelerator used for shotcrete at high pH environment often causes the dusting problem in practice. In this research, a slurry-type set accelerator was developed to avoid this problem and its effectiveness was investigated by applying it to shotcrete mortars. Set time, stiffening, compressive strength, and chloride ion penetration resistance were examined with different amounts of slag, used as partial replacement of cement. According to the experimental results, it was found that the earlier responses such as set time, stiffening, and 1-day compressive strength were probably affected by the amount of ettringite, formulated by the hydration between C12A7 and calcium sulfate polymorphs present in blast furnace slag. Whereas, it is believed that the result of compressive strength after 3 days was attributed to the hydration of tricalcium silicates. As for the results of a chloride ion penetration test, the partial replacement of cement with slag significantly reduced the total charge passed through the shotcrete mortar.