Jae-Suk Ryou

Influence of Titanium Dioxide Nanoparticles on the Sulfate Attack upon Ordinary Portland Cement and Slag-Blended Mortars

In this study, the effects of titanium dioxide (TiO2) nanoparticles on the sulfate attack resistance of ordinary Portland cement (OPC) and slag-blended mortars were investigated. OPC and slag-blended mortars (OPC:Slag = 50:50) were made with water to binder ratio of 0.4 and a binder to sand ratio of 1:3. TiO2 was added as an admixture as 0%, 3%, 6%, 9% and 12% of the binder weight. Mortar specimens were exposed to an accelerated sulfate attack environment. Expansion, changes in mass and surface microhardness were measured. Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), X-ray Diffraction (XRD), Thermogravimetry Analysis (TGA) and Differential Scanning Calorimetry (DSC) tests were conducted. The formation of ettringite and gypsum crystals after the sulfate attack were detected. Both these products had caused crystallization pressure in the microstructure of mortars and deteriorated the mortars. Our results show that the addition of nano-TiO2 accelerated expansion, variation in mass, loss of surface microhardness and widened cracks in OPC and slag-blended mortars. Nano-TiO2 containing slag-blended mortars were more resistant to sulfate attack than nano-TiO2 containing OPC mortars. Because nano-TiO2 reduced the size of coarse pores, so it increased crystallization pressure due to the formation of ettringite and gypsum thus led to more damage under sulfate attack.

Crack Healing Performance of PVA-Coated Granules Made of Cement, CSA, and Na2CO3 in the Cement Matrix

Various self-healing methods for concrete, such as the use of supplementary cementitious materials, adhesive agents, mineral admixtures, and bacteria, have been suggested to date, and each of these has merits and demerits. Among these, however, the use of cementitious materials may be appropriate due to their good healing efficiency, low cost, and compatibility with the cement matrix. In this study, granulation and coating methods were applied to a new cementitious composite material. The self-healing property of these materials was controlled by the polyvinyl alcohol (PVA) coating until cracks were created. Water dissolved the PVA coating after entering through the cracks, and reacted with the healing materials to generate healing products. The self-healing performance was evaluated at various elapsed times through the measurement of the crack widths, visual observation, and examination of the microscopic images. Simultaneously, a water permeability test was performed and the dynamic modulus of elasticity was measured to verify the recovery of the cracks. In addition, the healing products that had been formed in the cracks were analyzed via X-ray diffraction (XRD) and scanning electron microscopy (SEM).

A Study on Setting Time and Early Strength of Tablet-Shaped Accelerators

When concrete is worked in cold weather, the methods of using hot air, water and aggregate heating, accelerators are used to prevent early frosting and to improve early strength. But these methods raise problems such as implementation difficulty, high cost, and energy losses. Among the available cold weathering methods, accelerator method is the most economical but with the drawbacks of rapid setting and insufficient workability in the initial hydration stage. Therefore, the tablet method usually used for pharmaceutical field was applied to the accelerator method to compare the controlled reaction time of the new and old accelerator method. Based on the test results, physical and mechanical properties of concrete were tested and the possibility of delaying initial reactions to increase the total reaction time was evaluated. The results showed that when both accelerators and tablet were used, setting-time decreased. Physical properties of concrete were optimal for tablet 0.5% and 1.0%. Also, accelerator 0.5%, tablet 0.5% and 1.0% showed good early strengths.

New Surface-Treatment Technique of Concrete Structures Using Crack Repair Stick with Healing Ingredients

This study focused on the development of a crack repair stick as a new repair method along with self-healing materials that can be used to easily repair the cracks in a concrete structure at the construction site. In developing this new repair technique, the self-healing efficiency of various cementitious materials was considered. Likewise, a crack repair stick was developed to apply to concrete structures with 0.3 mm or lower crack widths. The crack repair stick was made with different materials, such as cement, an expansive material (C12A7), a swelling material, and calcium carbonate, to endow it with a self-healing property. To verify the performance of the crack repair stick for concrete structures, two types of procedures (field experiment and field absorption test) were carried out. As a result of such procedures, it was concluded that the developed crack repair stick could be used on concrete structures to reduce repair expenses and for the improved workability, usability, and serviceability of such structures. On the other hand, to evaluate the self-healing performance of the crack repair stick, various tests were conducted, such as the relative dynamic modulus of elasticity test, the water tightness test, the water permeability test, observation via a microscope, and scanning electron microscope (SEM) analysis. From the results, it is found that water leakage can be prevented and that the durability of a concrete structure can be improved through self-healing. Also, it was verified that the cracks were perfectly closed after 28 days due to application of the crack repair stick. These results indicate the usability of the crack repair stick for concrete structures, and its self-healing efficiency.

The Evaluation of Damage Effects on MgO Added Concrete with Slag Cement Exposed to Calcium Chloride Deicing Salt

Concrete systems exposed to deicers are damaged in physical and chemical ways. In mitigating the damage from CaCl2 deicers, the usage of ground slag cement and MgO are investigated. Ordinary Portland cement (OPC) and slag cement are used in different proportions as the binding material, and MgO in doses of 0%, 5%, 7%, and 10% are added to the systems. After 28 days of water-curing, the specimens are immersed in 30% CaCl2 solution by mass for 180 days. Compressive strength test, carbonation test, chloride penetration test, chloride content test, XRD analysis, and SEM-EDAX analysis are conducted to evaluate the damage effects of the deicing solution. Up to 28 days, plain specimens with increasing MgO show a decrease in compressive strength, an increase in carbonation resistance, and a decrease in chloride penetration resistance, whereas the S30- and S50- specimens show a slight increase in compressive strength, an increase in carbonation resistance, and a slight increase in chloride penetration resistance. After 180 days of immersion in deicing solution, specimens with MgO retain their compressive strength longer and show improved durability. Furthermore, the addition of MgO to concrete systems with slag cement induces the formation of magnesium silicate hydrate (M-S-H) phases.

A Characteristics of Fly-ash Concrete Incorporating Tablet-shaped Accelerators in Cold Weather

Although the accelerators are used at the early stage to control setting-time and strength of concrete when cold-weather concrete is utilized, no security of workability occurs because the early hydration makes them react rapidly. Therefore, the tablet used in previous study is applied in this study. In particular, because a small amount of fly-ash being replaced in cold weather concrete of domestic, fly-ash concrete incorporating the tablet is discussed in workability by elapsed time, early strength to ensure the development of adequate strength, and freezing-thawing resistance. As a result, both 0.5 and 1.0% tablets were found to be superior. Thus, it was verified in cold weather concrete incorporating fly-ash that workability can be secured, as well as the development of early strength to prevent early frost.

A Study on Properties of Retarder via Tabletting Method

When hot weather concrete is utilized, the cooling methods of cooling pipe, liquid nitrogen, ice, etc., are used to prevent the poor consistency and cold joint due to high temperature. These methods, however, spike the production cost and energy consumption, and make quality control difficult. Among these methods is one that involves the use of a retarder. Although economical, retarder is caused difficulty of retarded hardening and setting time control due to inaccurate weighing and poor working condition. Therefore, how to make a tablet for hot weather concrete, as with the existing pharmacy and foods, is discussed in this study, including the following items: mortar setting time, flow test by elapsed time, physical and mechanical properties of concrete. As a result, gluconic acid is superior to lignosulfonic acid and the possibility of using them for such purpose without quality degradation was confirmed in this study, when retarder is tabletting.