염해환경 내에서 MgO를 첨가한 산업부산물 기반의 콘크리트 실험에 관한 연구

Abstract

Abstract Concrete is widely used as the basic material for construction with high strength and high durability. However, due to various external factors, the performance and durability of concrete can be reduced. In order to improve durability in these various natural environments, researches have been conducted to develop high-performance, high-durability concrete using industrial by-products. In particular, the Republic of Korea is surrounded by the sea, and for this reason it is exposed to external factors harmful to roads and structures. In addition, snowy areas are exposed to harmful external factors using de-icing salts. These de-icing chemicals usually consist of chloride-based salts such as NaCl, CaCl2 and MgCl2. The seawater environment is composed mainly of SO4, Mg, Ca, and K in addition to Cl and Na. Therefore, in this study, concrete was prepared by adding 0%, 5%, 7% and 10% of MgO to 0%, 30%, 50% cement replacement of slag, and then cured for 28 days. After that, to analyze and evaluate the damage effects by chloride ions, all specimens were immersed in seawater and de-icing environment for 180 days. After that, compressive strength was measured after 30, 60, 120 and 180 days of the immersion. Futhermore, to analyze the damage effects of the de-icing solution and seawater, chloride content test according to depth was executed, and for evaluating damage effects from the surface of the specimen to the inside, XRD analysis was conducted after 180 days of the immersion. Also, for evaluating the damage effects on the surface exposed to the chloride ions, SEM-EDAX analysis was conducted. Compressive strength results showed that strength of all specimens were increased up to 60 days in seawater environment and deicing environment. However, it generally decreased after 120 days. S30 and S50 samples were generally higher in long-term strength than C-M samples. The addition ratio of MgO in C-M, S30-M, and S50-M mixes showed the highest compressive strength at 5%, 7%, and 7% respectively. Therefore, Slag-based concrete with MgO additive would have better chloride penetration resistance than other mixtures. The chloride content results after immersion in seawater and de-icing are the following: all specimens decrease in chloride content, and at 30 mm of depth, chloride content could not be observed. In C-M specimens with increased MgO addition, the total chloride penetration depth decreases. Similar trends can be found in S30-M and S50-M specimens. MgO would induce the total volume of pores in the cement matrix to decrease, and enhance the concrete system's resistance to chloride attack. The specimens with GGBFS show improved resistance to chloride attack, and chloride resistivity increases in accordance with increase in slag replacement level. This could be attributed to the increased chloride binding capacity of C-S-H phases formed by the addition of GGBFS. On the other hand, the peak intensity of M-S-H, MgCl2, and brucite of specimens with MgO additive is much higher than specimens without MgO additive. Due to the similar nature of M-S-H and C-S-H, chloride binding capacity was increased in specimens with MgO and GGBFS. Immersion in de-icing salt and seawater resulted in surface and surrounding crack formation along with the ITZ of all specimens after 180 days. C-M specimens with increasing MgO addition, the total crack size decreases. Similar trends can be found in S30-M and S50-M specimens. The chemical composition of all specimens on the paste is Al, Na, Ca, K, Cl, Mg, Si etc. From EDAX and SEM observation, MgO based compound could be verified. From these results, it could be verified that MgO based hydration products would be formed in the matrix. In addition, the presence of MgO additive could be attributed to the formation of Hydromagnesite and M-S-H phases. It could be verified that the main hydration products are C-S-H, M-S-H and MgCl2. From the result, it could be assumed that MgO based compound with Cl ions might not be attributed to the damage increase of the matrix. In addition, MgCl2, M-S-H or brucite could affect solid microstructure without expansion. These components could be filled to the porosity and then it would be protected by penetration of chloride ions and de-icing salt. Therefore, the addition of MgO additive could be attributed to the improvement of the durability and strength. Thus, the usage of MgO to the seawater or deicing environment could extend the service life of the construction structures.