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Advances in Materials Science and Engineering
Volume 2018, Article ID 5893254, 9 pages
https://doi.org/10.1155/2018/5893254
Research Article

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

1Environment and Resources Research Group, Research Institute of Industrial Science and Technology, 67 Chengnam-ro, Nam-gu, Pohang, Republic of Korea
2Department of Civil and Environmental Engineering, Gachon University, 1342 Seongnamdaero, Sujeong-gu, Seongnam-si, Gyeonggi-do 13120, Republic of Korea

Correspondence should be addressed to Young Cheol Choi; rk.ca.nohcag@eforez

Received 24 February 2018; Revised 22 April 2018; Accepted 30 April 2018; Published 17 May 2018

Academic Editor: Fernando Rubio-Marcos

Copyright © 2018 Bong Suk Cho and Young Cheol Choi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

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.

1. Introduction

The concrete shrinkage is a reduction of volume occurring when the relative humidity in capillary pores decreases during hydration. When a water/binder ratio is low and much blast furnace slag powder is used, the size of shrinkage increases and thus early-age cracking is more likely to happen [1, 2]. The conventional preventive methods of shrinkage cracks of concrete include expanding cement [3], surface treatments like prestressed technology [4], shrinkage-reducing admixtures [59], optimal temperature and humidity curing conditions [10], and expansive additives [11, 12].

Among these methods for controlling concrete shrinkage, the expansive additives can increase the volume of cement paste in concrete by utilizing specific chemical reactions. In other words, ettringite (3CaO·Al2O3·3CaSO4·32H2O) is created at an early age, and the growth of ettringite expands cement paste and compensates for shrinkage. CaO + CaSO4-based and calcium sulfoaluminate- (CSA-) based expansive additives are representative ones [1317]. These additives can prevent early-age cracking of concrete by inducing an appropriate expansion. However, since the expansive additives are much more expansive than the ordinary Portland cement, they cannot be widely applied to construction sites. Recent researches focus on enhancing the price competitiveness of inorganic expansive additives and utilizing industrial by-products for expansive additives [6].

As various types of steel products are demanded and supplied, various types of slag are produced in each process. Especially, when stainless steel, which is a high-value product, is fabricated, even a very small amount of oxygen mixed with steel needs to be removed. Aluminum, Fe-Si, and Fe-Mn, which have a high affinity with oxygen, are used as deoxidizers. The steel by-product of this process is refining slag (STS-refining slag (SRS)). SRS generated from POSCO in Korea is a by-product of the STS-refining process using aluminum as a main deoxidizer (Figure 1).

Figure 1: Raw material of STS-refining slag.

The SRS produced in Korea is finally transformed into a CaO-Al2O3 compound after the hot melt reaction of CaO (resulting from limestone, which is a supplementary material) included in the slag layer and aluminum inserted as a main reducing agent. As shown in Figure 1, SRS is produced in lumps. The annual SRS is around 140,000 tons. Among them, the amount of the flux material (less than 10 mm) used in the steel-manufacturing process is 50,000 tons. The remaining 90,000 tons are crushed to a size of 25 mm or 40 mm and simply used as landfill materials. POSCO in Korea wants to derive new high-value-added applications of SRS.

Since SRS contains a large amount of 3CaO·Al2O3 among main composition minerals of the ordinary Portland cement (OPC) like 3CaO·SiO2, 2CaO·SiO2, 3CaO·Al2O3, and 4CaO·Al2O3·Fe2O3, it can be effectively used as a cement additive. Moreover, as SRS also contains 12CaO·7Al2O3 along with 3CaO·Al2O3, if it is mixed with gypsum (CaSO4), ettringite can be generated in the initial phase of hydration. This phenomenon may induce an expansion in such an initial phase, which can reduce shrinkage. Slag cement is generally composed of OPC and ground-granulated blast furnace slag (BFS). Since slag cement undergoes a large shrinkage in the initial phase of hydration, it may very likely cause shrinkage cracks of concrete [1821]. Especially, in the case of slag cement with large BFS content, a large shrinkage may occur. Thus, when slag cement is applied to construction sites, an economic measure is necessary.

The aim of this research was to set up a method for reducing shrinkage of slag cement by using SRS and to find an effective method for recycling SRS, which is an industrial by-product. For this aim, physical and chemical properties of SRS were examined, and hydrate and physical properties of slag cement composites with only SRS or a mixture of SRS and gypsum were evaluated. Heat evolution analysis, XRD analysis and TG-DTA were conducted for various SRS-related parameters, and compressive strength and drying shrinkage were also evaluated according to age.

2. Experimental Details

2.1. Materials

Table 1 presents physical properties and chemical composition of raw materials used in this research. OPC with 3.13 g/cm3 density and 3,415 cm2/g Blaine was used as cement. BFS was produced by POSCO, and its density and Blaine were 2.99 g/cm3 and 4,340 cm2/g, respectively. SRS is produced as a by-product in the refining process of stainless steel. Since the STS-refining process used aluminum as a deoxidizer to remove oxygen from molten steel, SRS had a high content of Al2O3 (oxygen affinity: K > Ca > Mg > Al > Ti > C > Si > Mn > Cr > Fe > Ni > Cu). The density and Blaine of SRS were 3.04 g/cm3 and 4,010 cm2/g, respectively.

Table 1: Chemical composition and physical properties of the materials.

Figure 2 shows XRD patterns of the test materials. In the STS-refining process, calcium oxide included in a slag layer and aluminum, which is used as a deoxidizer, made a high-temperature melt reaction to create a CaO-Al2O3 compound, of which the main crystal phase consists of 12CaO·7Al2O3 and 3CaO·Al2O3. 12CaO·7Al2O3 and 3CaO·Al2O3 produced a rapid exothermic reaction against water to form xCaO-yAl2O3-zH2O [2226]. The pulverized particles of SRS had a plate shape, and the surface looked solid. The primary crystal phase of OPC was found to be 3CaO·SiO2 (alite) and 2CaO·SiO2 (belite). There were also small amounts of 3CaO·Al2O3 (calcium aluminate) and 4CaO·Al2O3·Fe2O3 (calcium aluminate ferrite).

Figure 2: XRD spectra of the test materials. (a) OPC. (b) Gypsum. (c) SRS. (d) BFS.

Calcium sulfate (CaSO4) was found to be the main crystal phase in the gypsum, based on which it was classified as anhydrous gypsum. BFS showed the typical noncrystalline XRD pattern due to wet quenching.

The SRS powder is a nonmagnetic slag collected through the processes of drying, crushing, and magnetic separation. The powdered SRS particles are similar to a rough plate type with a surface that appears to be hard as shown in Figure 3. In particular, the surface of the BFS was very slick due to the glassy state on the grain surface formed through the shock chilling process.

Figure 3: SEM image of the particle. (a) OPC. (b) Gypsum. (c) SRS. (d) BFS.
2.2. Mixture Proportions and Test Methods

The blast furnace slag cement conventionally used in Korea is generally composed of OPC and BFS in a ratio of 5 : 5 to 6 : 4 [27]. In this research, the slag cement with the mixture of 50% OPC and 50% BFS was set to O100, and a part of BFS was replaced by only SRS or the mixture of SRS and gypsum. The slag cement composites consisting of OPC (O), BFS (B), SRS (S), and gypsum (G) are called as O-B-S-G binders. As shown in Table 2, The O-B-S-G binders were mixed in the ratios of 100 : 0 : 0 : 0 (O100), 50 : 50 : 0 : 0 (B50), 50 : 40 : 10 : 0 (B40-S10), 50 : 40 : 5 : 5 (B40-S5-G5), 50 : 40 : 0 : 10 (B40-G10), 50 : 30 : 20 : 0 (B30-S20), 50 : 30 : 15 : 5 (B30-S15-G5), 50 : 30 : 10 : 10 (B30-S10-G10), 50 : 30 : 5 : 15 (B30-S5-G15), and 50 : 30 : 0 : 20 (B30-G20).

Table 2: Mixture proportions of binders (unit %).

To evaluate heat evolution, XRD, and TG and drying shrinkage, slag-cement paste specimens were fabricated with the water/binder ratio of 0.4. Mortar specimens for compressive strength testing were manufactured using the ISO standard sand (ISO 679).

The hydration heat was measured at 30-second intervals using the calorimeter (MMC-511SV6; Rico Corp., Tokyo) for 7 days. The TG (EXSTAR 6000; Seiko Instrument Corp.) analysis of the paste specimen was implemented at the age of 3 days and 91 days. The temperature in the instrument was raised from 20°C up to 1100°C under the atmospheric condition. The XRD (D/Max-2500V; Rigaku Corp.) and SEM (S-4300SE; Hitachi Corp.) analyses for paste specimens were performed in order to investigate hydration products.

The compressive strength of mortar was measured in compliance with ISO 679. A mortar specimen of dimensions 40 × 40 × 160 mm was made using a jolting machine and water-cured at 20 ± 2°C. The compressive strength of the specimen was then measured at 3, 7, 28, 56, and 91 days. In addition, the paste specimen (25.4 × 25.4 × 254 mm) for the drying shrinkage test was cured at 20 ± 3°C under the air condition for 1 day, after being detached from the mold. Then, the changes in length were measured under conditions of 20 ± 3°C and 60% humidity for 180 days [28].

3. Results and Discussion

3.1. Hydration Heat Evolution

Figure 4 shows the results of an evaluation of hydration heat of O100, B50, B30-S20, B30-S10-G10, and B30-G20. The hydration graph over time in Figure 4 was to analyze the hydration phase by phase.

Figure 4: Hydration heat evolution with binder types.

The graph depicts heat evolution rates and cumulative heat flows simultaneously. The first peak occurred within 30 minutes immediately after being mixed with water. The largest value of the first peak occurred in B30-S20 and O100 (about 1.40 J/hg), followed by B30-S10-G10 (1.27 J/hg), 50B30S0G20 (0.57 J/hg), and B50 (0.20 J/hg). Especially, the exothermic peak of B30S20 was about 7 times that of B50 and was also strong enough to maintain for about one hour. The reason why B30-S20 and B30-S10-G10 had a strong exothermic reaction was that large quantities of 3CaO·Al2O3 and 12CaO·7Al2O3, which were contained in SRS, caused a rapid exothermic reaction to water [19, 29]. 3CaO·Al2O3 and 12CaO·7Al2O3 in SRS create xCaO-yAl2O3-zH2O hydrate through the reaction with water.

In addition, when a mixture of SRS (3CaO·Al2O3 and 12CaO·7Al2O3) and gypsum (CaSO4) reacts with water, ettringite (3CaO·Al2O3·3CaSO4·32H2O) is generated. However, as opposed to the initial rapid exothermic reaction, B30-S20 showed the lowest exothermic value among the binders after about 6 hours. The second peaks occurred between 12 and 20 hr, and O100 showed the largest value of 0.28 J/hg around 12.8 hr, which was followed by B50 (0.18 J/hg at 19.6 hr), B30-G20 (0.17 J/hg at 13.0 hr), B30-S10-G10 (0.16 J/hg at 14.8 hr), and B30-S20 (0.07 J/hg at 49.9 hr). It was remarkable that B30-S20 had the strongest initial exothermic reaction after being mixed with water, but its second peak was very low and also occurred late. The largest cumulative heat for 160 hr was 295 J/g, which occurred in O100. When the cumulative heat of O100 was taken as 100%, B50, B30-S20, O50G30S10G10, and B30-G20 had the relative percentages of 68%, 76%, 78%, and 68%, respectively.

3.2. Compressive Strength

Figure 5 illustrates the compressive strength of the mortar with binder types. Until 7 days, O100 showed incomparably high compressive strength among all the specimens. However, after 28 days, the difference in compressive strength between O100 and the remaining specimens decreased gradually. The compressive strength of B50 was lower than that of OPC until 7 days and became similar to that of O100 at 28 days and finally had the highest value among all the specimens including O100. This is because the latent hydraulic properties of BFS made a long-term strength improvement. As compared to B40-G10, B40-S10, B30-S20, and B30-G20, to which either gypsum or SRS alone was added, B40-S5-G5, B30-S15-G5, and B30-S10-G10, to which the mixture of SRS and gypsum was applied, showed higher values of compressive strength.

Figure 5: Development of compressive strength with binder types. (a) O50B40 + S&G series. (b) O50B30 + S&G series.

Among O-B-S-G binders, B40-S5-G5 and B30-S15-G5 showed the highest overall compressive strength and had a similar value at each age. In particular, their compressive strength was similar to that of B50 at 1 day but became higher at 7 days. Then, they had a similar value to B50 again at 28 days. At 91 days, the compressive strength of B40-S5-G5 and B30-S15-G5 was about 10% lower than that of B50 but was similar to that of O100. B50 had the compressive strength of 62.9 MPa at 91 days, which was the highest value among all the binders. B40-S5-G5, B30-S15-G5, and O50B40G had similar values of 57.6 MPa, 57.5 MPa, and 57.0 MPa, respectively, taking the second place. Next, O100 (56.7 MPa) and B40-S10 (56.1 MPa) showed similar values, followed by B30-S10-G10 (55.4 MPa), B30-S20 (43.4 MPa), O50B30S5G15 (36.7 MPa), and B30-G20 (32.3 MPa). Among the binders to which SRS and gypsum were mixed, B30-S15-G5 and B30-S10-G10 had relatively higher values than O50B30S5G15. To be specific, under the condition that the content of SRS-gypsum mixture does not exceed 20%, when the content of gypsum exceeded 10%, the consequential compressive strength decreased. Consequently, as the compressive strength of ettringite is much affected by SRS and gypsum, it is necessary to optimize their mixing ratio and content and to maintain the quality.

3.3. Drying Shrinkage

Figure 6(a) shows the evaluation results of the drying shrinkage of the paste with O-B-R-G binders up to 180 days. Figure 6(b) shows the initial trend by illustrating the drying shrinkage until 14 days. The overall trend of drying shrinkage is clearly displayed according to the binder type. The early-age characteristics of shrinkage and expansion ultimately affect the shrinkage compensating at a later age. In other words, an appropriate compensation for expansion at an early age is essential to reduce shrinkage. As for the shrinkage characteristics of binders at 180 days, among the binders that did not contain gypsum, B40-S10 and B30-S20 had the shrinkage of −1,300 × 10−6 and −1,270 × 10−6, respectively, which were about 1.3 times the shrinkage of B50 (−1,010 × 10−6). However, B40-G10 that did not contain SRS decreased its shrinkage below that of B50 and had a similar result to O100 (−800 × 10−6). B30-G20 that contained more gypsum than B40-G10 had a still lower shrinkage of −620 × 10−6. Other specimens (B40-S5-G5, B30-S15-G5, O50B30S5G15, and B30-S10-G10), in which both SRS and gypsum were used, showed the drying shrinkage range of −690 × 10−6 to −500 × 10−6, indicating a significant decrease in shrinkage as compared to B50.

Figure 6: Drying shrinkage of paste specimens with binder types for (a) 180 days and (b) 14 days.

The early ages (1 to 4 days) revealed a remarkable expansion tendency in B30S10G10 and B30S5G15. Especially, B30S10G10 expanded more significantly than B30S5G15. In other words, when the mixture of SRS and gypsum was hydrated, ettringite was generated and developed (development of crystals) in capillary pores and thus a swelling pressure occurred, which resulted in expansion at the early ages.

In particular, the shrinkage of B30-S10-G10 was so low as to be only 62% and 50% of O100 and B50, respectively. The shrinkage of O50B30S5G15 at 180 days was 80% and 64% of O100 and B50, respectively, also indicating a good effect of reducing shrinkage. Accordingly, the appropriate application of SRS and gypsum could decrease the shrinkage of the existing slag cement by over 50%. On the contrary, O50B30RS20 and B40-S10, which used only SRS as an admixture, showed the highest shrinkage level. Although SRS is an industrial by-product of the steel-making process, it contains useful components like 3CaO·Al2O3 and 12CaO·7Al2O3. For this reason, SRS can be effectively used as a cement material. However, if SRS alone is used, an excessive shrinkage can occur, which will result in damages including cracks. Thus, a rigorous quality management is required for utilizing SRS.

As the reduction of the construction period is still the first priority in many construction sites in Korea, a Blaine value tends to be raised in order to develop the initial strength of slag cement. This frequently results in a large drying shrinkage, and such an adverse effect causes shrinkage cracks and increases the maintenance cost significantly. Consequently, the appropriate use of SRS and gypsum will solve the problem of large shrinkage, which is a disadvantage of slag cement.

3.4. XRD, TG, and SEM Analyses of Hydrates

The XRD spectra at 3 days and 91 days are illustrated in Figure 7. As shown in Figure 7, the hydrates had higher and clearer peaks at 91 days than at 3 days. B30-S10-G10 and B30-G20, to which gypsum was added, showed that B30-S10-G10 had slightly higher ettringite peaks than B30-G20. Overall, the ettringite peaks of B30-S10-G10 were observed to be the most intensive among all the specimens. In the case of B30-G20, where gypsum was put excessively, a peak of monosulfate was found. In B30-S20, which contained the largest amount of SRS, calcium aluminate hydrates (xCaO-yAl2O3-zH2O) and 3CaO·Al2O3 formed the main peaks. O100 and B50 showed an almost similar hydrate pattern, and their main hydrates were Ca(OH)2 and ettringite. In B50, the Ca(OH)2 peak appearing around 18° was lower than that of OPC. This may be because the amount of Ca(OH)2 generated from OPC was decreasing in the latent reaction with BFS.

Figure 7: XRD spectra of hydrates at (a) 3 days and (b) 91 days.

Figure 8 shows the results of thermal analysis for pastes of O100, B50, B30-S20, B30-S10-G10, and B30-G20 at the age of 3 days and 91 days. The overall trend was that the specimens showed a steeper gradient of the weight loss curve at 91 days than at 3 days. This is because the specimens generated more hydrates at 91 days, and dehydration of water molecules occurred remarkably under high-temperature condition [3033].

Figure 8: Thermogravimetric analysis with binder types at (a) 3 days and (b) 91 days.

The weight loss phenomenon around 100°C indicates dehydration (evaporation of crystallization water) of ettringite hydrates [3436]. In around 100°C, B30-G20 and B30-S10-G10 showed a relatively larger weight loss than O100 and B50. This is because B30-G20 and B30-S10-G10 contained a lot of ettringite hydrates. Especially, B30-S10-G10 had the largest weight loss and was followed by B30-G20 and B30-S20. The weight loss phenomenon around 250∼350°C means dehydration of xCaO-yAl2O3-zH2O hydrate. B30-S20 containing a large amount of xCaO-yAl2O3-zH2O showed the largest weight loss at 91 days.

The weight loss curves around 400∼500°C indicate the dehydration of H2O molecules due to the pyrolysis of Ca(OH)2. O100 showed a larger weight loss than other specimens, which indicates that O100 generated the largest amount of Ca(OH)2 hydrate.

The weight loss curves around 700°C indicate the decarbonation of the calcium carbonate in the hydrated compound [37, 38]. There was no significant difference at 3 days, but the weight loss curve of B30-G20 was remarkably steeper than that of the other specimens at 91 days. The final weight loss of the specimens at 1000°C was 81∼87% at 3 days and 74∼78% at 91 days.

Figure 9 presents SEM images of hydrates of O100, B30-S20, B30-S10-G10, and B30-G20 at 91 days. B30-S20 shows mainly xCaO-yAl2O3-zH2O and xCaO-ySiO2-zH2O hydrates, while the main hydrate of B30-S10-G10 and B30-G20 is ettringite. Besides, B30-S10-G10 has larger and thicker ettringite hydrates than B30-G20. As mentioned above, this demonstrates that the coexistence of SRS (CaO-Al2O3 compound) and CaSO4 is more effective in generating ettringite hydrates.

Figure 9: SEM images of hydrates. (a) O100 (×5k). (b) B30-S20 (×5k). (c) B30-S10-G10 (×5k). (d) B30-G20 (×5k). (e) B30-S10-G10 (×20k). (f) B30-G20 (×20k).

4. Conclusions

It was found that a more economical and eco-friendly shrinkage-reduced slag cement can be manufactured using SRS, which is a by-product of the steel-making secondary refining process. The findings of this research can be summarized as follows:(1)SRS is a by-product of the stainless steel-refining process. Its main components are CaO and Al2O3, which are constructed in the compound forms of 12CaO·7Al2O3 and 3CaO·Al2O3, respectively.(2)The binders with SRS as an admixture showed a strong exothermic reaction in the initial phase of hydration. This is closely related to ettringite hydrates generated by the intensive hydration of 12CaO·7Al2O3 and 3CaO·Al2O3 and the mixture of gypsum.(3)The compressive strength of B30-S10-G10 at 91 days was 91% of that of slag cement (B50) and was similar to that of OPC (O100). Besides, the drying shrinkage of B30-S10-G10 was 50% and 62% of that of slag cement and OPC, respectively. The slag cement composites with the admixture of SRS and gypsum tended to a large amount of bigger and thicker ettringite hydrate.(4)It is expected that the appropriate use of SRS, which is a by-product of the steel-making process, will solve the troublesome large shrinkage of the existing slag cement and facilitate the development of a new shrinkage reduction agent that could replace the existing expensive agents. Moreover, damage or defects caused by shrinkage cracks will also be reduced.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This research is supported by the Korea Ministry of Environment (MOE) as “The advancement of scientific research and technology development in environmental science program” (no. 2017000150001). And this research was also financially supported by the Technology Advancement Research Program (TARP) (Grant no. 18CTAP-C129989-02) funded by the Ministry of Land, Infrastructure and Transport of the Korean Government.

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