23. - 25. 5. 20, Brno, Czech Republic, EU HYDRAULIC PROPERTIES OF LADLE STEEL SLAGS Veronika HORÁKOVÁ*, Vlastimil MATĚJKA, Jana KUKUTSCHOVÁ, Jozef VLČEK VŠB TU Ostrava, 7. listopadu /272, 70 33 Ostrava Poruba, Czech Republic *veronika.horakova.st3@vsb.cz Abstract Steel slags were proved to have significantly lower stability during a contact with water and thus their utilization as inert aggregates in the building industry is almost impossible. However, this kind of slags could be reutilized in the course of steelmaking process after their compaction using suitable procedure. Chemical and phase composition of the three steel slags in as received state and in a course of their hydration using water were studied in the present work. Steel slags were characterized using infrared spectroscopy and X- ray diffraction analysis, the compressive strength tests of the hydrated slags were performed in selected time periods (2, 7, and 2 days). It was observed that the compressive strength of the hydrated steel slags samples differ significantly from each other as a result of different phase composition. To fulfill the efforts for using of steel slags as a secondary raw material, further investigation of hydration process of steel slags followed with detailed characterization of their microstructure and phase composition has to continue in the future. Keywords: steel slag, hydration, infrared spectroscopy, X-ray diffraction analysis, compressive strength. INTRODUCTION Slags are generally assigned as the by-products of thermal and combustion processes, whereas metallurgical slags resulting from smelting and refining of metals are the best known. The main types of metallurgical slags are: i) blast furnace slags, ii) steel slags produced in the course of the steel manufacturing, iii) foundry slags and iv) slags from production of non-ferrous metals. Steel slag is a by-product originating in the course of steel manufacturing. These slags are being produced in enormous quantities and there is strong interest for their further utilization instead of their taking to a dump, but study of their potential toxic character should by performed as shown by Gröplová []. Rapidly cooled slags show high content of vitreous phase which imparts so called latent hydraulic properties to the slags. This paper is focused on the evaluation of chemical and phase composition of three steel slags in as received state and in a course of their hydration using water. Prepared samples were evaluated by combination of X-ray diffraction analysis (XRD) and Fourier transform infrared spectroscopy (FTIR). 2. MATERIALS AND METHODS 2.. Raw slags and their hydration Three rapidly cooled ladle steel slags with different chemical composition were selected for experiments. The slags were assigned as S, S2 and S3. The slags were ground in a laboratory ball mill to receive fraction with particle sizes less than 0.09 mm. The chemical composition of the slags was obtained from the manufacturers and is listed in Tab.. 2.2. Characterization Methods The phase composition of the slags was analysed by X-ray diffraction (XRD) using a BRUKER D ADVANCE diffractometer with detector VANTEC. The samples were homogenized using the agate mortar.
23. - 25. 5. 20, Brno, Czech Republic, EU The powdered material was attached to the rotating holder for the XRD analysis. Software DIFFRACplus BASIC (Bruker AXS) was used for determination of the positions of diffraction lines on the X-ray powder diffraction patterns and database PDF 2 was utilized for identification of phases. Mid- infrared spectra of the surface of the raw slags and hydrated slags were acquired by Fourier transform infrared spectrometer NICOLET 6700 (Thermo Nicolet, USA) using single reflection ATR technique on diamond crystal with resolution 0. cm - and 32 scans. The samples were hydrated using water to give value 0.5 as a water/slag ratio. The mixtures were formed into molds with dimensions of 20 x 20 x 20 mm and stored in moist environment. During the hydration the samples were taken out during selected time of period (2, 7 and 2 days) and subjected to selected tests. The compressive strengths tests were performed using hydraulic press (BRIO Hranice). 3. RESULTS AND DISCUSSION 3.. Chemical and phase composition of raw slags The chemical composition of the slags differ significantly from each other and shows typical features of these slags like higher SiO 2 content observed for sample S, higher content of iron and MgO observed for steel slag S3. The lowest content of iron, SiO 2 and MgO was observed for sample S2, which has the highest content of Al 2 O 3 and CaO free. With respect to these differences in chemical composition the differences in phase composition are expected as well. The phase composition of the raw slags was studied using X-ray diffraction analysis. The mineralogical composition of steel slags change with its chemical composition. Main mineral phases identified at the sample: i) S are merwinite, β C 2 S (dicalcium silicate), γ C 2 S, akermanite, α Fe, RO phase (where R=Mn, Mg, Fe), ii) at sample S2 β C 2 S, CaO, periclase, portlandite, RO phase and iii) at sample S3 are gehlenite, merwinite, β C 2 S, CaO and RO phase. Chemical composition is an important parameter to determine the hydraulic activity of a steel slag. According to Wang and Yan [2] the alkalinity A=CaO/(SiO 2 + P 2 O 5 ), can be used to evaluate the hydration activity of a steel slag. If calculated alkalinity is higher than. the steel slag can be considered as a cementitious material. Table Chemical composition of the slags used in the study expressed in wt. % Sample Fe sum. SiO 2 CaO MgO Al 2 O 3 Fe 2 O 3 MnO TiO 2 P 2 O 5 S CaO free A S 3.72 2.97 4.29.5.69 0.6 3.77 0.37 0.04 0.05 0.5.97 S2.39.9 5.42 6.25 9.46 0.59 0.6 0.3 <0.02 0.05 7.2 5.2 S3 7.35 7.34 29. 2.56.55.64 5.99 0.37 0.3 0.09 0.3.7 3.2. Compressive strength The dependency of the compressive strength of the hydrated slag tested at different periods of their hydration is shown in Fig.. The highest value of compressive strength (3.4 MPa) was determined for slag S3 after 7 days hydration, after 2 days long hydration the strength slightly decreased to reach the value (3. MPa). The samples S and S2 shows similar values of compressive strength, which are significantly lower in comparison to these values measured for sample S3. All of the samples show acceptable strength after 2 days of hydration which enable gentle manipulation with them without the risk of their damage.
Compressive strength [MPa] 23. - 25. 5. 20, Brno, Czech Republic, EU 35 30 25 20 0 5 0 S S2 S3 2 days 7 days 2 days Fig. Development of compressive strength of the hydrated steel slags samples 3.3. Determination of non-evaporable water Wang and Yan [2] reported the procedure for determination of the amount of non-evaporable water, which is associated with the presence of hydrated phases. At this procedure the pulverized samples after 2 days long hydration are first put into an oven and heated at 65 C for 24 h, after the drying the samples are transferred into a muffle furnace and heated at 000 C for 2.5 hours. The obtained values of loss on ignition show that the highest amount of water bounded in the hydration products had S2 sample (20.6 %). For sample S it was 3. % and for sample S3 it was.0 %. 3.4. X-ray diffraction The presence of the phases at the studied samples was revealed using X-ray diffraction analysis. The diffraction patterns obtained for slags before and after their hydration are shown in Fig. 2-4. The intensity of the diffraction peaks of the identified mineral phases are of low intensity with respect to expected intensity for well crystalline materials. This fact is probably attributed to the presence of some portion of glassy phase in the samples as a result of their fast cooling. The mineral composition of studied steel slags differs significantly as proved by comparison of the diffraction patterns of raw S, S2 and S3, the legend identifying given number and phase is shown in Tab. 2. Merwinite Ca 3 Mg(SiO 4 ) 2 as a main crystalline phase was observed in the sample S. With respect to latent hydraulic properties of the slag S the presence of β-c 2 S and CaO was verified in the sample S as well. The Gehlenite Ca 2 Al 2 SiO 7, (MgO) 0.77 (FeO) 0.23 and Merwinite Ca 3 Mg(SiO 4 ) 2 was the main cristalline phase in the sample S3. As the main phases at the sample S2 Ca 9 (Al 6 O ), CaO and β-c 2 S were observed as main phases. The resulting new phases originating in the course of hydration process of given slag differ significantly as the result of different phase composition of raw slags whereas the new phases originating during hydration processes are shown below the line in Tab. 2. The highest content of free CaO in sample S2 led to formation of significantly higher number of hydration product as shown in Tab. 2. The compressive strength reached the lowest values for this sample, followed by the values of compressive strength observed for sample S. The highest values of compressive strength were observed for slag S3 after 7 and 2 days hydration. Surprisingly the slag S3 containing the lowest amount of free CaO, without hydraulic active β-c 2 S as well as Ca 9 (Al 6 O ) shows the highest values of compressive strength what could be explained by the formation of dense and amorphous C-S-H gel.
23. - 25. 5. 20, Brno, Czech Republic, EU Table 2 Phases identified in the samples S, S2 and S3 Phase S S2 S3 Phase S S2 S3 -Ca 3 Mg(SiO 4 ) 2 + + -Ca 3 (SiO 4 )O + 2-γ-C 2 S + + -Ca 2 Al 2 SiO 7 + 3-α-Fe + 3-(MgO) 0.77 (FeO) 0.23 + 4-Fe 3 O 4 + 4-FeO + 5-Ca 2 Mg(Si 2 O 7 ) + -Ca Al 4 O 4 CO 2.H 2 O + + 6-(MgO) 0.4 (MnO) 0.9 + 6-Ca 2 Al((AlSi). O 2 )(OH) (H 2 O) 2.25 + 7-β-C 2 S + + 7-Ca(OH) 2 + -Ca 9 (Al 6 O ) + -Ca 2.93 Al.97 Si 0.64 O 2.56 (OH) 9.44 + 9-CaO + 9-Ca 4 Al 2 O 7.9H 2 O + 0-MgO + 03009 20-Cavoda 2 Al 3 Si 3 O (OH) + 6 2 days 6 6 6 7 days 2 days 2 7 S 2 5 4 6 3 07309 H2O 2dny orig 0 20 30 40 50 60 70 0 2-Theta - Scale Fig. 2 X-ray diffraction patterns of the studied S slag and stored for 2, 7 and 2 days in moist environment 2 days 7 days 2 days 9 7 7,, 9 9 9 7 9 S2 7 9 9 7 0 9 0 20 30 40 50 60 70 0 2-Theta - Scale Fig. 3 X-ray diffraction patterns of the studied S2 slag and stored for 2, 7 and 2 days in moist environment
23. - 25. 5. 20, Brno, Czech Republic, EU 04509 voda 2 days 7 days 2 days 2, 9 S3 3 2 9 20 3 4 4 Transmitance (%) Transmitance (%) 2 3 5 0 20 30 40 50 60 70 2-Theta - Scale Fig. 4 X-ray diffraction patterns of the studied S3 slag and stored for 2, 7 and 2 days in moist environment 3.5. FTIR spectroscopy The samples of steel slags S, S2 and S3 were studied using Fourier transform infrared spectroscopy (FTIR). The registered IR spectra of the slags after their hydration using water after 2 days are shown in Fig. 5 and 6. The IR spectra can identify presence of silicates, carbonates and hydroxyl groups. Characteristic bands for Si-O bonds in [SiO 4 ] 4- are in the region between 970-940 cm - (stretching vibration) and 50-520 cm - (bending vibration) [3]. Presence of carbonates is evident according to the characteristic bands of C-O bonds around 450, 70 and 7 cm -. The C-O band of carbonates around 400 cm - in the S2 sample is significantly shifted to lower wavenumbers which may be given by different carbonate bonding in the silicates structure. The diffrerent carbonate bonding may be also manifested by shifted characteristic Si-O band to lower wavenumbers compared to samples S and S3. Characteristic stretching vibrations of Al-O bonds were detected at 50 cm - (AlO 4 tetrahedra) and 670-650 cm - (AlO 6 octahedra) [4]. The bands at 3200 3600 cm - and at 650 and 635 cm - are attributed to the presence of stretching and bending O-H vibrations in water molecules, respectively. In S2 sample, the absorption bands at 3644 and 650 cm - match with stretching and bending vibrations of structural OH groups in silicates and calcium hydroxide [5] detected also by XRD. The results of FTIR spectra accord with those of X-ray diffraction presented in Section 3.4. S S2 00 00 90 650 65 7 3670 352 3644 650 670 47 5 90 944 0 47 950 74 56 4 9 74 52 4000 3500 3000 00 000 500 Wavenumber (cm - ) 4000 3500 3000 00 000 500 Wavenumber (cm - ) Fig. 5 Mid- IR spectra of steel slags S, and S2 after 2 days hydration
Transmitance (%) 23. - 25. 5. 20, Brno, Czech Republic, EU S3 00 635 650 70 50 90 470 05 55 75 4 940 970 4000 3500 3000 00 000 500 Wavenumber (cm - ) Fig. 6 Mid- IR spectra of steel S3 after 2 days hydration 4. CONCLUSION Utilization of steel slags is problematic with respect to variation in their chemical and phase composition. Most often the slags are saved on landfill, the efforts to utilize them in building industry as aggregates for road construction revealed number of difficulties due to problems connected to their slow and delayed hydration ability. The steel slags containing high amount of iron are valuable source of this element and after their hydration with water they can be compacted and thus re-utilized in steel industry as was shown in this paper. ACKNOWLEDGEMENT The study was supported by Ministry of Education, Youth and Sports of the Czech Republic (project No. SP20/45 and No. SP20/2). REFERENCES [] Gröplová, K., Vlček, J., Čabanová, K., Kukutschová, J., Horáková, V., Eleková, H., Matějka, V., 200. The influence of metallurgical slags particle size on the inhibition of algal growth of demodesmus subspicatus. Metal 200. [2] Wang, Q., Yan, P., 200. Hydration properties of basic oxygen furnace steel slag. Construction and Building Matarials, doi:0.06/j.conbuidmat.2009..02. [3] Seung-Min Han, Jin-Gyun Park, Il Sohn, 20. Surface kinetics of nitrogen dissolution and ist correlation to the slag structure in the CaO-SiO2, CaO-Al2O3, and CaO-SiO2-Al2O3 slag system. Journal of Non-Crystalline Solids 357, p. 26-275. doi:0.06/j.jnoncrysol.20.03.023. [4] Gritco, A., Moldovan, M., Grecu, R., Simon, V., 2005. Thermal and infrared analyses of aluminosilicate glass system for dental implantants. Journal of Optoelectronics and Advanced Materials Vol. 7, No. 6, p. 245-247. [5] Jianxin Li, Qijun Yu, Jiangxiong Wei, Tongsheng Zhang, 20. Structural characteristics and hydration kinetics of modified steel slag. Cement and Concrete Research 4, p. 324-329. doi:0.06/j.cemconres.200..0.