MODIFICATION OF BOF SLAG FOR CEMENT MANUFACTURING João B. Ferreira Neto1, Catia Fredericci1, João O. G. Faria1, Fabiano F. Chotoli1, Tiago R.
Ribeiro1, Antônio Malynowskyj1, Andre N. L. Silva1, Valdecir A. Quarcioni1, Andre A. Lotto1
1Institute for Technological Research (IPT), 532 Av. Prof. Almeida Prado, São Paulo – SP, 05508-901, Brazil
Keywords: Steel Slag, Cement Slag, Pyrometallurgy
Abstract
The use of metallurgical slags in cement manufacturing depends on the phases that compose such slags, which are affected by changes in slag chemical composition as well as by cooling rates adopted during solidification. Two different slags were produced in a pilot scale metallurgical reactor by mixing additives into 300 kg of re-melted BOF slag followed by natural cooling or by adding of steel balls to it. Quantitative XRD, SEM and Raman analysis of slag samples revealed the relationship among cooling conditions and crystalline phases.
The modified slags had CaO/SiO2 between 1.8 and 2.0 which is lower than the 3.8 for Steelmaking slag. This reduced basicity resulted in the presence of di-calcium silicates (C2S) in higher amounts than in the Steelmaking slag. These di-calcium silicates were part composed of
-C2S and part of Bridigite when slag solidified slowly or ’-C2S when slag was cooled faster by the addition of steel balls.
Cement samples were produced by mixing 25% of treated steelmaking slag with 75% of Portland cement, resulting in expansion lower than 0,1% in the autoclave tests and compressive strength higher than 42 MPa after 28 days. The process indicates potential to be applied as a steelmaking slag treatment.
Introduction
The construction industry in Brazil has grown more than 5%/y, whereas the crude steel production has been steady at approximately 32 millions of tons in the last 10 years, and there is no expectation of growth. Therefore, a shortage of blast furnace slag is happening and the cement industry needs to find alternatives. Steelmaking slag could be an alternative in cement mineral admixture, as a substitute for blast furnace slag [1-4]. However, the use of steelmaking slag directly in cement admixture is not allowed because of expansion during hydration, caused by free CaO and MgO and Mg-rich wustite (RO phase) [5,6]. A pyrometallurgical process has been investigated to promote the modification of the chemical composition of molten steel slag to make it more appropriate for cement manufacturing. The hydraulic activity of slags is affected by the chemical composition, glass content and their combination [7]. Highly basic slags (CaO+MgO/SiO2+Al2O3 = 1.5) are mainly crystalline [7] and a glassy structure forms in acidic slags containing sufficient Al2O3. In addition, the development of glassy phase or crystalline, and crystal size, depends on cooling conditions [8].
Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
Methodology
The purpose of the experiments conducted in the present work was to study the effect of cooling rate and chemical composition on slag crystallization. Modified slags were produced in a metallurgical reactor [9] by adding modifying agents to 300 kg of BOF molten Steel Slag (SS) supplied by a Steelmaking Company in Brazil. The modifying agent used in this work was rich in SiO2.
Table I shows the chemical composition of BOF steel slag (SS) and of two modified steel slags (SS-M11 and SS-M12) determined by X-ray fluorescence (XRF).
The experiments were aimed at obtaining crystalline slags, especially Ca2SiO4 (C2S) formation.
Table I. Chemical compositions and basicity (%CaO/%SiO2) of Steel Slag (SS) and modified slags (wt%).
FeO: ASTM E 246-10 - Determination of Iron by Dichromate Titrimetry. Fe0: XU, Z et al[10], Fe3+ (%) = Fet FRX (%) Fe2+ (%) - Fe0, (*) CaO free.
After chemical modification, slag SS-M12 was cooled by steel balls, using the technique developed and patented by the company Paul Wurth, which has authorized the Institute for Technological Research (IPT), based on a cooperation agreement, to perform tests using such cooling conditions. The principle of cooling by steel balls is described in the patent [11] and in the literature [12]. Slag SS-M11 was naturally cooled in the same reactor in which the modification occurred, to evaluate the effect of cooling conditions on the slag crystallization.
After cooling, steel balls were separated from the slag by magnetic separation. The slags were milled and homogenized to obtain samples suitable for characterization. The mineralogical characterization was performed by XRD analysis with Rietveld quantification method.
Additionally, Raman spectroscopy (WiTec alpha 300R, =532 nm) was used to microscopically identify the phases and also validate the XRD analysis. Finally, samples were examined in SEM with EDS analysis in order to evaluate the chemical composition of phases.
The hydration heat of modified slags was determined by mixing 50 g of slag into 20 g of 20%
NaOH aqueous solution. The generated heat was measured for 72 hours.
Chemically modified steel slags were ground to a particle size less than 0.075 mm and mixed with Ordinary Portland Cement (OPC) resulting in Portland Slag Cements samples (PSC) in a proportion of 25 wt%/75 wt% (modified slag/Portland cement), named PSC-M11 and PSC-M12.
Cement manufactured with steelmaking (PSC-SS) was also prepared using the same ratio (25%
slag/75% Portland cement), for comparison.
Compressive strength, according to Brazilian Standard NBR 11578 (analogous to European Standards EN 197-1, ISO EN 196-1 and ISO EN 196-3) and volume soundness (slag expansion), according to autoclave expansion test (standard ASTM C), were performed in PSC samples.
Results and Discussion
Table II presents the mineralogical phases of slag samples, determined by X-ray diffraction (XRD), adopting the Rietveld methodology for quantification.
Table II. Mineralogical phases (%) of Steel Slag and modified slags determined by XRD (Rietveld). NF = not found.
Phases SS SS-M11 SS-M12
C2S 2CaO.SiO2 35.6 58.6 56.3
Brownmillerite 2CaO.(Al,Fe)2O3 38.7
RO FeO, MgO, MnO, CaO 21.5 29.4 31.3
Lime CaO 4.2 NF NF
Merwinite 3CaO.MgO.2SiO2 NF 4.5 NF
Melilite
2CaO.MgO.2SiO2 +
2CaO.Al2O3.SiO2
NF NF
3.1
Diopsita CaO.MgO.SiO2 NF NF 5.1
Gehlenite 2CaO.Al2O3.SiO2 NF 4.2 NF
Others - NF 1.3 4.2
Amorphous - NF 0 0
As shown in Table II, the steel slag (SS) is mostly crystalline, because this type of slag has high basicity (CaO/SiO2 = 3.8) and a high content of iron oxides, which may act as nuclei for crystallization [13]. The crystalline phases are those typically observed in steelmaking slags [14,15,4,8]: brownmillerite (Ca2(Fe,Al)2O5), larnite (Ca2SiO4), RO phase (solid solution among FeO, MnO, MgO and CaO) and lime. The free lime content (5.7 wt% in Table I), is too high to prevent volume soundness. The chemical composition of the RO phase determined through EDS analysis at different points of microstructure of Steel Slag, revealed a high MgO/FeO molar ratio (0.81), which is, together with free MgO and CaO, a limitation for the utilization of steelmaking slag in cement. Qian et al. [6] reported that a higher MgO/FeO ratio in the RO phase increases potential reactivity with water, affecting the expansion of the material. According to their results, when the RO phase is a Mg-rich wustite it has the potential to react with water to produce brucite (Mg(OH)2).
As shown in Table I, a significant fraction of Fe3+ content in the slags SS-M11 and SS-M12 was reduced, causing an increasing of Fe2+ compared to the SS slag. Most of Fe2+ was incorporated in the RO phase causing a decrease in the MgO/FeO ratio, preventing volume soundness. The chemical composition determined by EDS analysis in several points of RO phase, revealed lower MgO/FeO ratios than in the Steel Slag (SS). This ratio was 0.50 and 0.47 for SS-M11 and SS- M12 respectively. This is an indication of Mg-poor wustite formation, which presents less potential to react with water and consequent expansion, according to Qian et al [6]. Slag SS-M11 was naturally cooled in the metallurgical reactor where the modification process occurred, whereas SS-M12 was cooled by steel balls.
The identification and quantification of C2S by Rietveld method requires a deeper analysis as this phase has polymorphs. Figure 1 presents the diffractograms of samples SS-M11 and SS-M12, showing the main crystalline phases. The XRD patterns of α’-C2S and Bredigite (7CaO.MgO.4SiO2) according to JCPDS cards numbers 33-3003 and 36-0399, respectively, are very similar and it is difficult to conclude whether the samples contain C2S as the alpha
polymorph or Bredigite. For SS-M12 slag the peak close to 233º is significantly smaller than for SS-M11, which is an indication that the α’-C2S or Bredigite is present in less quantity in the slag that was cooled faster (SS-M12). Beta polymorph (β-C2S) was identified in both slags by the JCPDS card number 33-0302.
Figure 1. XRD patterns of samples (a) SS-M11 e (b) SS-M12
Figure 2 shows examples of the microstructure of SS-M11 and SS-M12, and Table III presents the chemical composition measured by EDS of some indicated points.
Figure 2. Microstructures of modified slag SS-M11 (naturally cooled) and SS-M12 (cooled by steel balls). Ge=Gehlenite, possible Br=Bredigite.
Table III – Chemical composition determined by EDS in the points indicated in Figure 2.
Points MgO Al2O3 SiO2 CaO TiO2 MnO FeO
Possible Br-SS-M11 2.56 0.0 33.4 61.2 0.0 0.9 2.0
RO-SS-M11 18.4 0.0 1.5 0.9 0.0 11.8 67.5
Ge-SS-M11 0.0 23.6 19.0 44.6 4.0 1.0 7.6
β-C2S-SS-M11 0.0 0.0 31.9 66.7 0.0 0.0 1.3
RO-SS-M12 20.2 0.0 0.0 0.7 0.0 11.7 67.4
(a)SS-M11
β-C2S
RO RO
Br Ge
β-C2S
(b)SS-M12
β-C2S-SS-M12 0.3 0.0 32.2 66.3 0.0 0.0 1.2 Raman spectroscopy analyses were performed in the same points indicated in Figures 2a and 2b.
These analyses give further information for validation of XRD data and for differentiate C2S polymorphs. Figure 3 shows the micrograph obtained by SEM and by optical microscopy (coupled to the Raman spectrograph) of the same region in SS-M11 slag.
Figure 3. (a) Microstructures of modified slag SS-M11 (naturally cooled): (a) by SEM and (b) by optical microscope acoppled to Raman spectrograph.
Figure 4 shows the Raman spectra of the phases presented in Figure 3 for SS-M11 slag. Figure 4a shows the typical peaks of Gehlenite: 240, 303, 626, 665 and 914 cm-1 [16], while the spectra of RO phase (Figure 4b) do not present any peak, possible due to the effect of fluorescence in phases with high amount of iron. Figure 4c presents the main Raman wavebands of the β-C2S phase showing the peak at 856 cm-1 of the highest intensity [17]. The last Raman spectra (Figure 4d) could not be properly identified as it was found only one peak of high intensity at 890 cm-1. This peak is dislocated 34 cm-1 in relation to the highest intensity peak of β-C2S (Figure 4c) showing this phase is not a -C2S. Bensted [18] reported the spectra of α’-C2S and attributed the main peaks of this phase at 828, 864 and 892 cm-1 all with high intensity, which differ from the spectra in Figure 4d. Unfortunately, the Raman spectra of Bredigite was not find in literature.
Although further studies are necessary to conclude that the pattern presented in Figure 4d could be from Bredigite, the EDS analysis indicates high amount of Mg (around 2.5 wt% of MgO) in this phase. Bredigite is a magnesium calcium silicate with stoichiometric composition of 58.3 CaO, 6.0 MgO, and 35.7 SiO2 (wt%), but Mg2+ can be substituted by other divalent cations in its structure such as Fe2+ and Mn2+.
The same study of EDS and Raman spectroscopy was carried out for the sample SS-M12 and the MgO content in the calcium silicates was smaller than 0.5 wt% which is inconsistent with the possible presence of Bredigite.
The presence of a higher intensity peak in 2=33.5º in the XRD and the presence of MgO in some calcium silicate particles are evidences of the presence of Bredigite in slag SS-M11. For slag SS-M12, the XRD pattern of Bredigite is found but the MgO content in the phase is very low. This is a possible indication of the presence of ’-C2S instead of Bredigite. Therefore, part of the calcium silicates in slag SS-M11 and SS-M12 are in the form of -C2S. The remaining is probably Bredigite for slag SS-M11 and ’-C2S for slag SS-M12.
RO
Ge
β-C2S Br
(a) (b)
Figure 4. Raman spectra of the phases indicated in figure 3.
The problem with Bredigite is that this phase shows low hydraulic reactivity when compared to β-C2S and α-C2S [19]. This would make the slag less proper to the use in cement manufacturing.
The hydration heat measured in NaOH aqueous solution is twice as high for SS-M12 as for SS- M11, as shown in Table IV. This difference can be caused by the difference in the mineral composition as discussed above and also by the C2S crystal size. The increase in cooling rate, by the presence of steel balls in slag SS-M12, caused a decrease of C2S crystals size, as shown in Figure 2.
Table IV. Heat generated in 72 h, for samples SS-M11 and SS-M12
SS PSC-M11 PSC-M12
Total Hydration Heat(J/g) ND 35 70
Table V presents the results for expansion determined according to autoclave test and compressive strength in ages of 3, 7, 28 and 91 days for cement samples produced with 75% of ordinary Cement and 25% of Steel Slag (PCS-SS) and with 75% of ordinary Cement and 25%
modified slags of (PCS-M11 and PSC-M12).
Cement samples produced by adding modified slags presented expansions 5 times lower than that determined for cement sample produced by adding steel slag without any modification. An increase of compressive strength in cement sample produced by adding SS-M12 slag was observed in comparison to PSC-M11. This is probably caused by a higher reactivity due to the presence of smaller C2S crystals and the absence of Bredigite formation. PCS-M12 sample has
(a) (b)
(c) (d)
also higher compressive strength than PCS-SS. This slag represents an improvement in volume soundness behavior and in development of compressive strength.
Table V. Expansion in autoclave and compressive strength in 3, 7, 28 and 91 days of cements samples produced with 25% of slag and 75% of ordinary cement.
PCS-SS PSC-M11 PSC-M12
Autoclave expansion (%) 0.44 0.07 0.08
R 3d (MPa) 27.0 29.4 29.8
R 7d (MPa) 34.1 34.0 35.4
R 28d (MPa) 41.1 40.7 42.4
R 91d (MPa) N/D 42.1 43.9
N/D: Not determined.
Conclusions
The main conclusions can be summarized as follows:
The partial reduction of Fe3+ to Fe2+ and a lowering in basicity from 3.9 to the range from 1.8 to 2.0 stabilized C2S and RO, with low MgO/FeO ratio, as the main crystalline phases of modified slags.
XRD, SEM-EDS and Raman spectroscopy analyses showed that the calcium silicate was part in the form of -C2S for modified slags. The remaining part was possibly in the form of Bredigite for the slag cooled naturally and in the form of ’-C2S for the slag cooled by steel balls.
The total hydration heat of the sample cooled by steel ball is twice as high as that of the sample cooled slowly, possible due the finer grain of β-C2S and the absence of Bredigite formation during solidification.
Cement samples produced by adding 25% of modified slag specially that cooled by steel balls, resulted in an increasing of 1,3 MPa (3,16%) of compressive strengths at 28 days compared with cement sample produced by adding Steel Slag. In addition, Steel Slag caused significant expansion in cement sample, whereas cements, produced by adding modified slags, presented values five times lower of volume soundness.
Acknowledgements
The authors acknowledge the financial support from InterCement and Embrapii.
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