2024 Volume 64 Issue 6 Pages 1089-1100
During the slow cooling process of steel slag, the crystals of inert mineral phases (C2F, RO phases) and active mineral phases (C2S, C3S) adhere and grow, and the irregular interlocking, embedding, filling, stacking, and coating between mineral phases seriously affect the hydration activity of active mineral phases in steel slag. Therefore, this article selects glycerol (C3H8O3) as a grinding aid to mechanically excite steel slag, exploring the mineral phase evolution and hydration activity excitation mechanism of steel slag under different process parameters. C3H8O3 mechanical excitation refined the steel slag particles, increased porosity, increased specific surface area, and caused peeling behavior between the rough surface active mineral phases (C2S, C3S) of the steel slag particles and the smooth surface inert mineral phase C2F with sharp angular protrusions. When the addition amount of C3H8O3 is 0.24 wt% and the ball milling time is 90 minutes, the mechanical excitation effect of steel slag is the best. The total mass fraction of C2S and C3S increases by 14.3 wt%, while the mass fraction of C2F decreases by 19.3 wt%. The mechanical excitation of C3H8O3 can cause the steel slag to germinate cracks at the interface of each phase, and a porous honeycomb structure composed of calcium hydroxide (CH) and calcium silicon hydrogel (C-S-H) appears during the hydration process, producing a large number of acicular ettringite (AFt), effectively improving the early hydration activity of steel slag.
As a byproduct in the steelmaking process, steel slag commonly contains mineral phases such as tricalcium silicate (Ca3SiO5, C3S), dicalcium silicate (Ca2SiO4, C2S), tetracalcium ferroaluminate (Ca8Fe4Al4O20, C4AF), iron oxide (Ca2Fe2O5, C2F), RO phase (FeO–MnO–MgO solid solution), and free calcium oxide (f-CaO). The presence of C3S and C2S endows steel slag with potential cementitious activity,1,2) and the chemical and mineral phase composition of steel slag is similar to that of cement clinker, making it largely suitable as a supplementary cementitious material. Therefore, the use of steel slag as a substitute for cement or as a cement admixture in different cementitious systems is one of the main ways of resource utilization of steel slag.3,4,5,6) However, the content of active mineral phases C2S and C3S in steel slag is relatively low, and after slow cooling treatment, the crystal of the mineral phase is dense, and the crystal is coarse and complete.7,8,9) The crystal growth of the inert mineral phase (C2F, RO are equal) and the active mineral phase (C2S, C3S) is dependent on each other, mainly manifested by irregular interlocking, embedding, filling, stacking, and coating between the mineral phases, which seriously affects the hydration activity of C2S and C3S in steel slag.10,11,12,13) At the same time, the addition of steel slag in Portland cement increases the concentration of Ca2+ in the pore solution, thereby inhibiting the nucleation and growth of C-S-H formed by the hydration of C2S and C3S, hindering the initial setting of cement slurry, and thereby reducing the early strength of cement mortar.14,15,16) Therefore, increasing the content of C2S and C3S in the active mineral phase to increase the hydration product C-S-H and peel off the encapsulation behavior of inert ore relative to the active mineral phase is an effective means to enhance the hydration activity of steel slag.
The main methods for stimulating the hydration activity of steel slag include high temperature excitation,17) chemical excitation,18) and mechanical excitation.19,20) Among them, mechanical excitation is the most typical excitation method due to its simplified process and low energy consumption. Grinding aids, as strong polar surfactants, have strong adsorption capacity and can adsorb on material surfaces and microcracks, promoting crack propagation and achieving efficient separation and disintegration.21,22,23,24) Glycerol (C3H8O3), as a common grinding aid, can improve the particle size distribution, specific surface area, and mechanical strength of Portland cement clinker and slag.25,26,27) At present, research on mechanical excitation of steel slag hydration activity mostly focuses on optimizing process parameters and exploring the influence of steel slag on the performance of cement hardened slurry. However, there are few reports on the mineral phase evolution behavior and hydration activity enhancement mechanism of steel slag during mechanical excitation. Therefore, in this work, glycerol was selected as a grinding aid to mechanically excite steel slag, and the mineral phase evolution behavior and hydration activity excitation mechanism of steel slag under different ball milling parameters (ball milling time, grinding aid addition amount) were explored. The structure-activity relationship between “process parameters-mineral phase evolution-hydration activity” was established, providing a theoretical and technical basis for the comprehensive utilization of steel slag as a resource.
Using analytically pure (>98.0%) glycerol produced by Tianjin Hengxing Chemical Reagent Manufacturing and block converter steel slag collected from a steel plant in China as raw materials. The obtained blocky converter steel slag is roughly crushed using a sealed crusher (6J-2B) every ten minutes, and the particle size distribution range of the crushed slag is 0.27–35.30 μm, and dry in an oven (DGG-9140B) at 105°C for 12 hours for future experimental use.
Table 1 shows the chemical composition of the raw steel slag measured by X-ray fluorescence spectrometer (XRF). The results show that the main oxides in the raw steel slag include CaO, Fe2O3, SiO2, MgO and Al2O3, with mass fractions of 45.05%, 29.04%, 11.79%, 6.24% and 1.94%, respectively. The trace oxides include SO3, P2O5, MnO and TiO2, its binary alkalinity R=(WCaO/WSiO2)=3.82.
| Oxide | CaO | Fe2O3 | SiO2 | MgO | Al2O3 | MnO | P2O5 | SO3 | TiO2 | others |
|---|---|---|---|---|---|---|---|---|---|---|
| Composition (wt%) | 45.05 | 31.04 | 11.79 | 6.24 | 1.99 | 1.26 | 1.07 | 0.63 | 0.31 | 2.62 |
Figure 1 shows the XRD diffraction pattern of the raw steel slag, combined with MDI Jade 9 and HighScore Plus 4.9 analysis software and COD 2021 database for mineral phase composition analysis. The mineral phases in raw steel slag mainly include Dicalcium Silicate (Ca2SiO4, C2S), Tricalcium Silicate (Ca3SiO5, C3S), Iron Calcium phase (Ca2Fe2O5, C2F), f-MgO, f-CaO, CaCO3, Ca(OH)2 and RO(solid solution of MgO, FeO and MnO), etc.
Ball milling of steel slag using a Planetary Ball Mill (XQM-4). Set the mass proportion of C3H8O3 and raw steel slag to be 0 wt%, 0.08 wt%, 0.16 wt%, 0.24 wt%, 0.32 wt%, 0.40 wt%, anhydrous ethanol as dispersant, agate ball (weight ratio is 10 mm:8 mm:6 mm:2 mm = 1:4:6:2) as the ball milling medium, the ball material ratio is 5:1, the rotating speed is 300 r/min, the ball milling time is 60 min, 90 min, 120 min. After the ball milling, the milled steel slag was placed in a drying oven at 105°C for 6 h, and then conduct further detection and analysis.
The steel slag is made into a clean slurry (L/S=0.3) test block according to the GB/T1346-2001 standard and carry out hydration curing. After demolding the clean slurry test block, it is placed in a constant temperature water bath at (20±1)°C for curing, and the changes in the hydration mineral phase at 1 day, 7 days, 28 days, and 90 days are measured. The experimental process is shown in Fig. 2.
The chemical composition of raw steel slag was analyzed by ARLAdvantX Intellipower TM3600 X-ray fluorescence spectrometer (XRF) produced by Thermo Fisher Corporation of the United States. The D8 ADVANCE X-ray diffractometer produced by German Brooke Company is used to analyze the mineral phase of the sample, the working conditions is Cu–Ka, the tube voltage is 40 kV, the tube current is 100 mA, the scanning angle is 10–80°, and the scanning speed is 10°/min. The microstructural morphology and element distribution of the samples were observed with a SU8020 field emission scanning electron microscope produced by Hitachi Company of Japan. The special functional groups of the samples were analyzed by Nicolet IS 10 Fourier Transform Infrared Spectrometer (FTIR) produced by Nicolet of the United States. The microscopic morphology and crystal structure were analyzed with a Tecnai G2 F20 S-TWIN transmission electron microscope produced by FEI Company of the United States. The particle size distribution of the coarse samples was measured in the range of 0.02–2000 μm with a Malvern Mastersizer 2000 laser particle size analyzer produced by Malvern, UK. The ASAP 3020 specific physical adsorption analyzer produced by American Mike Company was used to carry out the nitrogen adsorption and desorption test of the raw steel slag and the steel slag after ball milling to characterize the pore structure of the powder, and analyze and calculate the BET surface area value, pore size distribution curve, and pore size distribution curve. Volume value, average pore size value and peak pore size. The hydration weight loss of raw slag and steel slag after ball milling was analyzed by NETZSCH STA 449 F5 Jupiter synchronous thermal analyzer of German NETZSCH company.
Figure 3 shows the XRD diffraction patterns of the raw steel slag and the steel slag treated with different ball milling parameters (ball milling time, amount of grinding aid added). From the graph, it can be seen that the characteristic peaks of steel slag after mechanical excitation are higher than those of the raw steel slag, and under the same ball milling time, with the increase of grinding aid C3H8O3, the characteristic diffraction peaks in steel slag show a trend of first increasing and then decreasing. To further analyze the effect of mechanical excitation on the active mineral phase of steel slag, the characteristic peaks with the largest diffraction intensity of C2S and C3S were used to evaluate the optimal results of the active ore facies before and after ball milling, and the XRD pattern of diffraction angle 2θ in the range of 31.6°–32.5° was locally amplified, as shown in Figs. 3(b), 3(d) and 3(f). It can be seen from the results that the characteristic peak intensities of the active ore phase reached the maximum value when the C3H8O3 addition was 0.08 wt% ball milling time for 60 min, the C3H8O3 addition was 0.24 wt% ball milling time for 90 min, and the C3H8O3 addition was 0.32 wt% ball milling time for 120 min, corresponding to I=612.71 (2θ=32.11°), I=665.09 (2θ=32.03°) and I=591.10 (2θ=32.09°), respectively. Compared with the raw steel slag, the maximum intensity of the diffraction peaks of C2S and C3S is 285.94 at 2θ=32.16°. When the C3H8O3 addition amount was 0.24 wt%, and the ball milling time was 90 min, the diffraction intensity of the strongest characteristic peaks of C2S and C3S in the steel slag was the largest, which was 379.07 higher than that of the raw steel slag.
In order to further determine the content of active ore phase, the raw steel slag and the steel slag milled for 90 min with 0.24 wt% C3H8O3 were quantitatively analyzed, as shown in Fig. 4. The calculated pattern obtained from the refinement is shown as a black line, the violet line depicts the profile difference. The calculated Bragg positions of the refined phases are marked with vertical ticks, and the weighted averages Rwp are all below 10, it means that the results are reliable, and the mass fraction of each mineral phase is refined and refined, as shown in Table 2. It can be seen from the results that compared with the raw steel slag, the sum of the mass fractions of active ore phase C2S and C3S increased from 37.3 wt% to 51.6 wt% after mechanical excitation of steel slag, an increase of 14.3 wt%. The mass fraction of inert mineral phase C2F decreased from 38.3 wt% to 19.0 wt%, a decrease of 19.3 wt%. The Ca(OH)2, f-MgO, RO phase, CaCO3 and f-CaO showed slight changes.
| Mineral phase | Space group | Raw steel slag (wt%) | Steel slag with C3H8O3 addition of 0.24 wt% and ball milling time of 90 min (wt%) |
|---|---|---|---|
| Ca2Fe2O5 | Pnma | 38.3 | 19.0 |
| Ca2SiO4 | P1 | 2.5 | 20.6 |
| Ca3SiO5 | P-1 | 34.8 | 31.0 |
| Ca(OH)2 | P-3m1 | 1.1 | 1.3 |
| MgO | Fm-3m | 12.2 | 14.1 |
| CaO | Fm-3m | 0.4 | 0.3 |
| RO | P4/mmm | 10.7 | 13.6 |
Figure 5 shows the FTIR spectrum of raw steel slag and the steel slag milled for 90 min with 0.24 wt% C3H8O3. The vibrational peaks of the raw steel slag in the region of 800–1100 cm−1 correspond to the asymmetric and symmetrical vibrational peak bands of the Si–O bond in the [SiO4] of C2S.28) Under the action of mechanical excitation of C3H8O3, the internal defects of the crystal plane of C2S ore phase were increased, and the crystal structure was distorted, which increased the wavenumber of Si–O bond vibration peak band from 875 cm−1 and 904 cm−1 to 909 cm−1 and 985 cm−1, respectively. The spectral vibration interval is 1422–1440 cm−1 corresponding to the asymmetric tensile vibration of [CO32−] in the amorphous CaCO3,29) and the wave number of the raw steel slag and the steel slag milled for 90 min with 0.24 wt% C3H8O3 is the same, indicating that the mechanical excitation has no effect on the [CO32−] asymmetric tensile vibration in the steel slag under the ball mill parameters. The wavelength range of 3200–3650 cm−1 corresponds to the stretching vibration of favorable [OH−] groups in Ca(OH)2.30) After mechanical excitation, the peak of steel slag disappears at 3430 cm−1. This is due to the binding phenomenon of C3H8O3 and the formation of micelles by surfactants, which reduces and widens the stretching vibration peak of [OH−] groups and then stabilizes, but has no effect on the stretching vibration peak bands of [OH−] groups at high wavelength ranges of 3646 cm−1 and 3694 cm−1.
Figure 6 shows the microstructure of the raw steel slag and its EDS surface scan and spot scan analysis, and Fig. 6(a) shows the typical morphology of the C3S and C2S ore facies in which the rod particles and spherical particles are stacked on top of each other. The spherical particles in Fig. 6(b) were analyzed by surface scanning, as shown in Fig (d)–(g). As can be seen from the figure, Ca, Si and O are concentrated in the spherical region and together constitute the silicate phase, and the Fe element is uniformly distributed in the spherical and rod particles. To further analyze the mineral composition, EDS point scan analysis was performed on Point 1 in Fig. 6(b), as shown in Fig. 6(c) and the inserted table. The globular C3S particles contained Fe, Ca, Si, O and Mg, and their molar ratio was 27.78:13.41:2.95:53.94:1.92. The molar ratio of Ca and Si was 4.55:1, which was higher than the calcium-silicon ratio in C3S and C2S, and the molar content of Fe reached 27.78 wt%, combined with the XRD analysis in Fig. 1, it was determined that Fe and Ca in the raw steel slag constituted C2F, indicating that a large amount of C2F was stacked and wrapped on the surface of C2S and C3S ore phases.
Figure 7 shows the microstructure and EDS analysis of the steel slag milled for 90 min with 0.24 wt% C3H8O3. From Fig. 7(a), it can be seen that the mechanical excitation of C3H8O3 effectively refines the steel slag particles, which is manifested by the dispersion and attachment of the fine particle size around the large particles. Figure 7(b) shows the local enlargement of the X region of Fig. 7(a), from the results, it can be seen that the steel slag particles appear to have a combination of surface uneven morphology and smooth surface with incidental sharp-edged morphology, combined with the EDS mapping analysis of Figs. 7(c)–7(f), it can be seen that the distribution of Fe and Si elements on the surface of the particles shows obvious separation characteristics, and Ca and O elements are concentrated on the surface of the particles. In order to further determine the composition of the mineral phase, EDS point analysis was carried out on the uneven surface morphology in Fig. 7(b), and it can be seen from the point analysis results of Fig. 7(g) Point 1 that the mass ratios of Ca, Si and O elements are 26.12:10.62:63.12, and the atomic weight ratios of Ca, Si and O elements are 13.08:7.58:79.34, respectively, and the calcium-silicon ratio is about 2:1, which can determine that the mineral phase at the surface uneven morphology is C2S; from the point analysis results of Fig. 7(h) Point 2, it can be seen that the mass ratio of Fe, Ca, Si and O elements is 34.70:18.35:4.09:42.85, and the atomic weight ratio of Fe, Ca, Si and O elements is 15.92:11.73:3.73:68.62, and the iron-calcium ratio is about 1:1, it can be determined that the mineral phase at the surface smooth with sharp angular convex morphology is C2F.
Combined with the XRD diffraction pattern in Fig. 3 and the FTIR spectrum in Fig. 5, the mechanical excitation of C3H8O3 causes the peeling behavior between the active mineral phase (C2S, C3S) with uneven surface and the inert mineral phase (C2F) with sharp angular protrusions on the surface, thereby increasing the hydration contact area of steel slag.
3.1.3. Particle Size and Pore StructureFigure 8 shows the particle size parameters of raw steel slag and the steel slag milled for 90 min with 0.24 wt% C3H8O3. The results show that the particle size distribution intervals of raw steel slag and the steel slag milled for 90 min with 0.24 wt% C3H8O3, respectively, are 0.27–35.30 μm and 0.28–56.36 μm. D10 was 1.70 μm and 0.68 μm, respectively; D50 was 9.39 μm and 7.85 μm, respectively; D90 was 24.80 μm and 24.21 μm, respectively; the average particle sizes were 11.60 μm and 10.43 μm, respectively. Combined with the microscopic morphology analysis of SEM, it can be concluded that the particle size distribution range of steel slag becomes larger after mechanical excitation, and the median particle size D50 and average particle size are reduced by 1.54 μm and 1.17 μm, respectively, which can effectively break the steel slag and achieve particle refinement.
Figure 9(a) shows the N2 adsorption and desorption test curve of raw steel slag and the steel slag milled for 90 min with 0.24 wt% C3H8O3. From the results, it can be seen that the adsorption behavior of raw steel slag and the steel slag milled for 90 min with 0.24 wt% C3H8O3, in line with the typical multilayer BET adsorption curve, under the same pressure, the N2 adsorption capacity of the steel slag sample after the grinding aid ball milling is greater than that of the raw steel slag (5.76 cm3/g and 2.62 cm3/g at P/P0=1, respectively). The N2 adsorption and desorption curves of raw steel slag and the steel slag milled for 90 min with 0.24 wt% C3H8O3 did not completely overlap, and the phenomenon of “desorption hysteresis” appeared when the P/P0 value was greater than 0.8.
Figure 9(b) shows the pore size distribution curve calculated by the BJH model of N2 adsorption desorption curve, SBET (BET surface area value), DA (average pore size value) and DP (pore size peak). From the results, it can be seen that the specific surface area of the steel slag milled for 90 min with 0.24 wt% C3H8O3 is larger than that of the raw steel slag (1.152 m2/g and 0.618 m2/g, respectively), and combined with the results of XRD and SEM-EDS tests (as shown in Figs. 4, 6 and 7), the mechanical excitation of C3H8O3 causes the active and inert mineral phases in steel slag to peel off, resulting in an increase in specific surface area. The raw steel slag and mechanically excited steel slag exhibit concentrated pore size distribution at 2.241 nm and 20.226 nm, respectively, and the peak value of the concentrated distribution of mechanically excited steel slag is much higher than that of the raw steel slag, indicating that the peak pore size of mechanically excited steel slag is much higher than that of the raw steel slag. Meanwhile, the average pore size of steel slag after mechanical excitation is smaller than that of the raw steel slag (9.296 nm and 14.225 nm, respectively), while the pore volume is larger than that of the raw steel slag (0.0029 cm3/g and 0.0015 cm3/g, respectively). After mechanical excitation, a large number of small mesopores are formed in the particles of steel slag. By comparing the N2 adsorption desorption test and pore size distribution analysis before and after mechanical excitation of the raw steel slag, it can be confirmed that after mechanical excitation treatment with C3H8O3, the porosity of particles in the steel slag increases, the specific surface area increases, and thus the hydration activity of the steel slag is improved.
3.2. Hydration Properties 3.2.1. Phase CompositionFigure 10 is the XRD pattern of steel slag at different hydration times when the raw steel slag and the steel slag milled for 90 min with 0.24 wt% C3H8O3. The hydration products of C2S and C3S include calcium silicate hydrogel (C-S-H) and calcium hydroxide (Ca(OH)2, CH), because C-S-H is amorphous, and its characteristic peaks cannot be observed in the X-ray diffraction pattern. It can be seen from the results that as the hydration time increases from 7 days to 28 days, the diffraction peaks of each phase show a gradual decrease trend, and the characteristic peaks of C2S and C3S decrease particularly obviously. For example, the diffraction peak value of the raw steel slag at 2θ=32.09° decreased from I=641.62 at 7 days to I=254.01 at 28 days. When the steel slag milled for 90 min with 0.24 wt% C3H8O3, the diffraction peak value of the steel slag at 2θ=32.09° decreased from I=625.07 at 7 days to I=198.01 at 28 days. Further explanation shows that the hydration active mineral phases in steel slag are mainly C2S and C3S phases. In the hydration process, the C-S-H gel formed by the hydration of C2S and C3S is filled between the hydrated product and the no hydrated mineral phase, so that the system is continuously consolidated and dense, thus wrapping the rest of the mineral phase. Therefore, the characteristic peaks of C2F, RO, f-CaO, and CaCO3 also show a decreasing trend.
Figure 11 shows the microscopic morphology of steel slag at different hydration times. As shown in Figs. 11(a) and 11(d), after 1 day of hydration of the raw steel slag, there are still sharp and pointed protrusions on the surface and are relatively dense. When the steel slag milled for 90 min with 0.24 wt% C3H8O3, the steel slag particles are dispersed, and in Cracks germinated at various phase interfaces can promote the hydration effect inside the particles. It can be seen from the partial enlarged picture that the fine particles on the surface of steel slag have obvious hydration tendency after C3H8O3 mechanical excitation. It can be seen from Figs. 11(b) and 11(e) and their partial enlarged diagrams that a small amount of thin layer CH appeared after 7 days of hydration of the raw steel slag, while the steel slag milled for 90 min with 0.24 wt% C3H8O3 after 7 days of hydration, a porous honeycomb structure composed of thin layer CH and curled flakes C-S-H appeared. As the hydration time extended to 28 days, the number of needle shaped ettringite (AFt) on the surface of mechanically excited steel slag was significantly higher than that of the raw steel slag after 28 days of hydration, and the inert mineral phase C2F with a smooth surface and sharp angular protrusion morphology was still clearly visible, as shown in Figs. 11(c) and 11(f).
According to the different thermal decomposition intervals of steel slag and cement hydration by Huo et al.,9) Zhao et al.21) and Zhang et al.,25) C-S-H gel and AFt decompose between 50–300°C, at 400–500°C mainly removes the bound water of Ca(OH)2. In order to better study the hydration of steel slag before and after ball milling, the temperature range of the test is set at 20–650°C, and thermogravimetric analysis was performed on the steel slag before and after mechanical excitation, as shown in Fig. 12. When the raw steel slag and the steel slag milled for 90 min with 0.24 wt% C3H8O3, the thermal decomposition water loss rate of steel slag hydration 1d is very low, less than 0.5%. When the addition amount of C3H8O3 is 0.24 wt% and the ball milling time is 90 minutes, the water loss rates of steel slag hydration at 7d, 28d, and 90d are 3%, 10%, and 13.1%, respectively, while the water loss rates of raw steel slag hydration at 7d, 28d, and 90d are 1.6%, 3.7%, and 11.8%, respectively. The hydration weight of steel slag after mechanical excitation is significantly greater than that of the raw steel slag, and the increase in hydration products is greater than that of the raw steel slag. Therefore, mechanical excitation can significantly improve the hydration activity of steel slag.
Figure 13 shows the TEM test of raw steel slag and the analysis combined with Digital Micrograph. Figure 13(a) is the bright field morphology of the raw steel slag. It can be seen that the small particles of the raw steel slag are scattered and attached around the large particles, and the overall shape is amorphous. Figure 13(b) is the HRTEM diagram of the P1 region in Fig. 13(a). It can be seen from the diagram that the interface is tightly bonded and the crystal planes are staggered in different directions. Figure 13(c) is the crystal plane amplification diagram of the P2 region in Fig. 13(b). The crystal plane spacing is 3.05 Å corresponding to the (131) crystal plane of C2F. Figure 13(d) is the FFT electron diffraction pattern of Fig. 13(b), which shows a polycrystalline diffraction ring, indicating that this region is a diffraction superposition produced by the stacking of many small grains with different orientations. The distances from these diffraction rings to the center of the circle are 7.29 Å, 3.91 Å, 3.05 Å, 2.46 Å and 1.74 Å, respectively (Table 3). Figures 13(e) and 13(f) are the linear diagram and the intensity distribution of different diffraction rings converted by Digital Micrograph in Fig. 13(d). Compared with the standard COD card of calcium iron oxide based on the XRD identification results, it is found that the above crystal plane spacing corresponds to the {020}, {101}, {131}, {112} and {033} crystal plane family of Srebrodolskite (C2F, COD card: 01-071-2264) (crystal system: orthorhombic crystal system; space group: Pnma; a=5.425 Å, c=5.598 Å), so the mineral phase in the P1 region can be determined as the C2F phase.
| Rn | Measurement value/Å | Standard value/Å (Ca2Fe2O5) | hkl (Ca2Fe2O5) |
|---|---|---|---|
| R1 | 7.29 | 7.38 | {020} |
| R2 | 3.91 | 3.90 | {101} |
| R3 | 3.05 | 3.06 | {131} |
| R4 | 2.46 | 2.45 | {112} |
| R5 | 1.74 | 1.74 | {033} |
Figures 14 and 15 shows the TEM test of steel slag and the analysis combined with Digital Micrograph when the steel slag milled for 90 min with 0.24 wt% C3H8O3. Figure 14(a) shows the bright field morphology of steel slag after mechanical excitation, and Fig. 14(b) shows the HRTEM image of the P3 region in Fig. 14(a). Combining the two images, it can be seen that the steel slag after mechanical excitation exhibits obvious layering, interlacing, and stacking morphology. Figure 14(c) is an enlarged view of the crystal plane of the P4 region in Fig. 14(b), and the interplanar distances are 3.37 Å and 3.97 Å respectively, corresponding to the (020) crystal plane and (111) crystal plane of C2S. Figure 14(d) is the FFT electron diffraction pattern of Fig. 14(b), which also shows a polycrystalline diffraction ring, indicating that the region is a diffraction superposition generated by many fine grains stacked together with different orientations. The distances from these diffraction rings to the center of the circle are 3.97 Å, 3.17 Å, 2.56 Å, 2.41 Å and 1.97 Å, respectively (Table 4). Figures 14(e) and 14(f) are the linear diagram and the intensity distribution of different diffraction rings transformed by Digital Micrograph in Fig. 14(d), respectively. Compared with the standard COD cards of calcium iron oxide based on XRD identification results, it is found that the above interplanar spacings correspond to the {101}, {131}, {220}, {211} and {202} crystal face groups of Srebrodolskite (C2F, COD card: 01-071-2264) respectively (crystal system: orthorhombic system; space group: Pnma; a=5.425 Å, c=5.598 Å), and combined with the analysis of the interplanar spacing of P4, it can be obtained that the perovskite phase peeled off in the P3 region to expose the dicalcium silicate phase.
| Rn | Measurement value/Å | Standard value/Å (Ca2Fe2O5) | hkl (Ca2Fe2O5) |
|---|---|---|---|
| R1 | 3.97 | 3.90 | {101} |
| R2 | 3.17 | 3.06 | {131} |
| R3 | 2.56 | 2.55 | {220} |
| R4 | 2.41 | 2.41 | {211} |
| R5 | 1.97 | 1.95 | {202} |
Figure 15(b) shows the HRTEM image of the P5 region in the open field morphology of the steel slag in Fig. 15(a). It can be observed that after mechanical excitation, the steel slag exhibits a mosaic interface of two crystal phases, as indicated by the blue band in the figure. Figure 15(c) is the crystal plane amplification diagram of the P6 region in Fig. 15(b), and the crystal plane spacing is 3.07 Å corresponding to the (043) crystal plane of C3S. Figure 15(d) shows the FFT electron diffraction map of Fig. 15(b), again showing a polycrystalline diffraction ring, the distances from these diffraction rings to the center of the circle are 11.01 Å, 8.50 Å, 6.02 Å, 3.07 Å and 2.71 Å, respectively (Table 5). Figures 15(e) and 15(f) are respectively the polar coordinate linear diagram and the intensity distribution diagram of different diffraction rings converted by Digital Micrograph in Fig. 15(d), comparison with standard COD cards of calcium-silicon phase based on XRD identification results, it is found that the above interplanar spacings correspond to the {011}, {011}, {102}, {043} and {240} crystal plane families of Alite (C3S, COD card: 96-901-6126), respectively (crystal system: triclinic crystal system; space group: P-1; a=11.639 Å, c=14.172 Å). Combined with the interplanar spacing analysis of P6, it can be obtained that the P5 region is the exfoliated C3S phase.
| Rn | Measurement value/Å | Standard value/Å (Ca3SiO5) | hkl (Ca3SiO5) |
|---|---|---|---|
| R1 | 11.01 | 11.01 | {011} |
| R2 | 8.50 | 8.44 | {011} |
| R3 | 6.02 | 5.96 | {102} |
| R4 | 3.07 | 3.07 | {043} |
| R5 | 2.71 | 2.75 | {240} |
Figure 16 is a schematic diagram of the ball milling mechanism of steel slag adding C3H8O3. Combined with XRD and SEM-EDS analysis, it can be concluded that the dense surface of steel slag is mostly C2F phase, so the steel slag particles are simplified as surface dense spherical particles. Due to the continuous impact of steel slag particles during ball milling, molecular bonds and lattices are destroyed, resulting in cracking and particle size reduction. As the milling process proceeds, the resulting ionic bonds are broken, generating highly reactive positive and negative charges on the newly broken surface.24) Adhesion of individual particles due to van der Waals forces and electrostatic attraction, microplastic deformation at the particle contact interface leads to particle aggregation,31) causing them to adhere to the contact surface. Moreover, C3H8O3 has a strong adsorption capacity, which can be adsorbed on the surface of steel slag particles and micro-cracks to form an adsorption film, reduce the particle surface free energy to reduce the electrostatic force, promote the crack propagation. With the cracks caused by the collision caused by the mechanical excitation, the C2F phase is further shedding, and it is difficult for the ore phase separated and spalled under the action of C3H8O3 to reagglomerate, so as to improve the porosity and refine the steel slag particles. The stripping of inert facies C2F increased the relative mass of the active facies C2S and C3S, and the analysis of SEM and TG data of the steel slag at different hydration times before and after mechanical excitation further verified that mechanical excitation increased the hydration activity of the steel slag by increasing the content of the active ore phase.
In this work, polyhydric glycerol (C3H8O3) was used as a grinding aid to mechanically stimulate steel slag, and the mineral phase evolution behavior and hydration activity excitation mechanism of steel slag under different ball milling parameters (ball milling time, grinding aid addition) were studied. The structure-activity relationship between “process parameters-mineral phase evolution-hydration activity” was established to provide a theoretical and technical basis for the comprehensive utilization of steel slag resources. The specific conclusions are as follows:
(1) The mineral content of steel slag mechanically excited by different ball milling parameters indicates that, when the steel slag milled for 90 min with 0.24 wt% C3H8O3, the mechanical excitation effect of steel slag is the best. The sum of mass fractions of active mineral phase C2S and C3S increased from 37.3 wt% to 51.6 wt%, an increase of 14.3 wt%; the mass fraction of inert mineral phase C2F decreased from 38.3 wt% to 19.0 wt%, a decrease of 19.3 wt%.
(2) The C3H8O3 mechanical excitation refines the particle size, increases the porosity, and increases the specific surface area, the peeling behavior occurs between the active mineral phase (C2S, C3S) and the inert mineral phase (C2F).
(3) Mechanical excitation can cause steel slag to sprout cracks at multiple interfaces of different phases, and a porous honeycomb structure composed of hydration products CH and C-S-H appears during the hydration process, producing a large number of needle shaped AFt, making the structure of steel slag more dense.
(4) When the steel slag milled for 90 min with 0.24 wt% C3H8O3, the water loss rate of steel slag hydration curing for 7 days and 28 days is 1.4% and 6.3% higher than that of the raw steel slag, respectively, that is, the C3H8O3 mechanical excitation can effectively improve the early hydration activity of steel slag.
The work was supported by Innovative Research Group Project of Natural Science Foundation of Hebei Province (E2022209093), Central Guides Local Science and Technology Development Fund Projects (236Z3801G), Natural Science Foundation of Hebei Province (E2020209195), Science and Technology Project of Hebei Education Department (QN2021116), Science and Technology Project of Hebei Education Department (QN2022178), and the Graduate Student Innovation Fund of North China University of Science and Technology (CXZZSS2023067).
On behalf of all authors, the corresponding author states that there is no conflict of interest.
