Green Transforming Metallurgical Residue into Alkali-Activated Silicomanganese Slag-Based Cementitious Material as Photocatalyst

Silicomanganese slag is a solid waste in metallurgical industry and can be transformed into an alkali-activated silicomanganese slag-based cementitious-material (ASSC) for the first time. The ASSC shows quite low electro-conductivity and can be raised dramatically by incorporated carbon black (CB) in the matrix of ASSC to create an electro-conductive alkali-activated silicomanganese slag-based cementitious-composite (EASSC), served as a low cost and environmentally-friendly photocatalyst for the removal of dye pollutant in the paper. The interrelationships of mechanical, optical, electroconductive, microstructural, and photocatalytic properties are evaluated. The network of CB plays a critical role in the electron transfers. The electrical conductivity of EASSC doped 4.5% CB drastically increases by 594.2 times compared to that of ASSC. The FESEM, XRD, and XPS results indicated that the EASSC with mean grain size about 50 nm is composed of amorphous calcium silicate hydrate (CSH), alabandite (α-MnS) and CB. The UV–vis DRS and PL exhibit that the absorption edges of electro-conductive alkali-activated silicomanganese slag-based cementitious-composite EASSC samples are gradually blue-shifted and the photoluminescence intensities progressively decrease with increasing CB content. The activities of photocatalytic degradation of basic violet 5BN dye are positive correlated to the electro-conductivities. The separation efficiency of photo-generated electron-hole pairs is enhanced due to the electron transfers from α-MnS to the network of CB. The photocatalytic degradation of dye pollutant belongs to the second order kinetics via a reaction mechanism of superoxide radical (•O2−) intermediate.


Introduction
Much more attention has been focused on utilization of industrial by-products and wastes recycling due to the growing scarce resources and eco-environmental concerns. Silicomanganese slag (SS) is a by-product with higher manganese content generated by the smelting manganese ferroalloy plant for making manganese-steel pig-iron. In the recent three years from 2014 to 2016, the output of silicon-manganese alloy is about 35.7 million tons in China [1]. According to the statistics from the enterprises, about 1.2-1.3 tons silicon manganese slag will be discharged to produce per ton silicon-manganese alloy. A total of about 46.41 million tons of SS is emitted, but very few have been recycled owing to its low activity as a cementitious material [2]. With the rapid development of ferroalloy smelting industry in China, the emission of SS is gradually increased year by year and puts the pressures on the sustainable development of ecological environment and economy. Therefore, 31.5 × 31.5 × 50 mm 3 triplicate stainless steel mold by vibro-casting, and then four electrodes were inserted. The paste was sealed in a polyethylene film bag to cure at 80 • C for 6 h, and then kept at curing room for 18 h. After demolded, the paste sample was cured at curing room with a temperature of 20 • C and 95% relative humidity for different days. The sample incorporated 1.5 wt % carbon black was assigned as 1.5EASSC (Electroconductively Alkali-activated Silicomanganese Slag-based cementitious-composite by doped 1.5 wt % carbon black). According to similar experimental procedures, the samples doped 3.5 wt % and 4.5 wt % carbon black were prepared and designated as 3.5EASSC and 4.5EASSC, respectively. The electrical conductivity of samples was detected by four-electrode method. The EASSC sample was crushed and screened to obtain the photocatalyst with particle size distribution in the range of 0.16-0.315 mm.

Characterization of EASSC
Elemental analysis was carried out on an X-ray fluorescence (XRF) analyzer (S4 Pioneer, Bruker, Karlsruhe, Germany). Compressive strength was measured on a YAW-300 automatic pressure testing machine (YAW-300, Shaoxing, China) at a loading speed of 2.4 kN·s −1 . The morphology observation was performed on a field emission scanning electron microscopy (FESEM) (S-4800, Hitachi, Tokyo, Japan). X-ray diffraction patterns were measured on an X-ray diffractometer (D/MAX-2400, Rigaku, Tokyo, Japan) equipped with a rotation anode using a CuKα radiation with 0.02 • of 2θ step intervals and working electric current of 40 mA and voltage of 40 kV. UV-vis diffuse reflectance spectra (UV-vis DRS) were recorded on a UV-vis-NIR spectrophotometer (Cary 5000, Agilent, Santa Clara, CA, USA) equipped with an integrating sphere using BaSO 4 as a reference. Surface composition and chemical states were taken on an X-ray photoelectron spectroscopy (XPS) (AXIS SUPRA, Shimadzu, Tokyo, Japan) using AlKα (1486.6 eV) as an excitation source.

Evaluation of Photocatalytic Activity
The photocatalytic activities of samples were evaluated by degradation the basic violet 5BN dye, in which the initial absorbance A 0 was measured at the maximum absorption wavelength of 580 nm. 0.8 g of photocatalyst sample was put into the beaker that contains 100 mL aqueous solution of 4 mg·L −1 basic violet 5BN dye. After the adsorption-desorption equilibrium of dye molecules on solid sample achieved in the dark room, the solution suspending solid particles was irradiated by a UV-lamp (365 nm, 18 W) for different times under magnetic stirring at room temperature and the absorbance A t was detected. The photocatalytic degradation efficiency (PDE) of dye was calculated using Equation (1). Table 1 lists the oxide composition of SS and 4.5EASSC samples. The electrical conductivity at different cuing ages and the compressive strength at curing age of 28 days for all of samples are summarized in Table 2. It can be found from Table 2 that the electrical conductivity for each sample decreases slowly at curing ages of 3 days, 7 days, and 14 days, and the value approximately tends to be stable at curing age of 28 days, ascribing to that the ionic electrical conductivity gradually becomes stable when free water transforms into combined water during the hydration process of silicomanganese slag. The electrical conductivity is about 0.0005 (S·m −1 ) for the ASSC sample. When 1.5 wt % carbon black is incorporated into the ASSC, the stable electrical conductivity is about 0.0006 (S·m −1 ) for 1.5EASSC sample, 0.1155 (S·m −1 ) for 3.5EASSC sample and 0.2976 (S·m −1 ) for 4.5EASSC sample. The electrical conductivity increases by 0.20, 230, and 594.2 times compared to that of the ASSC sample at curing age of 28 days, respectively. The electrical conductivity of EASSC sample drastically increases with increasing of carbon black content, indicating that the carbon black particles overlap each other to form a cross-linking electro-conductive network, while carbon black conducts electric current in the plane of each covalently bonded sheet due to the delocalization of one outer electron in each atom to form π-cloud [11]. From Table 2, it can be observed that even though there is a progressive decrease for samples in compressive strengths from 45.6 MPa to 13.0 MPa, it meets the requirement of self-supporting strength of solid catalyst.

Interrelationship of Electrical Conductivity, Mechanical Strength and Microstructure
FESEM photographs of silicomanganese slag (SS), carbon black (CB), and 4.5EASSC samples are depicted in Figure 1. The irregular particles of SS sample vary in the size from about 1 µm to 10 µm. The spherical granules of carbon black in mean size of about 60 nm appear to be greatly aggregated. The morphology of 4.5EASSC sample consists of homogeneous particles in the size of about 50 nm and the microstructure is quite dense because the carbon black fills in the interstitial sites to decrease the porosity of ASSC. Generally, densification is one of the most significant methods to improve electrical conductivity of materials. Wagner et al. put forward a linear relationship to describe the porosity and electrical conductivity for polycrystalline graphite [12]. Some researchers developed models to formulate the relationship between porosity and electronic properties [13]. It can be found from Table 2 that the electrical conductivity for each sample decreases slowly at curing ages of 3 days, 7 days, and 14 days, and the value approximately tends to be stable at curing age of 28 days, ascribing to that the ionic electrical conductivity gradually becomes stable when free water transforms into combined water during the hydration process of silicomanganese slag. The electrical conductivity is about 0.0005 (S·m −1 ) for the ASSC sample. When 1.5 wt % carbon black is incorporated into the ASSC, the stable electrical conductivity is about 0.0006 (S·m −1 ) for 1.5EASSC sample, 0.1155 (S·m −1 ) for 3.5EASSC sample and 0.2976 (S·m −1 ) for 4.5EASSC sample. The electrical conductivity increases by 0.20, 230, and 594.2 times compared to that of the ASSC sample at curing age of 28 days, respectively. The electrical conductivity of EASSC sample drastically increases with increasing of carbon black content, indicating that the carbon black particles overlap each other to form a cross-linking electro-conductive network, while carbon black conducts electric current in the plane of each covalently bonded sheet due to the delocalization of one outer electron in each atom to form π-cloud [11]. From Table 2, it can be observed that even though there is a progressive decrease for samples in compressive strengths from 45.6 MPa to 13.0 MPa, it meets the requirement of self-supporting strength of solid catalyst. FESEM photographs of silicomanganese slag (SS), carbon black (CB), and 4.5EASSC samples are depicted in Figure 1. The irregular particles of SS sample vary in the size from about 1 μm to 10 μm. The spherical granules of carbon black in mean size of about 60 nm appear to be greatly aggregated. The morphology of 4.5EASSC sample consists of homogeneous particles in the size of about 50 nm and the microstructure is quite dense because the carbon black fills in the interstitial sites to decrease the porosity of ASSC. Generally, densification is one of the most significant methods to improve electrical conductivity of materials. Wagner et al. put forward a linear relationship to describe the porosity and electrical conductivity for polycrystalline graphite [12]. Some researchers developed models to formulate the relationship between porosity and electronic properties [13]. Figure 2 displays the XRD patterns of samples. The SS pattern appears a diffused wide band which extends from 20° to 38° and accompanies the hump at approximately about 30° of 2θ, in which the diffraction pattern has the characteristics of the short-range order of the C-S-A-M   [14]. The peaks placed at 34.19 • , 49.14 • , and 61.22 • of 2θ corresponding to the crystal faces of (200), (220), and (222) are identified as alabandite (α-MnS, JCPDS 72-1534). There are two peaks at 24.5 • and 43.6 • in the CB pattern belonging to the feature of graphitic microcrystalline structure [15,16]. When SS, CB and aqueous solution of NaOH are mixed under curing temperature of 80 • C for 6 h and then at curing room for 28 days, the amorphous SiO 2 , CaO and H 2 O react with NaOH to generate calcium silicate hydroxide (CSH) (Ca 5 (SiO 4 ) 2 (OH) 2 , JCPDS No. 84-0148) with the diffraction peak at 29.39 • of 2θ in the patterns of 1.5EASSC, 3.5EASSC, and 4.5EASSC samples, while the spherical granules of carbon black are evenly dispersed in the phase of C-S-H and concatenate each other into electro-conductive network to exhibit excellent electrical conductivity, as described in Table 2.  Figure 3 shows the high resolution C1s XPS spectrum of 4.5EASSC sample. The electron binding energy of C1s in carbon black from 280 eV to 295 eV, which is composed of six kinds of chemical oxygen-containing functional groups, corresponding to C-C at 284.6 eV, C-OH at 286.1 eV, C-O at 286.5 eV, C=O at 287.6 eV, O-C=O at 288.1 eV, and COOH at 289.6 eV [17]. The formations of different carbon-oxygen bonds probably results from two main reasons, one is the carbon black is exposed to air to form some carbon-oxygen groups, the other is the granular surface of carbon black is corroded by alkaline solution of activator and then is oxidized by air during the long curing time of 28 days.    Figure 3 shows the high resolution C1s XPS spectrum of 4.5EASSC sample. The electron binding energy of C1s in carbon black from 280 eV to 295 eV, which is composed of six kinds of chemical oxygen-containing functional groups, corresponding to C-C at 284.6 eV, C-OH at 286.1 eV, C-O at 286.5 eV, C=O at 287.6 eV, O-C=O at 288.1 eV, and COOH at 289.6 eV [17]. The formations of different carbon-oxygen bonds probably results from two main reasons, one is the carbon black is exposed to air to form some carbon-oxygen groups, the other is the granular surface of carbon black is corroded by alkaline solution of activator and then is oxidized by air during the long curing time of 28 days.  Figure 3 shows the high resolution C1s XPS spectrum of 4.5EASSC sample. The electron binding energy of C1s in carbon black from 280 eV to 295 eV, which is composed of six kinds of chemical oxygen-containing functional groups, corresponding to C-C at 284.6 eV, C-OH at 286.1 eV, C-O at 286.5 eV, C=O at 287.6 eV, O-C=O at 288.1 eV, and COOH at 289.6 eV [17]. The formations of different carbon-oxygen bonds probably results from two main reasons, one is the carbon black is exposed to air to form some carbon-oxygen groups, the other is the granular surface of carbon black is corroded by alkaline solution of activator and then is oxidized by air during the long curing time of 28 days.          Figure 5 depicts the photoluminescence spectra of samples. When the samples are excited by 280 nm wavelength from an emitter of xenon lamp, all samples appear at the biggest photo-fluorescence peak at 468 nm. The ASSC sample displays the strongest photoluminescence curve, demonstrating that the photo-generated electron and hole are easy recombination. The photoluminescence intensities for the samples doped carbon black is in the sequence of 1.5EASSC > 3.5EASSC > 4.5EASSC, implying that the photo-generated electron-hole pairs, created in the 4.5EASSC sample, are efficiently segregated, and the 4.5EASSC sample expects to have an excellent photocatalytic activity.  Figure 4. UV-vis diffuse reflectance spectra of specimens. Figure 5 depicts the photoluminescence spectra of samples. When the samples are excited by 280 nm wavelength from an emitter of xenon lamp, all samples appear at the biggest photo-fluorescence peak at 468 nm. The ASSC sample displays the strongest photoluminescence curve, demonstrating that the photo-generated electron and hole are easy recombination. The photoluminescence intensities for the samples doped carbon black is in the sequence of 1.5EASSC > 3.5EASSC > 4.5EASSC, implying that the photo-generated electron-hole pairs, created in the 4.5EASSC sample, are efficiently segregated, and the 4.5EASSC sample expects to have an excellent photocatalytic activity.   Figure 6b, manifesting that the photo-generated electron enable to transmit to the network of carbon black so that the photo-generated electron and hole pairs are efficiently separated and the photocatalytic degradation efficiency is improved.   Figure 6b, manifesting that the photo-generated electron enable to transmit to the network of carbon black so that the photo-generated electron and hole pairs are efficiently separated and the photocatalytic degradation efficiency is improved. The reaction kinetics equations and relative parameters for photocatalytic degradation of basic violet 5BN dye are summarized in Table 3. The correlation coefficients of R0 2 , R1 2 , R2 2 , and R3 2 corresponding to the zero, first, second, and third order kinetics equations. The correlation parameter is in the order of R2 2 > R3 2 > R1 2 > R0 2 , respectively, implying that the photocatalytic degradation of basic violet 5BN dye belongs to the second order kinetics. The 4.5EASSC catalyst displays the best correlation of 0.99147 and the smallest half-life of 13.01 min in Table 3. Generally, hydroxyl radical (•OH), superoxide radical (•O2 − ) and photo-generated hole (h + ) are considered to be the active species in the process of photocatalytic oxidization of dye [18,19]. EDTA-2Na is usually used as a scavenger of hole (h + ), tert-butyl alcohol (TBA) is served as a trapping agent of hydroxyl radical (•OH), and p-benzoquinone (BQ) is employed as a scavenger of superoxide radical (•O2 − ). From Figure 7 it is found that the degradation rate of basic violet 5BN dye over the 4.5EASSC is about 88.3% in the absence of free radical catching agent. When EDTA-2Na and TBA are separately dropped into the dye solution of basic violet 5BN suspending solid catalyst of 4.5EASSC, the degradation rates of dye slightly decrease by 0.3% and 1.8%, respectively, indicating that the active species for the degradation of dye is neither hole (h + ) nor hydroxyl radical (•OH). After BQ is added to the reaction system of dye solution for 100 min, it is found that the degradation rate of dye sharply decreases to 31.3%, manifesting that BQ has much better inhibition effect for the degradation of dye compared to EDTA-2Na and TBA. Therefore, it is deemed to that the superoxide radical (•O2 − ) plays a crucial role in photocatalytic degradation of basic violet 5BN dye. The reaction kinetics equations and relative parameters for photocatalytic degradation of basic violet 5BN dye are summarized in Table 3. The correlation coefficients of R 0 2 , R 1 2 , R 2 2 , and R 3 2 corresponding to the zero, first, second, and third order kinetics equations. The correlation parameter is in the order of R 2 2 > R 3 2 > R 1 2 > R 0 2 , respectively, implying that the photocatalytic degradation of basic violet 5BN dye belongs to the second order kinetics. The 4.5EASSC catalyst displays the best correlation of 0.99147 and the smallest half-life of 13.01 min in Table 3. are considered to be the active species in the process of photocatalytic oxidization of dye [18,19]. EDTA-2Na is usually used as a scavenger of hole (h + ), tert-butyl alcohol (TBA) is served as a trapping agent of hydroxyl radical (•OH), and p-benzoquinone (BQ) is employed as a scavenger of superoxide radical (•O 2 − ). From Figure 7 it is found that the degradation rate of basic violet 5BN dye over the 4.5EASSC is about 88.3% in the absence of free radical catching agent. When EDTA-2Na and TBA are separately dropped into the dye solution of basic violet 5BN suspending solid catalyst of 4.5EASSC, the degradation rates of dye slightly decrease by 0.3% and 1.8%, respectively, indicating that the active species for the degradation of dye is neither hole (h + ) nor hydroxyl radical (•OH). After BQ is added to the reaction system of dye solution for 100 min, it is found that the degradation rate of dye sharply decreases to 31.3%, manifesting that BQ has much better inhibition effect for the degradation of dye compared to EDTA-2Na and TBA. Therefore, it is deemed to that the superoxide radical (•O 2 − ) plays a crucial role in photocatalytic degradation of basic violet 5BN dye. Figure 8 shows the schematic mechanism of the photocatalytic degradation of dye over the EASSC sample. The covalent Si-O-Si and Si-O-Al bonds in the structure of SS powders are broken to generate the monomers of sodium orthosilicate Na + [SiO(OH) 3 ] − and sodium orthoaluminate Na + [OAl − (OH) 3 ] − by the alkaline activation of the aqueous NaOH solution, and then a new cementitious network structure is rebuilt by poly-condensation among monomers under the alkaline condition. The carbon black particles are simultaneously self-assembled to form the cross-linked electric-conduction networks in the interstitial sites of colloidal material, and α-MnS semiconductor disperse in the network of colloidal material. When the 4.5EASSC sample is irradiated by UV light, the electron in the valence band of α-MnS semiconductor absorbs the energy of photon which is bigger than band gap energy of 2.53 eV, and the electron jumps from the valence band to conduction band to generate both photo-induced electron (e − ) and hole (h + ). The positions of conduction and valence bands of α-MnS are located at −1.34 V and 1.19 V vs. NHE [20]. It is more negative than the potential of O 2 /•O 2 − (−0.33 V), which indicated that the photo-excited electrons on the conduction band could reduce the adsorbed oxygen molecules to produce superoxide radical (   Figure 8 shows the schematic mechanism of the photocatalytic degradation of dye over the EASSC sample. The covalent Si-O-Si and Si-O-Al bonds in the structure of SS powders are broken to generate the monomers of sodium orthosilicate Na + [SiO(OH)3] − and sodium orthoaluminate Na + [OAl − (OH)3] − by the alkaline activation of the aqueous NaOH solution, and then a new cementitious network structure is rebuilt by poly-condensation among monomers under the alkaline condition. The carbon black particles are simultaneously self-assembled to form the cross-linked electric-conduction networks in the interstitial sites of colloidal material, and α-MnS semiconductor disperse in the network of colloidal material. When the 4.5EASSC sample is irradiated by UV light, the electron in the valence band of α-MnS semiconductor absorbs the energy of photon which is bigger than band gap energy of 2.53 eV, and the electron jumps from the valence band to conduction band to generate both photo-induced electron (e − ) and hole (h + ). The positions of conduction and valence bands of α-MnS are located at −1.34 V and 1.19 V vs. NHE [20]. It is more negative than the potential of O2/•O2 − (−0.33 V), which indicated that the photo-excited electrons on the conduction band could reduce the adsorbed oxygen molecules to produce superoxide radical (•O2 − ). However, the photo-induced holes on the valence band cannot oxidize the adsorbed H2O molecules to form hydroxyl radical (•OH) because the valence band potential is much lower than the potential of •OH/H2O [21].   Figure 8 shows the schematic mechanism of the photocatalytic degradation of dye over the EASSC sample. The covalent Si-O-Si and Si-O-Al bonds in the structure of SS powders are broken to generate the monomers of sodium orthosilicate Na + [SiO(OH)3] − and sodium orthoaluminate Na + [OAl − (OH)3] − by the alkaline activation of the aqueous NaOH solution, and then a new cementitious network structure is rebuilt by poly-condensation among monomers under the alkaline condition. The carbon black particles are simultaneously self-assembled to form the cross-linked electric-conduction networks in the interstitial sites of colloidal material, and α-MnS semiconductor disperse in the network of colloidal material. When the 4.5EASSC sample is irradiated by UV light, the electron in the valence band of α-MnS semiconductor absorbs the energy of photon which is bigger than band gap energy of 2.53 eV, and the electron jumps from the valence band to conduction band to generate both photo-induced electron (e − ) and hole (h + ). The positions of conduction and valence bands of α-MnS are located at −1.34 V and 1.19 V vs. NHE [20]. It is more negative than the potential of O2/•O2 − (−0.33 V), which indicated that the photo-excited electrons on the conduction band could reduce the adsorbed oxygen molecules to produce superoxide radical (•O2 − ). However, the photo-induced holes on the valence band cannot oxidize the adsorbed H2O molecules to form hydroxyl radical (•OH) because the valence band potential is much lower than the potential of •OH/H2O [21].

Conclusions
A series of electro-conductively alkali-activated silicomanganese slag-based cementitiouscomposites were synthesized. The electrical conductivity of EASSC samples drastically increased and the compressive strength progressive decreased with the increasing of carbon black content. The EASSC with mean grain size about 50 nm consists of (CSH), alabandite (α-MnS), and carbon black.
The absorption edges of EASSC samples were gradually blue shifted with increasing carbon black content due to the literal darkening of samples, while the photo-fluorescence peak gradually decreased with increasing carbon black content due to the efficient separation of photo-induced electron-hole (e − /h + ) pairs. The photocatalytic degradation efficiency of basic violet 5BN dye increased with the increasing electronic conductivities of EASSC samples, implying that the photo-generated electrons quickly transfer to the network of carbon black so that the photo-generated electrons and holes were efficiently separated and the photocatalytic degradation rate was enhanced.

Conflicts of Interest:
The authors declare no conflicts of interest.