Production of Mayenite Nanoparticles from the Toxic Cement Dust

vicinity of the factories 8 ） . The main environmental issue with the cement industry its emissions or more specific cement dust which is known as BY-PASS. The latter is mainly composed of oxides of calcium, silicon, aluminum, ferric, magnesium, and other impurities 9 ） . These impurities according to XRF analysis obtained from different cement factories are mainly chlorides, sulphates, carbon dioxide, and NOx. All of them, especially chlorides and sulphates, have made this dust an environmental problem that poses a threat to the health of everyone who works in the cement industry 10 ） . Early literature showed that the gradual specific risks due to cement plant emissions are very low con-cerning each of the health effects, Toxicological and cancer dangers produced by cement kiln-emitting contaminants 11 ） , however, these conclusions have been challenged. Like-wise, earlier studies concluded that Long-term cement dust exposure doesn ’ t contribute to increased morbidity of serious respiratory diseases if compared with several types of blue-collar work 12, 13 ） . On the other hand, there are clear and noticeable rela-Abstract: To overcome the key challenges associated with cement dust, such as inhalable size, toxic ions, and the existence of large quantities of useless materials, researchers investigated an innovative and unusual conversion of toxic cement dust into Mayenite nanoparticles. Mayenite is a natural structure that can be used as a filler in a variety of industrial applications. The formation of Mayenite nanoparticles was achieved through a thermal reaction at 1000 ℃ for 2 h between cement dust and aluminum oxide. Different techniques were used to characterize the synthesized Mayenite nanoparticles, revealing the formation of the target phase as well as the reduction of toxic ions present in cement dust. According to Scherrer’s equation, the crystallite size of bypass and synthesized Mayenite nanoparticles is 45 and 30 nm, respectively. Also, with the aid of TEM analysis, the particle size distribution of the produced Mayenite nanoparticles was found to be 27±7 nm. The toxic ions, especially chlorides and sulphates, were reduced by 86% and 50%, respectively, according to X-ray fluorescence results. These findings are important for the future use of Mayenite, 12CaO.7Al 2 O 3 (C12A7), nanoparticles formed from toxic cement dust recycling.


Introduction
The cement manufacturing process well-achieved by mixing a preheated mixture of limestone, clay, and sand as sources of calcium, aluminum, iron, and silica. These raw materials at very high temperatures converted to clinker which grinded with other additives to finally for the cement 1 . During the last few decades, the widespread cement industry has been reported all over the world, which has resulted in a significant, but temporary, increase in CO 2 emissions and cement dust or by-pass 2 4 . Also, Egypt showed an industrial revolution in the cement industry. From 1975 till now cement industries have increased from only four factories producing 4 million tons per year to more than 16 factories producing more than 46 million tons of cement per year 5 7 . Of course, due to rapidly increasing demands for cement, the government still supports the expansion in cement industries. Surprisingly, about 6 of particulate matter with a diameter of 10 microns or less PM10 centered in Cairo is caused by the cement industry and this percentage reaches 30 in the vicinity of the factories 8 . The main environmental issue with the cement industry its emissions or more specific cement dust which is known as BY-PASS. The latter is mainly composed of oxides of calcium, silicon, aluminum, ferric, magnesium, and other impurities 9 . These impurities according to XRF analysis obtained from different cement factories are mainly chlorides, sulphates, carbon dioxide, and NOx. All of them, especially chlorides and sulphates, have made this dust an environmental problem that poses a threat to the health of everyone who works in the cement industry 10 . Early literature showed that the gradual specific risks due to cement plant emissions are very low concerning each of the health effects, Toxicological and cancer dangers produced by cement kiln-emitting contaminants 11 , however, these conclusions have been challenged. Likewise, earlier studies concluded that Long-term cement dust exposure doesn t contribute to increased morbidity of serious respiratory diseases if compared with several types of blue-collar work 12,13 .
On the other hand, there are clear and noticeable rela-tions between exposure to cement dust, persistent deficiency of lung functions, and human respirational symptoms. Not only skin and mucous membranes of the eyes and respiratory system are irritated by exposure to cement dust, but also its adsorption in the respiratory tract increases the pH value that irritates the exposed mucous membrane 14,15 . Various efforts have been devoted to controlling these emissions to minimize their hazards. Different systems are used separately or in combination to control cement dust discharges including mechanical and dust collectors, electrostatic precipitators, and fabric filters 16 . Zimwara et al. reported that different air pollution control technologies are used for example wet-scrubbers, electrostatic precipitators, and flexible pulsed jet filters. A gas stream is used by wet scrubbers to absorb contaminants until they are sprayed with a liquid to collect cement dust 17 . The Mayenite nanoparticles are also known as a calciumaluminate system that is commonly shown as a portion of the CaO-SiO 2 -Al 2 O 3 ternary system and comprises five stable CaO/Al 2 O 3 phases. Using cement notation the phases are denoted C 3 A, C 12 A 7 , CA, CA 2 , and CA 6 from the limerich end towards alumina 18 20 . Different preparation techniques could be applied for the preparation of calcium-aluminate phases. The widely used techniques are hightemperature solid-state preparation, sol-gel preparation from bauxite or/and lime, and combustion with specific raw materials 21 25 . Both the desired end-product and its application control the choice of preparation technique. The CACs are primarily used as construction cement and concrete. The mixing of varying phases of calcium-aluminate1 and 1calcium-aluminate-ferrite11 are the main components of these kinds of cement. The manufacturing method of CACs requires high temperatures, whereas the raw materials, bauxites, and limestones are inserted into the top of a rotary kiln to react during the melting. The final products are then tapped into the bottom of the kiln and cooled 26 .
The phase formation of Calcium aluminates gathered attention in several studies. Williamson and Glasser reported that no specific phases are favorably produced when equiweight ratios from CaCO 3 and Al 2 O 3 mixtures are fired up to 120 h at 1045-1405 27 . An additional study was completed by Mohamed and Sharp at 1150-1400 using CaCO 3 /Al 2 O 3 mixtures to confirm the conclusions of Williamson and Glasser 28 . S. Iftekhar et al. performed experiments including a holding step at 900 to allow the decomposition of CaCO 3 and ejection of its carbon dioxide. The variance in holding times at 900 did not affect the compositions of the obtained phase. I.e., only a meta-stable orthorhombic/hexagonal phase CA is formed at 900 27 .
This paper focuses on the reduction of toxic species present in cement dust, as well as the probability of Mayenite nanoparticle formation from cement dust at 1000 , which is lower than the majority of previously recorded re-action temperatures 27,28 . The structural and morphological properties are studied using different techniques. Besides, the particle size distribution, crystallite size, and dislocation density are calculated.

Materials
Cement dust was used as received from cement factories and Al 2 O 3 was bought from Sigma Aldrich and applied as received without additional purifications.

Apparatus
A closed electrical furnace Thermolyne Benchtop Muffle Furnace; model: F47910; 1200 was used for both phase formation and cement dust purification. The samples were handled and heated in a specially made porcelain vessel, which was used as a reactor for the reaction and purification in an environmentally benign technique.

Measurements
Elemental analyses were observed using the X-ray Fluorescence XRF technique THERMO-WDXRF SPECTROM-ETER 39 KV 80 Ma . According to the chemical analysis obtained from XRF of cement dust samples, a stoichiometric reaction was set up between cement dust and Al 2 O 3 and heated in the previously mentioned reactor at elevated temperatures of nearly 1000 and hold at this temperature for nearly 2 h. X-ray diffraction XRD analysis was accomplished by the PANalytical X-ray diffractometer Empyrean using Cu Kα radiation wavelength 0.154045 nm at 40 kV accelerating voltage, 35 mA current, and 20 -70 scan range with 0.02 step scan. The average size of the crystallites, Dc, of the produced nanoparticles was calculated by the Scherer equation 29 .
Where β is the corrected full width at half maximum FWHM , λ and θ are the X-ray wavelength and diffraction angle. Scanning electron micrographs SEM were captured using Quanta FEG 250 Switzerland . Transmission Electron Microscopy TEM images were captured by the JEOL JEM 1010 TEM, which operates at 100 kV. SEM and TEM samples were prepared by dispersing targeted powder in alcohol and dropping them onto a carbon film supported on a copper grid.

Structural properties
The phase determination and the purity of all prepared samples were analyzed by the p-XRD technique. Figures  1A-1C illustrates the X-ray diffraction patterns of cement dust Bypass and Mayenite 12CaO.7Al 2 O 3 hereinafter C12A7 phase. Figure 1B showed Rietveld refinement of the XRD data to indicate the phase purity of the samples.
In the case of the as-received cement dust, it is seen welldefined peaks that matched with mixed oxides of these known as Maynite, Portlandite, and others card no 00-009-0413 30 . In the case of the newly formed phase of Mayenite, it is seen a single phase of calcium aluminate with definite peaks, and these peaks matched with the card no 00-009-0413 . All the peaks of the produced calcium aluminate are assigned to the body-centered cubic phase 31 . It was noticed that new peaks appeared which confirm the reaction between the aluminum oxide and the dust. The crystallite sizes of the received Bypass and produced Mayenite nanoparticles were determined using two ways; Scherrer s equation and the Willamson-Hall equation 32,33 . The value of crystallite size based on Scherrer s equation was found to be 45 and 30 nm for bypass and produced Mayenite nanoparticles, respectively. I.e, the average crystallite size of the as-received cement dust was 45 nm while the addition of aluminum oxide to the dust under the action of heat decreases the average crystallite size to 30 nm. Hence, the introduced aluminum oxide under the action of high temperature has a significant effect on the grain size. Under the reaction conditions and during the growth of the new cubic calcium aluminate phase 34,35 . The concentrations of chlorine and sulfur ions began to decrease with the increase in the reaction temperature, which promotes the crystal growth in calcium aluminate through the dissolution/precipitation process. Figure 1C  Where Dc is the average crystallite size in nm, ε is the strain and m is a correction factor m 1 . Figure 2 shows the plot of β cosθ versus sinθ for a Bypass and b produced Mayenite nanoparticles. The crystallite size and strain could be determined from the intercept with the y-axis and the slope, respectively, of the linear fitting of  Positive strain values, 0.215 , and 0.554 were obtained for bypass and produced Mayenite nanoparticles, respectively 37 . This means the Mayenite nanoparticles strain is increased relative to the cement dust strain. The induced strain is likely to be responsible for the growth of Mayenite nanoparticles of smaller size with broad XRD peaks.

Surface characterization
TEM images were measured to study the sizes and shapes of the bypass and synthesized Mayenite nanoparticles. The samples were agitated ultrasonically in distilled water for 15 minutes to avoid aggregation of the particles. TEM images for cement dust Bypass and Mayenite 12CaO.7Al 2 O 3 hereinafter C12A7 are shown in Figs. 3a and 3b. The cement dust nanoparticles do not have a definite shape. Agglomerated clusters are observed in Fig. 2a. While Mayenite 12CaO.7Al 2 O 3 hereinafter C12A7 nanoparticles synthesized at 1000 for 2 h are almost spherical nanoparticles combined with closely packed polygonal particles without agglomeration Fig. 3b . Figure  3c shows the particle size distribution of Mayenite nanoparticles. The particle size is varied from 14 to 42 nm with an average value of 27 nm. Such a result demonstrates the development of a definite shape with characteristic features that allow these Mayenite nanoparticles to be applied over the fine cement dust for which high environmental and human health hazard effects are well known.
SEM images were also used to study the surface morphologies of the investigated samples. Figures 4a and 4b show SEM images of cement dust Bypass and Mayenite C12A7 . Highly condensed and agglomerated nanoparticles were observed for cement dust Bypass as observed in Fig. 4a. While the SEM image of Mayenite C12A7 , Fig.  4b, showed a high distribution of small dense nanoparticles. As shown in the inset image in Fig. 4b, the Mayenite nanoparticles self-aggregate together to form a nanoporous structure.

X-ray Fluorescence XRF analysis
The XRF analysis is carried out and the obtained data  are presented in Table 1. Also, the values of Loss on Ignition LOI were obtained during the measurement of XRF and provided in Table 1. In XRF analyses we obtained the LOI by measuring the crucible weight. The samples are heated at 105 to remove adsorbed water and then at 1000 for 1 h to remove both organic matter and carbonate. XRF results revealed a reduction in toxic ions especially chlorides and sulphates by 86 and 50 , respectively. Also, XRF data of our produced Mayenite nanoparticles are matched well with the XRF of cement calcium aluminate phase that is used in Europe especially in Spain which enables the latter to be directly used in construction. It is well-known that the calcium aluminate phase has property over the Portland cement phase in its high hardness and is five times much expensive.

Conclusion
In conclusion, nano-sized Mayenite, 12CaO.7Al 2 O 3 C12A7 , nanoparticles were successfully fabricated at a temperature of 1000 . The structural and morphological properties were studied by different techniques. The applied technique worked well in forming the calcium aluminate phase and reducing toxic ions in cement dust, such as chlorides and sulphates, by 86 and 50 , respectively. The Mayenite nanoparticles size is varied from 14 to 42 nm with an average value of 27 nm. XRF data is wellmatched with the cement calcium aluminate phase, which is widely used in Europe, especially in Spain. Mayenite C12A7 is a typical phase present in cement and used in concrete science, quite early introduced as a high-oxide conductor and catalyst for the biodiesel production process.