Red mud, an industrial waste obtained from alumina plants, is usually discharged into marine or disposed into a landfill polluting the surrounding water, atmosphere, and soil. Thus, disposal of red mud is an environmental concern and it should be recycled in an effective way. Since red mud consists of iron- and aluminum-rich phases, it can potentially be processed into cementitious material and can be used for a construction purpose. This research investigated the synthesis of nanoferrite (NF) clinker by using red mud as a raw material through chemical combustion technology for potential use in cement-based composite. Before the synthesis of NF, red mud was characterized by using XRF, XRD, and SEM techniques. From characterization results, the stoichiometric ratio of raw materials was calculated and experimentally optimized. The sample was then tested at various temperatures (815, 900, 1000, and 1100°C) to find the optimum synthesis temperature. Finally, the hydraulic activity of NF was verified and the contribution to mechanical properties was determined by replacing cement with NF at various substitution levels (0, 5, 10, and 20 wt%). Test results showed that the optimum condition for the synthesis of NF was found when the ratio of CaCO3/red mud was 1.5 and the sintering temperature was 815°C. The synthesized NF had an average diameter of 300 nm, and the main composition was brownmillerite (C4AF) with distinct hydraulic reaction. When NF was used as a substitute of Portland cement in mortar, the flexural strength with a 5% replacement level improved by 15%. Therefore, it can be concluded that the synthesis of NF provides an alternative approach to recycle red mud and could significantly help in reducing environmental pollution.
Red mud, produced by the Bayer process, is an industrial waste obtained during the production of aluminum. For every ton of alumina produced, approximately 1.6 tons of red mud is released and it is estimated that more than 66 million tons of this waste is annually generated worldwide [
Based on the composition of red mud, it can be recycled and used in a variety of fields. For example, it can be used as a recovery of metals or can be used as a potential alternative catalyst since it mainly consists of a mixture of oxides of Fe, Al, and Ti and a smaller amount of Si, Ca, and Na [
Ferrite cement has a variety of excellent properties such as high strength, impermeability, antifreeze, and corrosion resistance. Consequently, it can be used for various applications, such as emergency repair, winter construction, antiseepage, plugging, underground foundation, and high-strength concrete production [
In this research, iron-rich red mud was used to synthesize nanoferrite (NF) clinker through a chemical combustion method at a much lower temperature (≦1150°C) than traditional ferrite cement (1250-1300°C). The production conditions were theoretically designed and experimentally optimized. After optimization, the hydration activity and mechanical properties of NF admixture were determined.
Red mud and limestone were used as raw materials while urea and nitric acid were used as a fuel to produce NF. Red mud is an industrial waste produced by the Bayer process. The purity of CaCO3 in limestone and H2NCOCH2 in urea were greater than 95.5% and 99%, respectively, while the content of HNO3 in concentrated nitric acid was 65-68%. The raw material (limestone), urea, and concentrated nitric acid were purchased from Xilong Scientific Co. Ltd. (Guangzhou, China). In order to investigate the hydration and mechanical properties of NF, P·I type ordinary Portland cement (OPC) produced by China United Cement Qufu Co. Ltd. (Qufu, China) and having the following composition (Table
Composition of Portland cement (%).
SiO2 | Al2O3 | Fe2O3 | CaO | MgO | SO3 |
---|---|---|---|---|---|
22.12 | 4.51 | 3.45 | 64.92 | 3.35 | 0.4 |
The production procedure of NF is as follows: Firstly, raw materials (red mud and limestone) and fuel (urea and nitric acid) were mixed and placed in a muffle furnace. The heating temperatures of the samples in the muffle furnace were set at 815, 900, 1000, and 1100°C, respectively. The temperature of the samples was raised from room temperature to the set temperature (815, 900, 1000, and 1100°C) and held for 15 minutes. Then, the samples were rapidly taken out and allowed to cool in the air. Finally, the cooled samples were ground to obtain the final NF powder. The mass ratios of red mud to CaCO3 in the raw materials were 1 : 0.5, 1 : 0.6, 1 : 0.7, 1 : 0.8, 1 : 0.9, 1 : 1, and 1 : 2 while the raw materials-to-fuel ratio was 0.24 : 1. The role of the fuel was to provide the raw materials with the heat required for reaction so as to lower the sintering temperature and to refine the product. The mass ratio of urea to nitric acid in fuel was 2.02 : 1.
In this research, the secondary electron (SE), backscattered electron (BSE), and energy dispersive X-ray spectroscopy (EDS) in SEM were used to evaluate the morphology and composition of red mud and NF before and after hydration. The SEM observation was performed on FEI Quanta 250 (FEI Inc., Hillsboro, OR, USA) with a field emission gun and an accelerating voltage of 15 kV. The SE images provide the information on the morphology of the surface while BSE images combined with EDS mapping provide the cross-sectional microstructure and information about the chemistry of the analyzed samples.
The unhydrated and hydrated NF were observed directly by the SE technique. For BSE, the raw red mud powder and unhydated NF were prepared by adopting the following procedure. At first, mix the same mass of red mud and epoxy resin (containing a curing agent) to form viscous slurry. The slurry was poured into a cylindrical mold (10 mm diameter and 55 mm height) and cured at room temperature for 24 hours. The solidified sample was then cut by a low-speed diamond saw to expose the fresh surface. Thereafter, the surface was polished using diamond pastes of gradations 9, 6, 3, 1, and 0.25
In order to observe hydrated NF samples through BSE, the following procedure was adopted. Take the weight of NF powder and deionized water in a mass ratio of 1.0, and mix for 2 minutes. The mixture was poured into a mold (
To obtain the mineralogical composition of red mud powder and NF before and after hydration, XRD was performed on a Bruker D8 instrument (Bruker, Karlsruhe, Germany) using a CuK
The elemental composition of red mud powder was evaluated using a Bruker S4 Explorer XRF instrument (Bruker AXS, Germany) equipped with an X-ray tube with an operating voltage of 50 kV and a current of 50 mA. The exit spot at the end of the beamline was adjusted by the slit to
The specific surface area of unhydrated NF powder was tested by a BET instrument in a fully automated TriStar II Surface Area and Porosity Analyzer from Micromeritics Instrument Corporation. Before the experiment, each sample was degassed in a vacuum at 105°C for more than 6 hours. In this experiment, nitrogen was used as the adsorption gas.
In order to evaluate the mechanical properties of NF material, the compressive and flexural strength tests were conducted. For these tests, cement mortar samples having a size of
During the synthesis of NF, red mud acted as an iron and aluminum source while limestone acted as a calcium source. They reacted with the assistance of heat provided by both furnace and fuel (urea and nitric acid). It is known that the composition and characterization of limestone and fuel (urea and nitric acid) are simple, and the only factor that needs to be considered is the purity of the samples. However, the characterization of red mud raw material, which is a complex mixture of more than ten kinds of compounds, is not an easy task. The precise characterization of red mud is an important step before its applications are considered. According to the available literature [
Figure
BSE and element mapping results of red mud powder. The arrows indicate particles simultaneously containing elements Na, Al, Si, and O.
BSE
Fe
Al
Ca
Si
O
Na
Based on the elemental range as shown in Figure
XRD spectrum of red mud.
Composition of red mud from quantitative XRD analysis.
Compositions of red mud | Content (%) |
---|---|
Silica (SiO2) | 6.4 |
Sodalite (Na7.88(Al6Si6O24)(CO3)0.93) | 6.8 |
Sodium dihydrogen silicate tetrahydrate (Na2H2SiO4(H2O)4) | 24.3 |
Anorthite sodian (Na0.33Ca0.67Al1.67Si2.33O8) | 1.3 |
Calcite (CaCO3) | 5.0 |
Magnetite (Fe3O4) | 1.4 |
Hematite (Fe2O3) | 33.4 |
Boehmite (AlO(OH)) | 10.7 |
Goethite (FeOOH) | 2.8 |
Brookite (TiO2) | 4.5 |
Tialite (Al2TiO5) | 3.4 |
In order to verify the XRD results presented in Table
Comparison between XRD and XRF results of red mud.
Oxide | Fe2O3 | Al2O3 | SiO2 | Na2O | TiO2 | CaO |
---|---|---|---|---|---|---|
XRF result (%) | 37.69 | 24.4 | 16.8 | 11.7 | 5.33 | 2.83 |
XRD result (%) | 37.38 | 22.36 | 16.23 | 8.95 | 5.99 | 2.80 |
Deviation (%) | 0.82 | 8.36 | 3.39 | 23.50 | 12.4 | 1.06 |
In order to experimentally synthesize NF, stoichiometric calculation of raw materials was performed. According to the composition of NF (target compound was Ca2FeAlO5, abbreviated to C4AF) [
Assuming that CaO, Fe2O3, and Al2O3 are decomposed completely from red mud during the synthesis of NF [
It can be seen that in 100 g red mud, stoichiometrically, 16.80 g SiO2 reacts with 31.36 g CaO to synthesize C2S. Therefore, in total, 81.30 g of extra CaO is required to form C4AF and C2S, corresponding to 145.18 g of CaCO3. In other words, the mass ratio of CaCO3 added to red mud is about 1.5, and at this mass ratio, theoretically, 114.4 g of C4AF and 48.16 g of C2S can be generated.
In order to verify the above theoretical analysis, the ratios of CaCO3/red mud were experimentally adjusted to around 1.5 as 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.2, 1.5, 1.8, and 2.0, respectively, and optimized. Since the heating temperature might also influence the final composition of NF, the influence of various temperatures (815, 900, 1000, and 1100°C) was also investigated.
The raw materials and the fuel were mixed to synthesize NF. The influence of CaCO3/red mud on NF composition is shown in Figure
XRD spectra of NF produced at different CaCO3/red mud ratios.
Variation in content of major phases in NF at different CaCO3/red mud ratios (%).
CaCO3/red mud (wt%) | 0.5 | 0.6 | 0.7 | 0.8 | 0.9 | 1.0 | 1.2 | 1.5 | 1.8 | 2.0 |
---|---|---|---|---|---|---|---|---|---|---|
Ca2Fe2O5 (srebrodolskite) | 14.1 | 22.5 | 26.1 | 30.9 | 18.0 | 19.2 | 13.3 | 10.4 | 10.3 | 9.0 |
CaFe3O5 (calcium iron oxide) | 6.4 | 4.3 | 4.4 | 3.7 | 3.9 | 4.4 | 5.4 | 2.8 | 2.5 | 3.7 |
Ca3Fe2(SiO4)3 (andradite) | 2.1 | 3.5 | 2.7 | 0.6 | 2.6 | 2.7 | 0.6 | 1.7 | 0.3 | 2.1 |
Fe2O3 (hematite) | 14.8 | 17.2 | 12.8 | 9.4 | 5.5 | 3.3 | 5.0 | 2.5 | 2.3 | 3.4 |
14.0 | 9.5 | 6.3 | 5.2 | 7.0 | 7.5 | 6.9 | 7.4 | 7.2 | 6.2 | |
2.3 | 3.5 | 3.9 | 4.2 | 6.1 | 0.5 | 0.8 | 0.9 | 0.7 | 0.6 | |
1.5 | 0.2 | 0.2 | 0.1 | 0.2 | 1.4 | 0.2 | 0.3 | 0.2 | 0.2 | |
Ca3Si2O7 (rankinite) | 12.9 | 12.5 | 12.8 | 8.3 | 11.5 | 16.8 | 15.8 | 10.5 | 11.0 | 8.2 |
CaCO3 (calcite) | 0.6 | 0.1 | 0.1 | 0.1 | 0.1 | 2.2 | 2.5 | 6.2 | 15.1 | 16.2 |
CaO (lime) | 1.6 | 1.1 | 2.8 | 3.4 | 3.9 | 3.2 | 7.0 | 8.8 | 9.8 | 13.8 |
(CaO)12(Al2O3)7 (mayenite) | 0 | 0 | 0.5 | 0.1 | 0.3 | 1.2 | 0.4 | 0.6 | 0.1 | 0 |
NaAlSi4 (sodium aluminium silicon) | 1.2 | 1.8 | 1.7 | 1.0 | 1.7 | 3.2 | 0.6 | 0.7 | 0.7 | 1.7 |
Na8Al4Si4O18 (sodium aluminum silicate) | 9.7 | 1.8 | 2.3 | 0.9 | 0.3 | 0 | 0 | 0 | 0 | 0 |
Variation in contents of major phases in NF at different CaCO3/red mud ratios.
As far as silicate products are concerned, three types of dicalcium silicates (abbreviated as C2S), namely
The results (Figure
From the stoichiometric calculation of NF synthesis shown in Section
In conclusion, at a CaCO3/red mud ratio of 1.5, the content of C2S+C4AF in NF reached the maximum value. A further increase in CaCO3 content in the raw materials resulted in the decrease of required C4AF and unnecessary production of CaO and CaCO3 residue. Hence, in the following sections, the ratio of CaCO3/red mud would be taken as 1.5 and the heating temperature would be varied to evaluate the possibility of improving conversion efficiency of raw materials.
In order to study the effect of the heating temperature on NF prepared with a CaCO3/red mud ratio of 1.5, the XRD of NF samples synthesized at 815, 900, 1000, and 1100°C, respectively, was evaluated. The results are presented in Figure
XRD spectra of NF produced at different heating temperatures.
In addition, it can be observed that above 900°C, the peak of CaCO3 disappeared, while the peak of CaO gradually increased when the heating temperature was higher than 900°C, suggesting that extra heat was required to decompose CaCO3 into CaO. The peak intensities of other products shown in Figure
The morphology of typical NF synthesized at an optimal CaCO3/red mud ratio of 1.5 and heating temperature of 815°C is shown in Figure
SEM images of NF produced at CaCO3/red mud of 1.5 and temperature of 815°C. (a) NF (SE). (b) NF (BSE). (c) PC (BSE).
After determining the optimal synthesis temperature of NF, SEM and XRD techniques were used to determine the hydration products of NF. The SE image after 28 days of NF hydration presented in Figure
SEM images of hydrated NF at the age of 28 days. (a) Micrograph by SE. (b) Cross-sectional image by BSE.
The XRD spectra of NF hydrated at the age of 3, 7, 14, 28, and 56 days are presented in Figure
XRD spectra of hydrated NF cured at different ages.
In order to further investigate the properties of NF as an admixture, the mechanical properties of cement mortars were determined by replacing a specific content of PC by 0, 5, 10, 15, and 20% of NF, respectively. The results of mechanical properties at the age of 3, 7, 14, and 28 days are presented in Figure
The flexural and compressive strength of cement mortars prepared with different contents of NF.
The increase in strength at 5% NF when compared with that of control specimen is believed to be due to the following reasons: (1) NF particles having nanoscale size were effective in filling and improving packing density of mortar, and this in turn reduced the porosity of the system. (2) The appropriate amount of nanomaterials could improve the interfacial transition zone (ITZ) between sand and cement paste [
As mentioned earlier, when the dosage of NF exceeded 5%, the flexural and compressive strengths of mortar decreased. This can be attributed to the following reasons: (1) The hydration of C2S and C4AF in NF was still slower than that of PC, resulting in lower flexural and compressive strengths than the control mortar during the tested period (until 28 days). (2) It is believed that the NF particles might not have been sufficiently dispersed especially at high dosage and the aggregation of nanoparticles existed as defects in mortar and caused decrease in flexural and compressive strengths.
Therefore, the amount of NF dosage plays an important role in the strength development of cement mortar. Hence, higher compressive strength can be expected if the dispersion technique of NF in PC cement could be improved. However, further investigation is required and proposed for future research.
In this research, NF was synthesized by a chemical combustion method using red mud as one of the raw materials. By using stoichiometric calculation and experimental verification, the optimal conditions for NF preparation were found at a CaCO3/red mud ratio of 1.5 and a heating temperature of 815°C. The synthesized NF particles were roughly spherical in shape with an approximate size of 300 nm. The dominant composition of NF was found to be C4AF (47.2%) and C2S (8.6%). The decrease in the particle size of ferrite enhanced early hydration. Flaky hydration products of C(A,F)H10 and C2(A,F)H8 were formed and at later ages were gradually converted into a more stable cubic C3(A,F)H6. NF could enhance the mechanical properties when it was used as a replacement of cement in cement-based composite. At the 5% replacement level of cement with NF, the flexural and compressive strengths increased by 15% and 9%, respectively.
From the aspect of energy consumption and quality of product, NF was synthesized at a much lower temperature (≦1150°C) than traditional ferrite cement (1250-1300°C); thus, it would save a lot of sintering energy. The synthesized NF particles have a porous structure and are loosely agglomerated hence requiring slight grinding before it can be used as an admixture. This would greatly reduce the post grinding energy and further reduce the production cost. Additionally, the produced NF particles have a round shape with a relatively uniform particle size distribution, which is beneficial for the workability of concrete. In the future work, effort would be put to reduce the cost of fuels in order to reduce the overall production cost.
In general, red mud can be converted into hydraulic NF through a chemical combustion method and it provides a new approach for the utilization of red mud in construction industry.
The data used to support the findings of this study are included within the article.
The authors declare that there is no conflict of interest regarding the publication of this paper.
This research work was supported by the National Natural Science Foundation of China (Grant Nos. 51878412, 51520105012, 51878413, 51678368, and 51508338), the (Key) Project of Department of Education of Guangdong Province (Grant No. 2014KZDXM051), and the Shenzhen R&D Fund (Grant No. JCYJ20170818100641730). We also thank the Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, College of Civil and Transportation Engineering, Shenzhen University, for providing facilities and equipment.