Journal of Nanomaterials
Volume 2019 (2019), Article ID 3617050, 12 pages
https://doi.org/10.1155/2019/3617050

Research Article

Synthesis and Properties of Red Mud-Based Nanoferrite Clinker

Hongfang Sun,¹ Chuyu Chen,¹ Li Ling,¹ Shazim Ali Memon,² Zhu Ding,¹ Weiwen Li,¹ Lu-ping Tang,³ and Feng Xing¹

¹Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, College of Civil and Transportation Engineering, Shenzhen University, Shenzhen 518060, China
²Department of Civil Engineering, Nazarbayev University, Astana 010000, Kazakhstan
³Division of Building Technology, Chalmers University of Technology, Gothenburg 41296, Sweden

Correspondence should be addressed to Feng Xing; xingf@szu.edu.cn

Received 1 May 2019; Accepted 8 July 2019; Published 29 July 2019

Guest Editor: Yunpan Ying

Copyright © 2019 Hongfang Sun et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

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.

1. Introduction

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 [1, 2]. The red mud is usually discharged into marine [3] or disposed into land [4] polluting the surrounding water, atmosphere, and soil, especially in the areas where this industry is located [5]. Therefore, steps must be taken to recycle this waste in an eco-friendly way.

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 [6, 7]. Red mud can also act as a pigment since its iron-rich phases have a red color [8]. However, a large amount of red mud still needs to be recycled and more effective applications should be developed. Recently, the application of red mud as building materials has attracted the research community [9]. Calcined red mud usually consists of elements Si, Al, and Fe and has shown pozzolanic reactivity [10]. Thus, it has a potential to be used as a cementitious material. Till to date, a variety of red mud-based cements have been fabricated, such as Portland cement [11], self-compacting cement [12], and alkali-activated cement [13]. However, it could greatly enlarge the application area of red mud if new cement-based composites containing red mud are developed.

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 [14]. However, the hydration rate of ferrite cement is slow (several years or even decades to fully hydrate) [15], which greatly limits its popularity. One of the effective approaches to accelerate the early hydration is to reduce the particle size of ferrite cement [16], especially down to nanoscale [17–20]. The most commonly used technique to reduce the particle size of ferrite cement is grinding, which consumes a lot of energy. The ground particles usually have a wide particle size distribution with an irregular shape which might not be conducive to the workability of concrete. Furthermore, it is an uphill task to reduce the particle size to a nanometer scale [21]. Therefore, it is necessary to develop an effective and economical way to refine the particle size of ferrite cement.

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.

2. Materials and Methods

2.1. Materials

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 1) was used.

Table 1: Composition of Portland cement (%).

SiO₂
Al₂O₃
Fe₂O₃
CaO
MgO
SO₃

22.12
4.51
3.45
64.92
3.35
0.4

2.2. Synthesis of NF

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.

2.3. Testing

2.3.1. Scanning Electron Microscopy (SEM)

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 μm on top of Texmet papers.

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 (), consolidated, and cured at room temperature (). After curing for 3, 7, 14, 28, and 56 days, the samples were taken out for testing (only the 28-day sample was used for SEM observation while samples cured at other ages as well as 28 days were used for XRD examination). The 28-day samples were then cut into 2 cm thick pieces and vacuum-saturated with epoxy for 24 h at room temperature. Thereafter, the specimens were cut with a slow speed diamond saw to expose a fresh surface by following the procedure as mentioned in the previous paragraph. Before loading the specimen into the SEM chamber, the specimens were sputter coated using gold to form a conductive surface.

2.3.2. X-Ray Diffraction (XRD)

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 CuKα X-radiation () at 40 kV and 40 mA while Jade program 6.5 was used for quantitative analysis.

2.3.3. X-Ray Fluorescence (XRF)

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 sample was kept 1 m away from the slit at a 45° angle to the X-ray beam, while the detector was 3-10 cm away at an angle of 45° with the sample. The effective exposure time of each sample in the experiment was 600 s.

2.3.4. BET Surface Area

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.

2.3.5. Mechanical Properties

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 were prepared with NF at different levels of cement substitution (0, 5, 10, 15, and 20 wt% replacement of OPC by NF). The water-to-binder ratio and the binder-to-sand ratio used were kept as 0.35 and 1, respectively, while the dosage of the superplasticizer was 0.55%. In order to reduce the influence of large sand particle size on the test specimens, the sand having a size of more than 1 mm was removed by sieving. A YZH-300 testing machine (Luda Machinery Instrument Co. Ltd., Zhejiang, China) was used to determine the mechanical properties at the age of 3, 7, 14, and 28 days. The strength value represented the average of three specimens.

3. Results and Discussion

3.1. Synthesis of NF

3.1.1. Characterization of Red Mud Raw Material

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 [22], most of the time, red mud is usually characterized by only XRD or XRF methods. However, XRD spectrum of the red mud sample is difficult to analyze due to the presence of multiphases in red mud, which leads to overlapping peaks. As far as XRF is concerned, it can only identify the relative proportion of elements instead of phases, although it is important for the stoichiometric calculation. Therefore, in order to accurately characterize the composition of red mud, firstly, an element mapping technique (EDS combined with BSE in SEM) was used to narrow down the range of elements. Then, phase identification and quantitative analysis were performed by the XRD. Finally, for a verification purpose, the results were compared with XRF.

Figure 1 presents a typical BSE image and corresponding element mapping results of red mud. It is clear that red mud powder contains a large amount of Fe and Al elements as well as some Na, Si, Ca, and O. The mapping results of elements H and C are not shown here since it is believed that part of them came from epoxy resin and was introduced during a sample preparation process. Moreover, element H was also beyond the detection limit of this technique. However, actually, they might possibly exist in red mud. Therefore, it was speculated that elements with possible positive valency were C, Si, H, and the aforementioned metals, while the element with negative valence was O. Therefore, red mud might contain components possibly having Al-Si-Na-O (-H and/or -C), Al-O (-H and/or -C), Fe-O (-H and/or -C), and Ca-O (-H and/or -C). It is suggested that Ti-O (-H and/or -C) and Fe-Ti-O (-H and/or -C) might also have formed. Based on the above analysis, the XRD spectrum was obtained and analyzed to further characterize the composition of red mud.

(a) BSE

(b) Fe

(c) Al

(d) Ca

(e) Si

(f) O

(g) Na

Figure 1: BSE and element mapping results of red mud powder. The arrows indicate particles simultaneously containing elements Na, Al, Si, and O.

Based on the elemental range as shown in Figure 1, the phase identification and quantitative analysis of red mud were performed through XRD and the results are presented in Figure 2 and Table 2. It can be seen that Fe-related phases in red mud are in the forms of Fe₂O₃ (33.4%), Fe₃O₄ (1.4%), and FeOOH (2.8%, decomposition temperature of 400°C) [23]. The Al-related phases consist of AlO(OH) (10.7%, decomposition temperature of 430°C) [24], Na7.88(Al₆Si₆O₂₄)(CO₃)0.93 (6.8%, decomposition temperature of 520°C) [25], Al₂TiO₅ (3.4%, decomposition temperature of 800°C) [26], and Na0.33Ca0.67Al11.67Si2.33O₈ (1.3%, decomposition temperature of 800°C) [27]. Moreover, a small amount of Ca-related phases was detected in the forms of CaCO₃ (5.0%) and Na0.33Ca0.67Al11.67Si2.33O₈ (1.3%). Minor SiO₂ (6.4%) and TiO₂ (4.5%) were also found. Based on the above analysis, it can be seen that all Al-related phases Na7.88(Al₆Si₆O₂₄)(CO₃)0.93, Na0.33Ca0.67Al1.67Si2.33O₈, and Al₂TiO required extra heat to synthesize NF. As far as Fe-related phases are concerned, FeOOH required extra heat to synthesize NF while the other forms of Fe (Fe₂O₃ and Fe₃O₄) can directly be used to form NF. Therefore, decomposition temperature of both Fe- and Al-related phases was above 800°C and was set to be the minimal temperature required for sintering NF. It should also be noticed that SiO₂ and CaO components obtained from either red mud or decomposition reaction (such as decomposition of CaCO₃ at 900°C) may also produce belite with hydraulic activity.

Figure 2: XRD spectrum of red mud.

Table 2: 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 2, the composition of red mud was converted to oxide composition and compared with XRF results as shown in Table 3. The comparison indicates that the XRD and XRF results are in good agreement with percentage errors in most cases within 10%. Despite Na₂O having an error of 23.5%, the difference between the content determined by XRF (11.7%) and XRD (8.95%) is acceptable. This mutual verification confirmed the accuracy of the composition of analyzed red mud raw material, which forms foundation for the synthesis of NF.

Table 3: 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

3.1.2. Stoichiometric Calculation of NF Synthesis with Red Mud

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) [28], the raw materials are required to consist of oxides of CaO, Fe₂O₃, and Al₂O₃, part of which can be provided by red mud while the remaining has to be provided by supplementary materials.

Assuming that CaO, Fe₂O₃, and Al₂O₃ are decomposed completely from red mud during the synthesis of NF [29], according to the following formula [30]:

it can be seen that theoretically in 100 g of red mud, maximum 37.38 g of Fe₂O₃ and 52.33 g of CaO can be used to produce C₄AF with the assistance of 25.23 g of Al₂O₃. By looking at the oxide composition of red mud in Table 3, it seems that extra Fe₂O₃ exists in red mud while the quantity of CaO (2.83 g) required is far from enough. However, it should be noticed that during sintering, SiO₂ may consume CaO to generate silicate. Thus, more calcium source should be added so that it can possibly react with both Fe₂O₃ and SiO₂. It is worth noting that at the synthesis temperature of C₄AF (1000-1100°C) [31], it is possible to generate hydraulic C₂S, which does not have a negative effect on C₄AF. The production of C₂S is based on the following formula:

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.

3.1.3. Influence of CaCO₃/Red Mud Raw Materials

The raw materials and the fuel were mixed to synthesize NF. The influence of CaCO₃/red mud on NF composition is shown in Figure 3 and Table 4, respectively, while the change in the content of main phases in NF with varying CaCO₃/red mud ratios is shown in Figure 4. It can be seen that with the increase in the ratio of CaCO₃/red mud from 0.5 to 2.0, the content of the target product Ca₂FeAlO₅ (brownmillerite, C₄AF) firstly increased and then decreased. The peak value of 47.2% was achieved when the ratio of CaCO₃/red mud reached 1.5. The intermediate products of Ca₂FeAlO₅, i.e. Ca₂Fe₂O₅ and CaFe₃O₅, can also be seen. The content of Ca₂Fe₂O₅ firstly increased from 14.1% to 30.9% and then decreased to 9.0%, while the content of CaFe₃O₅ did not change much and remained at about 4%. In addition, the product containing Fe phase also had Ca₃Fe₂(SiO₄)₃; however, its content was minor (less than 4%).

Figure 3: XRD spectra of NF produced at different CaCO₃/red mud ratios.

Table 4: 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

Ca₂FeAlO₅ (brownmillerite)
19.2
21.7
23.4
32.1
39.0
36.3
41.5
47.2
39.8
34.9

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

β-Ca₂SiO₄ (dicalcium silicate)
14.0
9.5
6.3
5.2
7.0
7.5
6.9
7.4
7.2
6.2

α-Ca₂SiO₄ (dicalcium silicate)
2.3
3.5
3.9
4.2
6.1
0.5
0.8
0.9
0.7
0.6

α-Ca₂SiO₄ (dicalcium silicate)
1.5
0.2
0.2
0.1
0.2
1.4
0.2
0.3
0.2
0.2

Ca₂SiO₄ (sum(α+α+β)) (dicalcium silicate)
17.8
13.2
10.4
9.5
13.3
9.4
8.0
8.6
8.1
7.0

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

Figure 4: 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 β, α, and α, were found in NF. Among these three types, β showed the highest content ranging from 5.2 to 14.0%, while the content of α and α types was less than 6.1%. Since all three types of C₂S have hydration activity, therefore, a total amount of C₂S would be used for discussion. With the increase in the level of CaCO₃ raw materials, the total C₂S content decreased slightly. At a CaCO₃/red mud ratio of 1.5, the C₂S content was 8.1% while the total amount of the hydraulic contents (C₄AF+C₂S) was about 55%. The silicate also contained a certain amount of Ca₃Si₂O₇ (abbreviated as C₃S₂); however, it did not change much (range 8-16%) with the increase in the ratio of CaCO₃/red mud. It can also be seen from Figure 4 that at the 1.5 CaCO₃/red mud ratio, the content of C₃S₂ was found to be 10.5%. Although C₃S₂ does not have hydration activity, its volume expands in the carbonization environment [32], which can be beneficial to control the cracking and strength decrease caused by carbonization shrinkage.

The results (Figure 3 and Table 4) also show the presence of CaCO₃ residue and CaO (decomposition product of CaCO₃) in NF. Their content increased (CaCO₃: 0.1-16.2%; CaO: 1.1-13.8%) with the increase of the ratio of CaCO₃/red mud. At a CaCO₃/red mud ratio of 1.8 and higher, the content of CaCO₃ and CaO increased sharply, indicating that the effective use of raw materials was drastically reduced. At the 1.5 CaCO₃/red mud ratio, the CaCO₃ and CaO contents were found to be 6.2% and 8.8%, respectively. The contents of NaAlSi₄ and Na₈Al₄Si₄O₁₈, which are the residuals from red mud and having no hydration activity, were below 2%. Therefore, their influence on NF was negligible.

From the stoichiometric calculation of NF synthesis shown in Section 3.1.2, it can be seen that at the 1.5 CaCO₃/red mud ratio, the theoretical content of C₄AF in NF was 63.2%. However, the actual content of C₄AF in the synthesized NF was 47.2%, which was approximately 16% lower than the theoretical value. This is due to the reason that part of Fe in NF is in the form of Ca₂Fe₂O₅, Ca₃Fe₂(SiO₄)₃, and CaFe₃O₅ while the remaining unreacted part remained as Fe₂O₃. It should be mentioned here that the total amount of element Fe in Fe-related phases was 16.03%, which was close to the theoretical value of 14.55%, confirming the reliability of quantitative analysis. However, it is suggested that the conversion efficiency of raw materials to NF could be further improved by using the current experimental conditions. With regard to element Al, it was almost completely consumed to generate C₄AF. It is also noticed that the actual content of C₂S is 8.1%, which is quite different from the calculated value of 26.59%. The difference between the theoretical and experimental values is attributed to the formation of 10.5% Ca₃Si₂O₇(C₃S₂) and 1.7% Ca₃Fe₂(SiO₄)₃, which in turn influenced Si-related phases. It is believed that Si-related phases in red mud were too stable to completely release SiO₂ required for the synthesis of NF.

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.

3.1.4. Influence of Heating Temperature

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 5. It can be seen that the peak intensity corresponding to brownmillerite (Ca₂FeAlO₅) increased slightly with the increase in temperature. However, above 900°C, the change was not significant. It is known that the synthesis temperatures of α-C₂S, α-C₂S, and β-C₂S are higher than 1450, 725, and 500°C, respectively [33]. Therefore, the α-C₂S level increased sharply with the increase in temperature while α-C₂S and β-C₂S were not influenced significantly by the increase in temperature.

Figure 5: 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 5 remained unchanged. Therefore, it can be deduced that from the perspective of energy saving and efficiency, the temperature required to convert raw materials to target product (synthesis of NF) is 815°C.

3.1.5. Morphology and Fineness of NF

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 6. It can be seen from Figure 6(a) that the sintered NF has a particle size of approximately 300 nm and its shape is nearly spherical. The comparison of BSE images of NF (Figure 6(b)) and PC (Figure 6(c)) for PC also shows that the size of the NF was much smaller than that of the PC particles.

(a)

(b)

(c)

Figure 6: 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).

3.2. Hydration of NF

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 7(a) shows that the morphology of the main hydration product is flaky. The BSE image of hydrated NF presented in Figure 7(b) shows that the prepared NF had been significantly hydrated at the age of 28 days. It suggests that the reduction of the particle size of ferrite to nanoscale could effectively accelerate hydration when compared with a traditional ferrite material, which has not shown significant hydration at the age of 28 days [34].

(a)

(b)

Figure 7: 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 8. At this time, a large number of early hydration products C(A,F)H₁₀ and a small amount of C₂(A,F)H₈ were formed. This confirmed that the prepared NF had hydrated significantly at 3 days. At this age, the content of C₂S in NF decreased and the content of hydration product Ca(OH)₂ increased, indicating that C₂S had also hydrated. However, during the first 56 days, the content of C₂S decreased slowly, which confirmed that the early hydration of C₂S was still slow. At the age of 7 days, the content of C(A,F)H₁₀ decreased and the content of C₂(A,F)H₈ increased due to the gradual transformation of C(A,F)H₁₀ to C₂(A,F)H₈ crystals. At 14 days, the content of C(A,F)H₁₀ decreased and the content of C₂(A,F)H₈ increased further. Meanwhile, C₃(A,F)H₆ started to appear [35]. At 28 days, the content of C₃(A,F)H₆ further increased while the contend of Ca(OH)₂ decreased. At this age, a diffraction peak of CaCO₃ appeared due to carbonation. At the age of 56 days, the C(A,F)H₁₀ crystal almost disappeared, C₂(A,F)H₈ content decreased significantly, and C₃(A,F)H₆ content increased significantly. This indicated that C(A,F)H₁₀ and C₂(A,F)H₈ were so unstable that they transformed into more stable C₃(A,F)H₆ at later ages and is consistent with the findings presented in references [15, 36].

Figure 8: XRD spectra of hydrated NF cured at different ages.

3.3. Mechanical Properties of NF

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 9. The figure indicates that the flexural and compressive strengths of all samples increased with the increase in the age of testing. As the replacement level of NF increased from 0% to 20%, the flexural and compressive strengths first increased and then decreased, reaching the maximum compressive and flexural strengths at 5% substitution level. Moreover, specimens prepared with 5% replacement level achieved the highest flexural and compressive strengths than the control mix (specimens with 0% replacement) at all ages. Compared with the control specimens, the flexural and compressive strengths of the 5% substitution specimens increased by 15.4% and 9.0%, respectively, at the age of 3 days, and by 9.0% and 6.7%, respectively, at 28 days. The increase in strength is much higher than that of mortar prepared with 5% traditional ferrite additive produced by red mud (compressive strength enhanced by 5% at 28 days) [37] as well as mortar prepared with 3.5% as-received red mud (compressive strength enhanced by 0% at 3 days and by 4.5% at 28 days) [38]. It indicates that reprocessing of red mud and nanocrystallization could effectively improve the mechanical properties on cementitious materials. Unfortunately, at other replacement levels, such as 10%, 15%, and 20%, the overall flexural strength and compressive strength were lower than control specimens. Therefore, with increasing NF content in cement mortar, the mechanical properties of cement mortar did not always increase but achieved maximum mechanical strength at the 5% replacement level.

(a)

(b)

Figure 9: 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 [39].

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.

4. Conclusions

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.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this paper.

Acknowledgments

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.

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