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Synthesis and characterization of low temperature (<800 °C) ceramics from red mud geopolymer precursor
Construction and Building Materials 131 (2017) 564–573
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Synthesis and characterization of low temperature (<800 °C) ceramics from red mud geopolymer precursor Patrick N. Lemougna a,b,⇑, Kai-tuo Wang a, Qing Tang a, Xue-min Cui a,⇑ a School of Chemistry and Chemical Engineering and Guangxi Key Lab of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China b Local Materials Promotion Authority, MINRESI/MIPROMALO, P.O. Box 2396, Yaounde, Cameroon
h i g h l i g h t s Compressive strength of up to 40 MPa was obtained from red mud geopolymer. Curing at 60 °C for weeks was essential for better strength development. Post heating to 700 °C increased the strength to 55 MPa and the mechanical stability. The use of sodium silicate instead of NaOH was essential for the material stability.
a r t i c l e
i n f o
Article history: Received 13 July 2016 Received in revised form 14 October 2016 Accepted 22 November 2016
Keywords: Red mud Geopolymer Temperature Ceramic Structural applications
a b s t r a c t This paper presents a way to valorize red mud for the production of potential structural materials, using geopolymer technology. Several compositions of red mud geopolymers were prepared with sodium silicate solutions. After sintering the red mud geopolymer products at 300–800 °C, their stability in water has been improved. The starting red mud was found to contain hematite, katoite, cancrinite and a few amount of diaspore, which hardly dissolved and participated in the geopolymerization reaction. However, the geopolymer gel formed was sufficient to bind the unreacted phases and form a high strength material with about 40 MPa after appropriate curing. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Red mud (RM) or bauxite residue is the major waste produced by the alumina refining industry where the Bayer process is used to extract alumina from bauxite ores [1–3]. The process involves the use of highly concentrated NaOH solution for the ore digestion at temperatures up to 240 °C and 1–6 atm pressure [3,4]. The production of 1 ton of aluminum metal produces as a by-product 4–5 tons of red mud [4]. Fresh RM slurry is usually transported to waste lakes for impoundments, followed by dewatering and drying to reduce its volume and maintenance costs [3]. The exact composition of the mud depends on the origin of bauxite and on processing conditions [5], but essentially consists of oxides and hydroxides of ⇑ Corresponding authors at: School of Chemistry and Chemical Engineering and Guangxi Key Lab of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China. E-mail addresses: [email protected] (P.N. Lemougna), [email protected]. cn (X.-m. Cui). http://dx.doi.org/10.1016/j.conbuildmat.2016.11.108 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
Fe, Al and Si along with some quantities of CaO, TiO2, Na2O, and is alkaline in nature [5,6]. The worldwide bauxite residue disposal areas contains an estimated 2.7 billion tons of residue, with an increase of approximately 120 million tons per annum, representing an environmental and economic problem [1–3,7]. To date, although enormous efforts have been made on RM treatment, recycling, and utilization [1,3,7–10], only a small fraction, most probably <5 wt% is being used in few countries in specific industrial processes such as cement production, the rest being stored [9]. An economical widely accepted technology for the recycle and reuse of RM has yet to be developed [2,3,9,10], hence the need of further researches to explore possibilities of valorizing red mud [1,10]. Among the utilization options, construction and building materials pose lower risk for implementation and the manufacture of geopolymers based on RM including controlled low strength materials were suggested as interesting area of research to be explored [1,2]. Geopolymers are a class of inorganic polymer materials which have attracted interest during the last decades due to their inter-
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esting physical, structural and thermal properties [11–15]. Most of previous reports on the use of red mud for the development of potential building materials using geopolymer synthesis have been limited to alkali-activated composites of red mud with other aluminosilicate materials [16–19], likely due to the difficulty of obtaining good structural properties from a pure RM geopolymer based system. Attempts to produce traditional ceramic materials showed a substantial inertia of RM up to 900 °C, resulting in a cost disadvantage need of high temperatures of 1000–1200 °C to obtain potential useful structural properties [7,8,20,21]. The present work investigates the possibility to produce inorganic polymers using red mud as the aluminosilicate precursor, and the effect of post heating of the products to produce low temperature ceramics. Several compositions of the red mud geopolymers were prepared by varying the modulus (R = SiO2/Na2O) of the activating solution from 1.6 to 2.2. The geopolymer samples were subjected to different curing regimes (sealed and humid) and the effect of post heating (200–800 °C) was assessed on the optimally formulated products. The starting red mud was also treated at some temperatures for comparative studies. The resulting products were characterized by X-ray diffraction, Scanning Electron Microscopy, Differential Scanning Calorimetry and Infrared spectroscopy. The wet and dry compressive strengths, the leaching test, water absorption and porosity were then performed to assess the suitability of the synthetized harmless products for potential structural applications. 2. Experimental
open but the samples were maintained inside the bag and, the third day, the samples were removed from the plastic bag to the oven atmosphere. This low and long curing procedure was adopted to reduce the sensitivity of crack formation observed on the red mud. Part of the 7 days dried specimens was subjected to heat treatment from 200 to 800 °C at 100 °C interval, in an electric furnace, heating rate of 2 °C/min with a dwell time of 2 h at each temperature. The heating rate of 2 °C/min was adopted to reduce the sensitivity to form cracks, which was observed at higher heating rates, with negative effect on the mechanical properties. The temperature 60 °C was chosen for curing because relatively poor performances were observed during preliminary investigations on specimens cured at 25 and 40 °C. The post heating temperature range of 200–800 °C was chosen because it is below the usual sintering temperatures of traditional ceramics and below the temperature at which red mud without admixtures generally presents a substantial inertia [7,8]. On the basis of the phases identified by XRD, the pure red mud was also treated at 300 °C, 500 °C, 700 °C and 800 °C for X-ray comparative study. 2.3. Characterization method 2.3.1. XRD and FTIR analyses The samples were powdered and examined by X-ray diffraction with a Rigaku Mini Flex 600 instrument with Ni-filtered Cu (Ka) radiation, a step size of 0.02°, operated at 40 kV and 15 mA, with a dwell time of 0.5 s and a 2h range of 5–80°. The powdered samples were also pressed into KBr pellets for FTIR analysis using a Thermo Scientific FTIR spectrometer.
2.1. Materials The red mud used in this study was from Guangxi Province, China. The specific surface area determined by the BET method was 8.04 m2/g. The oxide composition determined by X-ray fluorescence is reported in Table 1. The alkaline activating solutions with silica moduli (R = SiO2/ Na2O) of 1.6–2.2 with 0.2 interval were prepared by dissolving solid sodium hydroxide in a commercial sodium water glass with R = 3.3. The alkaline activating solutions were sealed and stored for a minimum of 24 h prior to use. 2.2. Specimens preparation The preparation of the fresh mixture was performed by mixing red mud, water glass with different moduli (R = 1.6; 1.8; 2.0 and 2.2), and some amount of deionized water. The details on the mix proportioning are presented in Table 2. The mixing process was performed for about 10 min, using an electric mixer at 600 rpm, up to obtaining a homogenous paste. The samples were then casted in cubic alloy molds of 20 20 20 mm3, covered with a thin layer of plastic to facilitate the removal of the hardened paste upon curing. The alloy molds were vibrated on a vibration table for 2 min to remove air bubbles and sealed afterwards. The specimens were stored at 25 °C for 48 h, unmolded and kept for further 48 h sealed at 25 °C. After this period, the specimens were transferred sealed at 60 °C. Some of them were maintained sealed until the 7 and 28 days strength testing while others were progressively dried at 60 °C in the following manner: the first day, the samples were kept sealed; the second day, the plastic bag was
2.3.2. TG/DTA analysis TG/DTA analysis was performed with a simultaneous STA409PC TG/DTA measurement in air, at a constant heating rate of 5 °C/min. The sample was heated from room temperature to 1000 °C. 2.3.3. SEM/EDX analysis Scanning Electron Microscopy (SEM) and Energy Dispersive Xray Spectroscopy (EDX) were used to analyze the microstructure of the powdered red mud and the polished surfaces of the specimens with an S-3400N device (Japan Hitachi Limited Company). Specimens were impregnated using absolute ethyl alcohol, polished with SiC paper, and then coated with gold. 2.3.4. Compressive strength testing Compressive strength testing was performed on specimens using a DNS100 universal testing machine. The displacement rate used was 0.5 mm/min. The test was performed on specimens cured for 7 and 28 days and for specimens post heated at 200–800 °C. The dry strength is for specimens after the curing or post heating treatment, and the wet strength is for these specimens immersed for 24 h in water. The values were determined as the average of three samples of each composition. 2.3.5. ICP analysis, water absorption and porosity In the realization of the ICP analysis, the samples were soaked in deionized water for 96 h. The sample/water weight ratio was 1/3. The water was collected for analysis after 48 h, then renewed and collected again for analysis after an additional 48 h for some samples. This experiment was carried out for the samples treated
Table 1 Chemical composition of red mud (wt%). Fe2O3
Al2O3
CaO
SiO2
TiO2
Na2O
MgO
SO2
K2O
MnO
33.99
18.47
14.19
9.39
5.42
5.11
0.32
0.33
0.10
0.091
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Table 2 Mixture proportioning. Composition No
Modulus of the liquid water glass
Na2O/Al2O3 (molar)
SiO2 /Al2O3 (molar)
Red mud (g)
Liquid water glass (g)
Water (g)
1 2 3 4 5 6 7 8 9
1.6 1.8 2.0 2.2 1.6 1.8 2.0 2.2 –
0.96 0.91 0.87 0.84 0.75 0.73 0.71 0.69 0.99
1.66 1.68 1.70 1.72 1.34 1.36 1.37 1.38 0.86
90 90 90 90 90 90 90 90 90
30 30 30 30 18 18 18 18 36 (NaOH 6M)
6 6 6 6 18 18 18 18 /
at 60, 300, 500, 700 and 800 °C. The elemental composition of the soaked water was performed with an inductively coupled plasma optical emission spectrometer (ICP-OES, optima 5300DV, Perkin Elmer, USA) under a plasma gas flow of 15 L/min and a nebulizer gas flow of 0.6 L/min. The percentage of water absorption was obtained after immersing the specimens for 24 h in deionized water and the porosity of the samples was assessed using a PoreMaster GT Quantachrome instrument mercury porosimetry analyser (model PM 33–18). 3. Results and discussions 3.1. XRD analysis Fig. 1 presents the results on XRD pattern of the pure red mud (1-A) and alkali silicate activated red mud prepared with a silicate solution of R = 2.0 (1-B). From these results, it is observed that the main crystalline phases present in red mud are hematite, Fe2O3, PDF 33-0664; cancrinite, Na6Ca1.5 Al6Si6O24 (CO3)1.6, PDF 340176, and a few amount of katoite, Si-rich, Ca3Al2(SiO4)(OH)8, PDF no. 38-0368 and diaspore, AlO(OH), PDF 05-0355. The hematite phase is not perturbed by the heat treatment up to 800 °C, while it is observed a disappearance of the crystalline picks associated to diaspore and katoite above 300 °C and, cancrinite above 700 °C. At 800 °C, there is the appearance of a new phase, gehlenite, Ca2Al2SiO7, PDF 35-0755, likely formed from the decomposition of cancrinite (Fig. 1A). The comparison of the spectra of the pure
A)
red mud heated at 300–800 °C and the inorganic polymers heated in the same temperature range shows that there is only little difference in the qualitative crystalline composition of the two series below 800 °C. At 800 °C, we have the appearance of nepheline in the heated inorganic polymer, which is not the case for the heated starting red mud at the same temperature. No major additional crystalline phase was formed upon alkaline treatment suggesting the amorphous character of the newly formed phase resulting from the mixing of red mud with the alkaline silicate reagent. However, the possibility of formation of some additional katoite is not excluded as the formation of this mineral was suggested to be favorable in poor silica CaO–SiO2–Al2O3–H2O system [22], which is close to the experimental conditions of our study. The few change observed in the XRD spectra of the starting red mud and the inorganic polymer could be attributed to the initial alkaline character of the starting red mud and the relative mild alkalinity of the alkaline reagent used, which hardly dissolved the initial crystalline phases in the red mud. The red mud and the inorganic polymer subjected to heat presented much simpler diffractograms suggesting an increase of the amorphous content upon heating. These observations are in agreement with previously reported studies in red mud. The presence of katoite and few diaspore was reported in several red mud samples, especially those from China [10,19,23]. Katoite was also reported to be present as hydratation product in calcium aluminate cementitious materials [24,25]. Some studies on the thermal behavior of red mud attributed the decomposition of cancrinite below 900 °C to the CO2 evolu-
B) 800°C-( j)
800°C-(e)
700°C-(i) 700°C-(d) 500°C-(h) 500°C-(c) 300°C-(g) 300°C-(b)
60°C-(f)
(a)
2 (°)
(a)
2 (°)
= Katoite, Si-rich, Ca3Al2(SiO4)(OH)8 , PDF n° 38-0368 = Hemate, Fe2O3, PDF n° 33-0664 = Cancrinite, Na6Ca1.5 Al6Si6O24 (CO3)1.6, PDF n° 34-0176 = Gehlenite, Ca2Al2SiO7, PDF n° 35-0755 = Diaspore, AlO(OH) , PDF n° 05-0355 = Nepheline,NaAlSiO4 , PDF n°35-0424 Fig. 1. XRD analysis of pure red mud (A) and inorganic polymer (B), treated at indicated temperatures.
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tion, hematite constituting the fundamental phase up to 1100 °C [7]. The presence of the starting crystalline phases in inorganic polymers prepared from red mud, mainly hematite and cancrinite has been also previously reported [3,16,17,26], suggesting the absence or low dissolution of these minerals in the alkaline condition of the inorganic polymer synthesis.
3.2. FTIR analysis The infrared spectra of red mud and synthetized products are presented in Fig. 2. The large band at 1000 cm1 is assigned to the stretching vibrations of Si(Al)–O groups, and it is sensitive to the content of structural Si and Al [27,28]. It is observed that this band enlarges after treatment of the sample with sodium silicate solution, likely suggesting an increase in the amorphous content in the material. Another observation is the relatively great difference in the shape of the band in the specimen treated at 800 °C compared to those treated at 60–700 °C, suggesting a relatively important change in the structure of the material between 700 and 800 °C. This change is attributed to the onset of solid state reactions for the formation of high temperature mineral such as nepheline as observed in the XRD section. The band around 1110 C m1 arises from the presence of Si-O-Si bond [2]. This band also decreases in the inorganic polymer specimens, suggesting the substitution of Si/O/Si bonds with Si/O/Al bonds during the formation of the inorganic polymer network [2,29]. The stretching vibrations of the FeAO bands of the hematite structure are observed around 450 and 550 cm1 [27,28,30]. The band around 1400– 1500 cm1 indicates the presence of OACAO [2,27]. It is noted that this band is progressively reduced as the temperature increases. The bands around 1600–1700 cm1are attributed to water, since they correspond to the characteristic regions of O–H stretching and H–O–H bending in H2O [2,27,28,30]. The high intensity of this band in the inorganic polymer treated at 60 °C is attributed to the presence of some structural water in the sample. It is observed that this band decreases on heating, but do not disappears completely, the remaining intensity at high temperature likely arising from the adsorption of water in the surrounding air by the sample, as often observed in red mud or hematite samples [27,28,30].
800ºC (f) 700ºC (e)
500ºC (d)
300ºC (c) 60 ºC (b) (a)
Wavenumbers (cm-1) Fig. 2. FTIR analysis of the pure red mud (a) and inorganic polymers (b–f), treated at indicated temperatures.
Exo
Fig. 3. TG/DTA analysis of inorganic polymer cured at 60 °C.
3.3. TG/DTA analysis Fig. 3 shows the TG/DTA analysis and the derivative weight loss of the inorganic polymer. From this Figure, we can see that the total weight loss after heating the inorganic polymer at 1000 °C is about 13%, about half of that obtained with metakaolin-based inorganic polymer [31,32]. The derivative weigh loss indicates that the weight loss upon heating is not constant. We have four distinct regions of weight loss. On the basis of the chemical and the mineralogical composition of the sample, we can say that the weight loss around 100 °C corresponds to the loss of residual free water in the sample. The one around 250–450 °C is likely to arise from the loss of structural water in the inorganic polymer as well as the decomposition of katoite and diapore [19,24]. The one around 500–600 °C is likely to arise from a partial release of CO2 from cancrinite and/or the decomposition of residual structural water in the inorganic polymer while the one around 650–700 °C is assigned to the decomposition of cancrinite, which completely disappears above 700 °C, in consistence with the X-ray analysis and previously reported study on red mud [7]. 3.4. SEM/EDX analysis Scanning Electron microscopy analysis of the red mud powder (Fig. 4) shows that red mud particles are very fine, with many particles of size below 1 lm. Polished samples showed some similarities in the microstructure of the inorganic polymer cured at 60 °C for 28 days (a) and its homologues post heated at 300 and 800 °C (Fig. 5). These similarities are likely to arise from the persistence of less reactive phases such as hematite in this temperature range in the geopolymer. The more obvious change observed in the microstructure evolution upon heating is marked by the change in color of the post heated sample which became less dark in color (Figs. 5 and 6). From the weight and atomic composition obtained by EDX analysis (Fig. 6), we can say that the composition of the inorganic polymer mixture is almost homogenous at a microscopic scale. The heterogeneous distribution of elements observed on EDX map (not shown here) is more marked for iron and is in agreement with the presence of not dissolved hematite in the XRD analysis. Si,
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20µm
3µm
Fig. 4. SEM of red mud powder.
a
a’
Pores
500 µm
b
3 µm
b’
Pores Air bubble
3 µm
500µm
c
c’
Pores
500µm
3 µm
Fig. 5. SEM analysis at lower and higher magnification at different temperatures: a and a0 , 60 °C; b and b0 , 300 °C; c and c0 , 800 °C.
Al, Na and Ca are more homogeneously distributed, likely to suggest their participation in the inorganic polymer formation. This result is in agreement with some previous studies on alkali activation of different red muds [2–4], and suggests that the inertia of iron, usually in the form of hematite, is likely to be a common fact during alkali activation of red mud. Hairi et al., [4] studied the role of iron during alkali activation of a red mud from Canada and observed a similarity between the Mössbauer spectra of the starting materials and their corresponding geopolymers, indicating that
the iron components, mainly from hematite and goethite in their case, was largely present as spectator species rather than becoming involved in the geopolymer structure. Raw materials for inorganic polymer synthesis can been classified into three groups depending on the amount of Ca present: high Ca system, low Ca system and intermediate Ca system [13]. Based on the chemical composition of the system, red mud can be classified as intermediate Ca system which can form a mixture of C-A-S gel and N-A-S gel during alkaline activation [13]. When combining
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a
Cracks
20 µm b
20 µm c
20 µm Fig. 6. SEM analysis at middle magnification at different temperatures: a, 60 °C; b, 300 °C; c, 800 °C.
the results from XRD, FTIR and SEM, we can say that the crystalline phase in the red mud hardly dissolved and participated in the formation of the gel. However, the amount of gel formed was sufficient to bind the unreacted crystalline phases and formed a high strength material with increasing stability to water when transformed into low temperature ceramic upon heating, as shown in the compressive strength section. 3.5. Compressive strength analysis The 28 days compressive strength values for the dry and wet samples from compositions 1–4 are presented in Fig. 7. The error
bar indicates the standard deviation from three replicate specimens. From this figure, it is observed that the values of the dry compressive strength of the samples prepared with different moduli of the activating solution vary between 30 and 40 MPa. The best values are obtained for the composition prepared with the silicate solution with the moduli R = 1.8 and 2.0, with a better wet performance observed with R = 2.0. This trend is maintained for the values of the wet strength which are in the range 5– 25 MPa. The reduction of strength upon 24 h immersion in water is more significant for the compositions with the activation solution moduli of 1.6 and 2.2, which exhibited a decrease of about 85% and 65% respectively. The reduction of strength for the
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Fig. 10. Effect of post heating on the dry and wet compressive strength. Fig. 7. Dry and wet 28 days compressive strength of samples cured at 60 °C.
Fig. 8. Dry and wet compressive strength of samples post heated at 300 °C.
Fig. 9. Effect of sealed and open curing on the 7 and 28 days compressive strength.
composition with R = 1.8 and 2.0 is less significant, about 61 and 49% respectively. The strength reduction after immersing the specimens in water is often observed for some inorganic polymers and can be attributed to the hydration of some SiAOASi bonds to SiAOH bonds, weakening the structure [33,34]. The composition prepared with the solution with R modulus of 2.0 still presented the best strength at 300 °C (Fig. 8). Further investigation on the strength development from this composition is presented in Figs. 9 and 10. From Fig. 9, it is observed that curing in sealed environment led to low strength when the products are not dried afterward. A 7 days compressive test (4 days at 25 °C and 3 day at 60 °C as described in the experimental section) only led to compressive strength of about 4 and 15 MPa for sealed and dry curing respectively. Prolonging curing time does not significantly improve strength in sealed environment which led to a 28 days compressive strength of about 10 MPa at 60 °C, hence, about four times smaller than their counterparts cured in open environment. Curing in sealed environment is essential at the beginning of the curing regime to reduce the sensitivity to cracks formation, but the products should be progressively dried to achieve their maximum strength performance. The inorganic polymers from red mud are sensitive to humidity and water which significantly affect the strength. Fig. 10 shows that post heating of the sample does not significantly improve the strength from the well cured and dried sample, but improve the product stability in water. A maximum strength of about 55 MPa is however observed on the products heated at 700 °C. Exposure of geopolymers to heat is not always leading to an improvement of the mechanical properties [35]. However, metakaolin geopolymers prepared with sodium or potassium alkaline reagents were reported to be fire resistant, with thermal stability up to about 900 °C [36]. Kong and Sanjayan [37] also observed that some fly ash-based geopolymer can consolidated further when exposed to elevated temperatures up to 800 °C. Considering the results from XRD, SEM and FITR, the strength development after curing is associated to the formation of amorphous gel surrounding the unreacted phases in the red mud. The strength and stability of this amorphous gel increases with prolonging curing temperature and by post heating the products. The post heating improves the stability of the amorphous gel, possibly by akin reaction, leading to the formation of low temperature ceramics with lower or no strength loss after water immersion for well designed compositions. Actually, it was observed that the composition N° 9 presented some instability in water while the compositions N° 5–8 were very sensitive to crack formation and resulted in variable low
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strength values after 28 days of curing or upon heating, mainly below 15 MPa. These strengths were in the range 3–10 MPa for geopolymers and 5–15 MPa for post heated geopolymer samples up to 800 °C. This shows that concentrated sodium silicate medium is essential to dissolve the reactive part of red mud for a better performance of the synthesized materials. However, the amount of sodium in the best compositions did not exceed the requirement for the equilibrium of all the aluminum in red mud by sodium during the geopolymerization reaction, as shown by the Na2O/Al2O3 molar ratios in Table 2. Hence, the performance of the red mud based geopolymer is not only driven by the alkalinity of the mixture; an appropriate use of silicate solution instead of NaOH solution is essential to improve the mechanical performance, likely by an increase of the SiAOASi bonds in the mixture, as often observed in metakaolin geopolymer [38]. 3.6. ICP analysis, water absorption and porosity The ICP analysis of water collected after soaked the samples for 48 h in deionized water is presented in Table 3. From this table, we can see that the main elements diffusing in water are Al, Na, As and
Fig. 12. Water absorption and porosity of the inorganic polymer and its counterparts post heated at indicated temperatures.
Table 3 ICP analysis of the soaked water for the samples treated at indicated temperatures. Elements
Different concentration in collected soaked water (mg/l) 60 °C
300 °C
500 °C
700 °C
800 °C
Initial water after the first 48 h Al As Cd Cu Mn Fe Na Ni Pb Ti Zn
0.26 8.49 <0.01 <0.01 0.01 0.02 5790 <0.01 0.32 0.02 <0.01
1.57 3.65 <0.01 <0.01 <0.01 0.02 4936 <0.01 0.38 0.01 <0.01
60 °C
500 °C
Renewed water the 2nd 48 h 370 3.82 <0.01 <0.01 0.01 0.06 2466 <0.01 <0.01 <0.01 <0.01
981 1.69 <0.01 <0.01 0.62 0.17 1781 <0.01 <0.01 <0.01 <0.01
1572 4.51 <0.01 0.19 <0.01 0.56 3731 <0.01 0.76 0.01 0.02
0.28 0.36 <0.01 <0.01 <0.01 0.01 531 <0.01 0.09 0.01 <0.01
Fig. 11. Soaked samples for the leaching test (A: after the first 48 h; B: after the second 48 h with renewed deionized water).
117 1.60 <0.01 <0.01 <0.01 <0.02 946 <0.01 <0.01 <0.01 <0.01
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Fig. 13. Pore size distribution of the inorganic polymer and its counterparts post heated at indicated temperatures.
to a lesser extent Fe. From these analyses we can say that only part of the aluminum in red mud took part in alkali activation, forming a gel that bonded the less reactive phases. Upon post heating, there was an increase of the sensitivity to diffuse of the aluminum, mainly from the remaining crystalline phases such as diaspore, leading to an increase of Al diffusing in water with the increase of the post heating temperature. This is in agreement with the increase of the amorphous character (much simpler diffractogram) of red mud and geopolymers upon heating, and suggests an interest for the study of the effect of preheating red mud at 300–800 °C prior to alkaline activation in future research work. The breakage of AlAO bonds, with increasing post heating temperature is not excluded, mainly above 700 °C, and could explain the decrease observed in the compressive strength between 700 and 800 °C. The relatively high amount of Na released is associated with the fact that only part of aluminum present in red mud reacted and the global reduction of released Na with increasing temperature is in consistence with the increase of the stability of the formed phase and the material upon heating. The soaked samples after the first 48 h in deionized water and after the second 48 h with renewed water are presented in Fig. 11 A and B respectively. The water absorption test (Fig. 12) shows that the percentage of water absorption increase with the post heating of the sample up to a maximum value of 18% at 700 °C. A slight decrease is observed at 800 °C and is attributed to the onset of solid state reaction usually observed in the traditional ceramic process. It is worth pointing out that the overall linear shrinkage of the specimens after heating at 800 °C was below 1.6%. The consideration of the value of water absorption of 17% at 800 °C and the low linear shrinkage suggests that the glassy phase is yet to be achieved at this temperature. The increase in water absorption value is attributed to the decomposition reactions presented in the TG/DTA section, increasing the porosity in the sample. This is supported by the porosity data presented in Figs. 12 and 13. From these Figures, it is observed that the porosity of the samples is in the range 24–35%, globally following the trend of water absorption, albeit the maximum of porosity is observed at 500 °C (Fig. 12). It is also noted that most of the pores of all the samples are in the range 0.03–0.5 lm, with the main population around 0.2 lm (Fig. 13). These porosity data are relatively similar to those observed in some ceramics and fly ash based geopolymer [39,40]. In summary, we can say that concentrated sodium silicate solution is needed to dissolve the reactive part of red mud, although all sodium in the mixture is not taking part in the reaction. The release of some elements from this red mud geopolymer suggests the need of washing the products prior to their use for structural applications. This option is however realistic when considering the increasing issue associated to red mud
disposal [41]. The washed water can then be dried and the residual solid which will be insignificant compared to the initial volume of the red mud could be treated appropriately.
4. Conclusions The valorization of red mud remains an important issue to be addressed. The present studies showed that it is possible to develop consolidated materials with interesting structural properties with some red mud, using inorganic polymers chemistry. The relatively little change between the structure of the starting red mud and the inorganic polymer was attributed to the alkaline character of red mud, the low sensitivity of the minerals present in the red mud to the alkaline condition of the inorganic polymer synthesis and the relatively small amount of the newly formed phase, binding the less reactive phases. Long curing, low drying and appropriate use of sodium silicate solutions were found to be essential for better strength development. The value of the 28 days dry strength for the best composition obtained with a liquid water glass with a modulus R = 2.0 was about 40 MPa. The strength reduction when immersing the products in water was not significant after post heating of the samples which transformed them into low temperature ceramics with a maximum strength of about 55 MPa at 700 °C. The need for long curing, the relatively high sensitivity to form cracks and the leaching of some elements were found to be the main concerns for the valorization of red mud as aluminosilicate precursor using geopolymer technology. However, efforts in overcoming these concerns appear realistic when considering the increasing issues associated to red mud harmless disposal. Acknowledgments This work was supported by the Chinese Natural Science Fund (Grant: 51262002, 21566006 and 51561135012) and Postdoctoral Project from Guangxi University. References [1] C. Klauber, M. Gräfe, G. Power, Bauxite residue issues: II. Options for residue utilization, Hydrometallurgy 108 (2011) 11–32. [2] N. Ye, J. Yang, S. Liang, Y. Hu, J. Hu, B. Xiao, Q. Huang, Synthesis and strength optimization of one-part geopolymer based on red mud, Constr. Build. Mater. 111 (2016) 317–325. [3] J. He, Y. Jie, J. Zhang, Y. Yu, G. Zhang, Synthesis and characterization of red mud and rice husk ash-based geopolymer composites, Cem. Concr. Compos. 37 (2013) 108–118.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Synthesis and characterization of low temperature (<800 °C) ceramics from red mud geopolymer precursor Patrick N. Lemougna a,b,⇑, Kai-tuo Wang a, Qing Tang a, Xue-min Cui a,⇑ a School of Chemistry and Chemical Engineering and Guangxi Key Lab of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China b Local Materials Promotion Authority, MINRESI/MIPROMALO, P.O. Box 2396, Yaounde, Cameroon
h i g h l i g h t s Compressive strength of up to 40 MPa was obtained from red mud geopolymer. Curing at 60 °C for weeks was essential for better strength development. Post heating to 700 °C increased the strength to 55 MPa and the mechanical stability. The use of sodium silicate instead of NaOH was essential for the material stability.
a r t i c l e
i n f o
Article history: Received 13 July 2016 Received in revised form 14 October 2016 Accepted 22 November 2016
Keywords: Red mud Geopolymer Temperature Ceramic Structural applications
a b s t r a c t This paper presents a way to valorize red mud for the production of potential structural materials, using geopolymer technology. Several compositions of red mud geopolymers were prepared with sodium silicate solutions. After sintering the red mud geopolymer products at 300–800 °C, their stability in water has been improved. The starting red mud was found to contain hematite, katoite, cancrinite and a few amount of diaspore, which hardly dissolved and participated in the geopolymerization reaction. However, the geopolymer gel formed was sufficient to bind the unreacted phases and form a high strength material with about 40 MPa after appropriate curing. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Red mud (RM) or bauxite residue is the major waste produced by the alumina refining industry where the Bayer process is used to extract alumina from bauxite ores [1–3]. The process involves the use of highly concentrated NaOH solution for the ore digestion at temperatures up to 240 °C and 1–6 atm pressure [3,4]. The production of 1 ton of aluminum metal produces as a by-product 4–5 tons of red mud [4]. Fresh RM slurry is usually transported to waste lakes for impoundments, followed by dewatering and drying to reduce its volume and maintenance costs [3]. The exact composition of the mud depends on the origin of bauxite and on processing conditions [5], but essentially consists of oxides and hydroxides of ⇑ Corresponding authors at: School of Chemistry and Chemical Engineering and Guangxi Key Lab of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China. E-mail addresses: [email protected] (P.N. Lemougna), [email protected]. cn (X.-m. Cui). http://dx.doi.org/10.1016/j.conbuildmat.2016.11.108 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
Fe, Al and Si along with some quantities of CaO, TiO2, Na2O, and is alkaline in nature [5,6]. The worldwide bauxite residue disposal areas contains an estimated 2.7 billion tons of residue, with an increase of approximately 120 million tons per annum, representing an environmental and economic problem [1–3,7]. To date, although enormous efforts have been made on RM treatment, recycling, and utilization [1,3,7–10], only a small fraction, most probably <5 wt% is being used in few countries in specific industrial processes such as cement production, the rest being stored [9]. An economical widely accepted technology for the recycle and reuse of RM has yet to be developed [2,3,9,10], hence the need of further researches to explore possibilities of valorizing red mud [1,10]. Among the utilization options, construction and building materials pose lower risk for implementation and the manufacture of geopolymers based on RM including controlled low strength materials were suggested as interesting area of research to be explored [1,2]. Geopolymers are a class of inorganic polymer materials which have attracted interest during the last decades due to their inter-
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esting physical, structural and thermal properties [11–15]. Most of previous reports on the use of red mud for the development of potential building materials using geopolymer synthesis have been limited to alkali-activated composites of red mud with other aluminosilicate materials [16–19], likely due to the difficulty of obtaining good structural properties from a pure RM geopolymer based system. Attempts to produce traditional ceramic materials showed a substantial inertia of RM up to 900 °C, resulting in a cost disadvantage need of high temperatures of 1000–1200 °C to obtain potential useful structural properties [7,8,20,21]. The present work investigates the possibility to produce inorganic polymers using red mud as the aluminosilicate precursor, and the effect of post heating of the products to produce low temperature ceramics. Several compositions of the red mud geopolymers were prepared by varying the modulus (R = SiO2/Na2O) of the activating solution from 1.6 to 2.2. The geopolymer samples were subjected to different curing regimes (sealed and humid) and the effect of post heating (200–800 °C) was assessed on the optimally formulated products. The starting red mud was also treated at some temperatures for comparative studies. The resulting products were characterized by X-ray diffraction, Scanning Electron Microscopy, Differential Scanning Calorimetry and Infrared spectroscopy. The wet and dry compressive strengths, the leaching test, water absorption and porosity were then performed to assess the suitability of the synthetized harmless products for potential structural applications. 2. Experimental
open but the samples were maintained inside the bag and, the third day, the samples were removed from the plastic bag to the oven atmosphere. This low and long curing procedure was adopted to reduce the sensitivity of crack formation observed on the red mud. Part of the 7 days dried specimens was subjected to heat treatment from 200 to 800 °C at 100 °C interval, in an electric furnace, heating rate of 2 °C/min with a dwell time of 2 h at each temperature. The heating rate of 2 °C/min was adopted to reduce the sensitivity to form cracks, which was observed at higher heating rates, with negative effect on the mechanical properties. The temperature 60 °C was chosen for curing because relatively poor performances were observed during preliminary investigations on specimens cured at 25 and 40 °C. The post heating temperature range of 200–800 °C was chosen because it is below the usual sintering temperatures of traditional ceramics and below the temperature at which red mud without admixtures generally presents a substantial inertia [7,8]. On the basis of the phases identified by XRD, the pure red mud was also treated at 300 °C, 500 °C, 700 °C and 800 °C for X-ray comparative study. 2.3. Characterization method 2.3.1. XRD and FTIR analyses The samples were powdered and examined by X-ray diffraction with a Rigaku Mini Flex 600 instrument with Ni-filtered Cu (Ka) radiation, a step size of 0.02°, operated at 40 kV and 15 mA, with a dwell time of 0.5 s and a 2h range of 5–80°. The powdered samples were also pressed into KBr pellets for FTIR analysis using a Thermo Scientific FTIR spectrometer.
2.1. Materials The red mud used in this study was from Guangxi Province, China. The specific surface area determined by the BET method was 8.04 m2/g. The oxide composition determined by X-ray fluorescence is reported in Table 1. The alkaline activating solutions with silica moduli (R = SiO2/ Na2O) of 1.6–2.2 with 0.2 interval were prepared by dissolving solid sodium hydroxide in a commercial sodium water glass with R = 3.3. The alkaline activating solutions were sealed and stored for a minimum of 24 h prior to use. 2.2. Specimens preparation The preparation of the fresh mixture was performed by mixing red mud, water glass with different moduli (R = 1.6; 1.8; 2.0 and 2.2), and some amount of deionized water. The details on the mix proportioning are presented in Table 2. The mixing process was performed for about 10 min, using an electric mixer at 600 rpm, up to obtaining a homogenous paste. The samples were then casted in cubic alloy molds of 20 20 20 mm3, covered with a thin layer of plastic to facilitate the removal of the hardened paste upon curing. The alloy molds were vibrated on a vibration table for 2 min to remove air bubbles and sealed afterwards. The specimens were stored at 25 °C for 48 h, unmolded and kept for further 48 h sealed at 25 °C. After this period, the specimens were transferred sealed at 60 °C. Some of them were maintained sealed until the 7 and 28 days strength testing while others were progressively dried at 60 °C in the following manner: the first day, the samples were kept sealed; the second day, the plastic bag was
2.3.2. TG/DTA analysis TG/DTA analysis was performed with a simultaneous STA409PC TG/DTA measurement in air, at a constant heating rate of 5 °C/min. The sample was heated from room temperature to 1000 °C. 2.3.3. SEM/EDX analysis Scanning Electron Microscopy (SEM) and Energy Dispersive Xray Spectroscopy (EDX) were used to analyze the microstructure of the powdered red mud and the polished surfaces of the specimens with an S-3400N device (Japan Hitachi Limited Company). Specimens were impregnated using absolute ethyl alcohol, polished with SiC paper, and then coated with gold. 2.3.4. Compressive strength testing Compressive strength testing was performed on specimens using a DNS100 universal testing machine. The displacement rate used was 0.5 mm/min. The test was performed on specimens cured for 7 and 28 days and for specimens post heated at 200–800 °C. The dry strength is for specimens after the curing or post heating treatment, and the wet strength is for these specimens immersed for 24 h in water. The values were determined as the average of three samples of each composition. 2.3.5. ICP analysis, water absorption and porosity In the realization of the ICP analysis, the samples were soaked in deionized water for 96 h. The sample/water weight ratio was 1/3. The water was collected for analysis after 48 h, then renewed and collected again for analysis after an additional 48 h for some samples. This experiment was carried out for the samples treated
Table 1 Chemical composition of red mud (wt%). Fe2O3
Al2O3
CaO
SiO2
TiO2
Na2O
MgO
SO2
K2O
MnO
33.99
18.47
14.19
9.39
5.42
5.11
0.32
0.33
0.10
0.091
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Table 2 Mixture proportioning. Composition No
Modulus of the liquid water glass
Na2O/Al2O3 (molar)
SiO2 /Al2O3 (molar)
Red mud (g)
Liquid water glass (g)
Water (g)
1 2 3 4 5 6 7 8 9
1.6 1.8 2.0 2.2 1.6 1.8 2.0 2.2 –
0.96 0.91 0.87 0.84 0.75 0.73 0.71 0.69 0.99
1.66 1.68 1.70 1.72 1.34 1.36 1.37 1.38 0.86
90 90 90 90 90 90 90 90 90
30 30 30 30 18 18 18 18 36 (NaOH 6M)
6 6 6 6 18 18 18 18 /
at 60, 300, 500, 700 and 800 °C. The elemental composition of the soaked water was performed with an inductively coupled plasma optical emission spectrometer (ICP-OES, optima 5300DV, Perkin Elmer, USA) under a plasma gas flow of 15 L/min and a nebulizer gas flow of 0.6 L/min. The percentage of water absorption was obtained after immersing the specimens for 24 h in deionized water and the porosity of the samples was assessed using a PoreMaster GT Quantachrome instrument mercury porosimetry analyser (model PM 33–18). 3. Results and discussions 3.1. XRD analysis Fig. 1 presents the results on XRD pattern of the pure red mud (1-A) and alkali silicate activated red mud prepared with a silicate solution of R = 2.0 (1-B). From these results, it is observed that the main crystalline phases present in red mud are hematite, Fe2O3, PDF 33-0664; cancrinite, Na6Ca1.5 Al6Si6O24 (CO3)1.6, PDF 340176, and a few amount of katoite, Si-rich, Ca3Al2(SiO4)(OH)8, PDF no. 38-0368 and diaspore, AlO(OH), PDF 05-0355. The hematite phase is not perturbed by the heat treatment up to 800 °C, while it is observed a disappearance of the crystalline picks associated to diaspore and katoite above 300 °C and, cancrinite above 700 °C. At 800 °C, there is the appearance of a new phase, gehlenite, Ca2Al2SiO7, PDF 35-0755, likely formed from the decomposition of cancrinite (Fig. 1A). The comparison of the spectra of the pure
A)
red mud heated at 300–800 °C and the inorganic polymers heated in the same temperature range shows that there is only little difference in the qualitative crystalline composition of the two series below 800 °C. At 800 °C, we have the appearance of nepheline in the heated inorganic polymer, which is not the case for the heated starting red mud at the same temperature. No major additional crystalline phase was formed upon alkaline treatment suggesting the amorphous character of the newly formed phase resulting from the mixing of red mud with the alkaline silicate reagent. However, the possibility of formation of some additional katoite is not excluded as the formation of this mineral was suggested to be favorable in poor silica CaO–SiO2–Al2O3–H2O system [22], which is close to the experimental conditions of our study. The few change observed in the XRD spectra of the starting red mud and the inorganic polymer could be attributed to the initial alkaline character of the starting red mud and the relative mild alkalinity of the alkaline reagent used, which hardly dissolved the initial crystalline phases in the red mud. The red mud and the inorganic polymer subjected to heat presented much simpler diffractograms suggesting an increase of the amorphous content upon heating. These observations are in agreement with previously reported studies in red mud. The presence of katoite and few diaspore was reported in several red mud samples, especially those from China [10,19,23]. Katoite was also reported to be present as hydratation product in calcium aluminate cementitious materials [24,25]. Some studies on the thermal behavior of red mud attributed the decomposition of cancrinite below 900 °C to the CO2 evolu-
B) 800°C-( j)
800°C-(e)
700°C-(i) 700°C-(d) 500°C-(h) 500°C-(c) 300°C-(g) 300°C-(b)
60°C-(f)
(a)
2 (°)
(a)
2 (°)
= Katoite, Si-rich, Ca3Al2(SiO4)(OH)8 , PDF n° 38-0368 = Hemate, Fe2O3, PDF n° 33-0664 = Cancrinite, Na6Ca1.5 Al6Si6O24 (CO3)1.6, PDF n° 34-0176 = Gehlenite, Ca2Al2SiO7, PDF n° 35-0755 = Diaspore, AlO(OH) , PDF n° 05-0355 = Nepheline,NaAlSiO4 , PDF n°35-0424 Fig. 1. XRD analysis of pure red mud (A) and inorganic polymer (B), treated at indicated temperatures.
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tion, hematite constituting the fundamental phase up to 1100 °C [7]. The presence of the starting crystalline phases in inorganic polymers prepared from red mud, mainly hematite and cancrinite has been also previously reported [3,16,17,26], suggesting the absence or low dissolution of these minerals in the alkaline condition of the inorganic polymer synthesis.
3.2. FTIR analysis The infrared spectra of red mud and synthetized products are presented in Fig. 2. The large band at 1000 cm1 is assigned to the stretching vibrations of Si(Al)–O groups, and it is sensitive to the content of structural Si and Al [27,28]. It is observed that this band enlarges after treatment of the sample with sodium silicate solution, likely suggesting an increase in the amorphous content in the material. Another observation is the relatively great difference in the shape of the band in the specimen treated at 800 °C compared to those treated at 60–700 °C, suggesting a relatively important change in the structure of the material between 700 and 800 °C. This change is attributed to the onset of solid state reactions for the formation of high temperature mineral such as nepheline as observed in the XRD section. The band around 1110 C m1 arises from the presence of Si-O-Si bond [2]. This band also decreases in the inorganic polymer specimens, suggesting the substitution of Si/O/Si bonds with Si/O/Al bonds during the formation of the inorganic polymer network [2,29]. The stretching vibrations of the FeAO bands of the hematite structure are observed around 450 and 550 cm1 [27,28,30]. The band around 1400– 1500 cm1 indicates the presence of OACAO [2,27]. It is noted that this band is progressively reduced as the temperature increases. The bands around 1600–1700 cm1are attributed to water, since they correspond to the characteristic regions of O–H stretching and H–O–H bending in H2O [2,27,28,30]. The high intensity of this band in the inorganic polymer treated at 60 °C is attributed to the presence of some structural water in the sample. It is observed that this band decreases on heating, but do not disappears completely, the remaining intensity at high temperature likely arising from the adsorption of water in the surrounding air by the sample, as often observed in red mud or hematite samples [27,28,30].
800ºC (f) 700ºC (e)
500ºC (d)
300ºC (c) 60 ºC (b) (a)
Wavenumbers (cm-1) Fig. 2. FTIR analysis of the pure red mud (a) and inorganic polymers (b–f), treated at indicated temperatures.
Exo
Fig. 3. TG/DTA analysis of inorganic polymer cured at 60 °C.
3.3. TG/DTA analysis Fig. 3 shows the TG/DTA analysis and the derivative weight loss of the inorganic polymer. From this Figure, we can see that the total weight loss after heating the inorganic polymer at 1000 °C is about 13%, about half of that obtained with metakaolin-based inorganic polymer [31,32]. The derivative weigh loss indicates that the weight loss upon heating is not constant. We have four distinct regions of weight loss. On the basis of the chemical and the mineralogical composition of the sample, we can say that the weight loss around 100 °C corresponds to the loss of residual free water in the sample. The one around 250–450 °C is likely to arise from the loss of structural water in the inorganic polymer as well as the decomposition of katoite and diapore [19,24]. The one around 500–600 °C is likely to arise from a partial release of CO2 from cancrinite and/or the decomposition of residual structural water in the inorganic polymer while the one around 650–700 °C is assigned to the decomposition of cancrinite, which completely disappears above 700 °C, in consistence with the X-ray analysis and previously reported study on red mud [7]. 3.4. SEM/EDX analysis Scanning Electron microscopy analysis of the red mud powder (Fig. 4) shows that red mud particles are very fine, with many particles of size below 1 lm. Polished samples showed some similarities in the microstructure of the inorganic polymer cured at 60 °C for 28 days (a) and its homologues post heated at 300 and 800 °C (Fig. 5). These similarities are likely to arise from the persistence of less reactive phases such as hematite in this temperature range in the geopolymer. The more obvious change observed in the microstructure evolution upon heating is marked by the change in color of the post heated sample which became less dark in color (Figs. 5 and 6). From the weight and atomic composition obtained by EDX analysis (Fig. 6), we can say that the composition of the inorganic polymer mixture is almost homogenous at a microscopic scale. The heterogeneous distribution of elements observed on EDX map (not shown here) is more marked for iron and is in agreement with the presence of not dissolved hematite in the XRD analysis. Si,
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20µm
3µm
Fig. 4. SEM of red mud powder.
a
a’
Pores
500 µm
b
3 µm
b’
Pores Air bubble
3 µm
500µm
c
c’
Pores
500µm
3 µm
Fig. 5. SEM analysis at lower and higher magnification at different temperatures: a and a0 , 60 °C; b and b0 , 300 °C; c and c0 , 800 °C.
Al, Na and Ca are more homogeneously distributed, likely to suggest their participation in the inorganic polymer formation. This result is in agreement with some previous studies on alkali activation of different red muds [2–4], and suggests that the inertia of iron, usually in the form of hematite, is likely to be a common fact during alkali activation of red mud. Hairi et al., [4] studied the role of iron during alkali activation of a red mud from Canada and observed a similarity between the Mössbauer spectra of the starting materials and their corresponding geopolymers, indicating that
the iron components, mainly from hematite and goethite in their case, was largely present as spectator species rather than becoming involved in the geopolymer structure. Raw materials for inorganic polymer synthesis can been classified into three groups depending on the amount of Ca present: high Ca system, low Ca system and intermediate Ca system [13]. Based on the chemical composition of the system, red mud can be classified as intermediate Ca system which can form a mixture of C-A-S gel and N-A-S gel during alkaline activation [13]. When combining
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a
Cracks
20 µm b
20 µm c
20 µm Fig. 6. SEM analysis at middle magnification at different temperatures: a, 60 °C; b, 300 °C; c, 800 °C.
the results from XRD, FTIR and SEM, we can say that the crystalline phase in the red mud hardly dissolved and participated in the formation of the gel. However, the amount of gel formed was sufficient to bind the unreacted crystalline phases and formed a high strength material with increasing stability to water when transformed into low temperature ceramic upon heating, as shown in the compressive strength section. 3.5. Compressive strength analysis The 28 days compressive strength values for the dry and wet samples from compositions 1–4 are presented in Fig. 7. The error
bar indicates the standard deviation from three replicate specimens. From this figure, it is observed that the values of the dry compressive strength of the samples prepared with different moduli of the activating solution vary between 30 and 40 MPa. The best values are obtained for the composition prepared with the silicate solution with the moduli R = 1.8 and 2.0, with a better wet performance observed with R = 2.0. This trend is maintained for the values of the wet strength which are in the range 5– 25 MPa. The reduction of strength upon 24 h immersion in water is more significant for the compositions with the activation solution moduli of 1.6 and 2.2, which exhibited a decrease of about 85% and 65% respectively. The reduction of strength for the
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Fig. 10. Effect of post heating on the dry and wet compressive strength. Fig. 7. Dry and wet 28 days compressive strength of samples cured at 60 °C.
Fig. 8. Dry and wet compressive strength of samples post heated at 300 °C.
Fig. 9. Effect of sealed and open curing on the 7 and 28 days compressive strength.
composition with R = 1.8 and 2.0 is less significant, about 61 and 49% respectively. The strength reduction after immersing the specimens in water is often observed for some inorganic polymers and can be attributed to the hydration of some SiAOASi bonds to SiAOH bonds, weakening the structure [33,34]. The composition prepared with the solution with R modulus of 2.0 still presented the best strength at 300 °C (Fig. 8). Further investigation on the strength development from this composition is presented in Figs. 9 and 10. From Fig. 9, it is observed that curing in sealed environment led to low strength when the products are not dried afterward. A 7 days compressive test (4 days at 25 °C and 3 day at 60 °C as described in the experimental section) only led to compressive strength of about 4 and 15 MPa for sealed and dry curing respectively. Prolonging curing time does not significantly improve strength in sealed environment which led to a 28 days compressive strength of about 10 MPa at 60 °C, hence, about four times smaller than their counterparts cured in open environment. Curing in sealed environment is essential at the beginning of the curing regime to reduce the sensitivity to cracks formation, but the products should be progressively dried to achieve their maximum strength performance. The inorganic polymers from red mud are sensitive to humidity and water which significantly affect the strength. Fig. 10 shows that post heating of the sample does not significantly improve the strength from the well cured and dried sample, but improve the product stability in water. A maximum strength of about 55 MPa is however observed on the products heated at 700 °C. Exposure of geopolymers to heat is not always leading to an improvement of the mechanical properties [35]. However, metakaolin geopolymers prepared with sodium or potassium alkaline reagents were reported to be fire resistant, with thermal stability up to about 900 °C [36]. Kong and Sanjayan [37] also observed that some fly ash-based geopolymer can consolidated further when exposed to elevated temperatures up to 800 °C. Considering the results from XRD, SEM and FITR, the strength development after curing is associated to the formation of amorphous gel surrounding the unreacted phases in the red mud. The strength and stability of this amorphous gel increases with prolonging curing temperature and by post heating the products. The post heating improves the stability of the amorphous gel, possibly by akin reaction, leading to the formation of low temperature ceramics with lower or no strength loss after water immersion for well designed compositions. Actually, it was observed that the composition N° 9 presented some instability in water while the compositions N° 5–8 were very sensitive to crack formation and resulted in variable low
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strength values after 28 days of curing or upon heating, mainly below 15 MPa. These strengths were in the range 3–10 MPa for geopolymers and 5–15 MPa for post heated geopolymer samples up to 800 °C. This shows that concentrated sodium silicate medium is essential to dissolve the reactive part of red mud for a better performance of the synthesized materials. However, the amount of sodium in the best compositions did not exceed the requirement for the equilibrium of all the aluminum in red mud by sodium during the geopolymerization reaction, as shown by the Na2O/Al2O3 molar ratios in Table 2. Hence, the performance of the red mud based geopolymer is not only driven by the alkalinity of the mixture; an appropriate use of silicate solution instead of NaOH solution is essential to improve the mechanical performance, likely by an increase of the SiAOASi bonds in the mixture, as often observed in metakaolin geopolymer [38]. 3.6. ICP analysis, water absorption and porosity The ICP analysis of water collected after soaked the samples for 48 h in deionized water is presented in Table 3. From this table, we can see that the main elements diffusing in water are Al, Na, As and
Fig. 12. Water absorption and porosity of the inorganic polymer and its counterparts post heated at indicated temperatures.
Table 3 ICP analysis of the soaked water for the samples treated at indicated temperatures. Elements
Different concentration in collected soaked water (mg/l) 60 °C
300 °C
500 °C
700 °C
800 °C
Initial water after the first 48 h Al As Cd Cu Mn Fe Na Ni Pb Ti Zn
0.26 8.49 <0.01 <0.01 0.01 0.02 5790 <0.01 0.32 0.02 <0.01
1.57 3.65 <0.01 <0.01 <0.01 0.02 4936 <0.01 0.38 0.01 <0.01
60 °C
500 °C
Renewed water the 2nd 48 h 370 3.82 <0.01 <0.01 0.01 0.06 2466 <0.01 <0.01 <0.01 <0.01
981 1.69 <0.01 <0.01 0.62 0.17 1781 <0.01 <0.01 <0.01 <0.01
1572 4.51 <0.01 0.19 <0.01 0.56 3731 <0.01 0.76 0.01 0.02
0.28 0.36 <0.01 <0.01 <0.01 0.01 531 <0.01 0.09 0.01 <0.01
Fig. 11. Soaked samples for the leaching test (A: after the first 48 h; B: after the second 48 h with renewed deionized water).
117 1.60 <0.01 <0.01 <0.01 <0.02 946 <0.01 <0.01 <0.01 <0.01
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Fig. 13. Pore size distribution of the inorganic polymer and its counterparts post heated at indicated temperatures.
to a lesser extent Fe. From these analyses we can say that only part of the aluminum in red mud took part in alkali activation, forming a gel that bonded the less reactive phases. Upon post heating, there was an increase of the sensitivity to diffuse of the aluminum, mainly from the remaining crystalline phases such as diaspore, leading to an increase of Al diffusing in water with the increase of the post heating temperature. This is in agreement with the increase of the amorphous character (much simpler diffractogram) of red mud and geopolymers upon heating, and suggests an interest for the study of the effect of preheating red mud at 300–800 °C prior to alkaline activation in future research work. The breakage of AlAO bonds, with increasing post heating temperature is not excluded, mainly above 700 °C, and could explain the decrease observed in the compressive strength between 700 and 800 °C. The relatively high amount of Na released is associated with the fact that only part of aluminum present in red mud reacted and the global reduction of released Na with increasing temperature is in consistence with the increase of the stability of the formed phase and the material upon heating. The soaked samples after the first 48 h in deionized water and after the second 48 h with renewed water are presented in Fig. 11 A and B respectively. The water absorption test (Fig. 12) shows that the percentage of water absorption increase with the post heating of the sample up to a maximum value of 18% at 700 °C. A slight decrease is observed at 800 °C and is attributed to the onset of solid state reaction usually observed in the traditional ceramic process. It is worth pointing out that the overall linear shrinkage of the specimens after heating at 800 °C was below 1.6%. The consideration of the value of water absorption of 17% at 800 °C and the low linear shrinkage suggests that the glassy phase is yet to be achieved at this temperature. The increase in water absorption value is attributed to the decomposition reactions presented in the TG/DTA section, increasing the porosity in the sample. This is supported by the porosity data presented in Figs. 12 and 13. From these Figures, it is observed that the porosity of the samples is in the range 24–35%, globally following the trend of water absorption, albeit the maximum of porosity is observed at 500 °C (Fig. 12). It is also noted that most of the pores of all the samples are in the range 0.03–0.5 lm, with the main population around 0.2 lm (Fig. 13). These porosity data are relatively similar to those observed in some ceramics and fly ash based geopolymer [39,40]. In summary, we can say that concentrated sodium silicate solution is needed to dissolve the reactive part of red mud, although all sodium in the mixture is not taking part in the reaction. The release of some elements from this red mud geopolymer suggests the need of washing the products prior to their use for structural applications. This option is however realistic when considering the increasing issue associated to red mud
disposal [41]. The washed water can then be dried and the residual solid which will be insignificant compared to the initial volume of the red mud could be treated appropriately.
4. Conclusions The valorization of red mud remains an important issue to be addressed. The present studies showed that it is possible to develop consolidated materials with interesting structural properties with some red mud, using inorganic polymers chemistry. The relatively little change between the structure of the starting red mud and the inorganic polymer was attributed to the alkaline character of red mud, the low sensitivity of the minerals present in the red mud to the alkaline condition of the inorganic polymer synthesis and the relatively small amount of the newly formed phase, binding the less reactive phases. Long curing, low drying and appropriate use of sodium silicate solutions were found to be essential for better strength development. The value of the 28 days dry strength for the best composition obtained with a liquid water glass with a modulus R = 2.0 was about 40 MPa. The strength reduction when immersing the products in water was not significant after post heating of the samples which transformed them into low temperature ceramics with a maximum strength of about 55 MPa at 700 °C. The need for long curing, the relatively high sensitivity to form cracks and the leaching of some elements were found to be the main concerns for the valorization of red mud as aluminosilicate precursor using geopolymer technology. However, efforts in overcoming these concerns appear realistic when considering the increasing issues associated to red mud harmless disposal. Acknowledgments This work was supported by the Chinese Natural Science Fund (Grant: 51262002, 21566006 and 51561135012) and Postdoctoral Project from Guangxi University. References [1] C. Klauber, M. Gräfe, G. Power, Bauxite residue issues: II. Options for residue utilization, Hydrometallurgy 108 (2011) 11–32. [2] N. Ye, J. Yang, S. Liang, Y. Hu, J. Hu, B. Xiao, Q. Huang, Synthesis and strength optimization of one-part geopolymer based on red mud, Constr. Build. Mater. 111 (2016) 317–325. [3] J. He, Y. Jie, J. Zhang, Y. Yu, G. Zhang, Synthesis and characterization of red mud and rice husk ash-based geopolymer composites, Cem. Concr. Compos. 37 (2013) 108–118.
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