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Preparation of glass-ceramic foams using extracted titanium tailing and glass waste as raw materials
Construction and Building Materials 190 (2018) 896–909
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Preparation of glass-ceramic foams using extracted titanium tailing and glass waste as raw materials Cuiping Xi a, Feng Zheng a,⇑, Jiahe Xu a, Weiguang Yang a, Yuqing Peng a, Yang Li b, Peng Li c, Qiang Zhen a,⇑, Sajid Bashir d, Jingbo Louise Liu d a
Research Center of Nano Science and Technology, School of Materials Science and Engineering, Shanghai University, Shanghai 200444, PR China The State Key Laboratory for Refractories and Metallurgy, School of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, PR China School of Iron and Steel, Soochow University, Suzhou 215021, PR China d Department of Chemistry, Texas A&M University-Kingsville, MSC 161, 700 University Boulevard, Kingsville, TX 78363, United States b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Glass-ceramic foams are prepared
using extracted titanium tailing as raw materials. The pore size, number and distribution of the foams could be controllable prepared. The optimized condition for preparing low density glass-ceramic foams is obtained. The formation mechanism of glassceramic foams is reasonably given.
a r t i c l e
i n f o
Article history: Received 24 April 2018 Received in revised form 20 September 2018 Accepted 24 September 2018
Keywords: Glass-ceramic foams Extracted titanium tailing Glass waste Apparent density Compressive strength
a b s t r a c t Extracted titanium tailing, a large amount of solid waste produced from Panzhihua in China, not only occupies a lot of lands, but also seriously pollutes the environment. Glass-ceramic foams could be synthesized using extracted titanium tailing as a raw material, mixed with glass waste, foaming agent Na2CO3 and fluxing agent B2O3 by a simple powder foaming method. The effects of preparing conditions such as sintering temperature (760–790 °C), sintering time (10–40 min), Na2CO3 content (0–3 wt%) and B2O3 content (0–3 wt%) on the micromorphology have been systematically investigated by scanning electron microscopy. The crystallinity, compressive strength, thermal conductivity, and thermal expansion coefficient are investigated by X-ray diffraction, electronic universal testing machine, heat flow method and thermal dilatometer. The pore size, percent, and distribution of the glass-ceramic foams can be controllably prepared by adjusting the preparing conditions. And the pore structure plays an important role in determining the apparent density, compressive strength, thermal conductivity, and thermal expansion coefficient. The glass-ceramic foam has a low apparent density of 0.30 ± 0.01 g cm3, an available compressive strength of 1.0 ± 0.1 MPa, a thermal conductivity of 0.060 ± 0.002 W m1 °C 1, a high porosity of 88.0% and an average thermal expansion coefficient of 5.27 106 m m1 °C1 under the optimal preparation parameters, indicating that the glass-ceramic foam could be widely used in construction and building industry. Ó 2018 Elsevier Ltd. All rights reserved.
⇑ Corresponding authors. E-mail addresses: [email protected] (F. Zheng), [email protected] (Q. Zhen). https://doi.org/10.1016/j.conbuildmat.2018.09.170 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
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In this paper, glass-ceramic foams have been prepared by using extracted titanium tailing and glass waste as raw materials. We demonstrate that the sintering temperature, sintering time, Na2CO3 content and B2O3 content have great impacts on the micromorphology and apparent density. The effects of pore size, number, and distribution on the compressive strength of the glass-ceramic foams have been systematically investigated. And the formation mechanism of the glass-ceramic foams prepared by extracted titanium tailing has also been discussed. Preparation of glass-ceramic foams using extracted titanium tailing and glass waste as raw materials are developed in order to minimize the environmental issues and effective utilization of resources. The glass-ceramic foam sintered in the optimism condition has a compressive strength of 1.0 ± 0.1 MPa and a low thermal conductivity value of 0.060 ± 0.002 W m1 °C1 and an average thermal expansion coefficient of 5.27 10-6 m m1 °C1, which could be widely used as roofs, walls, floors, ceilings, fireplaces, and grills.
1. Introduction In the Panxi area of southwestern China, a large amount of high titanium blast furnace slag (HTBFS, 250 million tons per year) [1] is generated in the process of steel-making and iron-making using vanadium-titanium magnetic iron ore that belongs only to Panzhihua. These piled HTBFS not only occupies a lot of lands, but also seriously pollutes the environment. The HTBFS contains 20 wt%–27 wt% TiO2 that will be a great loss if it cannot be fully utilized [2]. To address this concern, many efforts have been made to develop the extraction technology of titanium from HTBFS [3– 5]. Wang et al. [6] and Zhang et al. [7] investigated the enrichment conditions of perovskite phase (CaTiO3) from HTBFS, respectively. Sui et al. [8] prepared nano-TiO2 photocatalysts from Ti-bearing slag. And this photocatalysts showed a good photocatalysis activity for the degradation of Rhodamine B. Li et al. [9] synthesized potassium hexatitanate (K2Ti6O13) whiskers from Ti-bearing electric arc furnace molten slag. And this K2Ti6O13 whiskers had a excellent thermal stability which could not decompose up to 1430 °C. Although the above-mentioned works make contributions to the enrichment and extraction of titanium (Ti) element from Tibearing furnace slag, the large amount of extracted titanium tailing has not been processed effectively. Most of the residue is used for low-value products, such as filling and paving material. There are also a small number of studies prepared high value-added products. Li et al. [10] synthesized NaA zeolite and sodalite using the residue as the raw material. However, the requirement of extracted titanium tailing is limited. Through the process of titanium extraction from HTBFS, the residues are mainly silicon dioxide (SiO2), calcium oxide (CaO), magnesium oxide (MgO), aluminum oxide (Al2O3) and titanium dioxide (TiO2) in the extracted titanium tailing [11]. The main elemental composition of the extracted titanium tailing is similar to the components of the ceramic phase [12]. And the SiO2 and CaO are also the main components of glass phase [13]. Thus the extracted titanium tailing can be used as a raw material for the manufacture of glass-ceramic foams. Glass-ceramic foams are mixtures of glass phase and the ceramic phase with a large number of micro pores [14]. It is a high value-added product, which not only has advantages of both glass phase and ceramic phase, but also some unique properties, such as low density and dielectric loss, high strength and mechanical property, good erosion resistance and thermal-shock resistance, excellent heat insulation and controllable thermal expansion [15–17]. Thus the glass-ceramic foams can be widely used in the fields of building insulation, acoustic insulating and environmental chemistry [18,19]. Many researchers have got to start preparing glass-ceramic foams from various solid wastes. Zhou et al. [20] prepared ceramic foams using fly ash by powder foaming method. The apparent density could be as low as 0.32 g cm3. Li et al. [21] fabricated glass-ceramic foams by using coal gangue as the raw material. The apparent density and flexible strength varied between 0.59 and 0.68 g cm3 and 4.5– 6.0 MPa, respectively. Tang et al. [22] prepared glass-ceramic foams using municipal solid waste. Their products had a high strength (2.5 MPa) with a low density (0.3 g cm3). Although the glass-ceramic foams have been prepared by various solid wastes. Few kinds of literature reported glass-ceramic foams prepared by extracted titanium tailing. The optimal preparation parameters and formation mechanism of glass-ceramic foams were not clear.
Fig. 1. The sintering process versus sintering time.
Fig. 2. Differential scanning calorimetric curve of the extracted titanium tailing, glass waste and additives mixture.
Table 1 The chemical composition of extracted titanium tailing and glass waste (wt%).
Extracted titanium tailing. Glass waste.
SiO2
CaO
Na2O
MgO
TiO2
Al2O3
Fe2O3
C
others
21.3 70.0
35.2 12.7
0.5 11.5
9.5 3.9
11.0 /
8.9 0.9
2.5 0.4
9.6 /
1.5 0.6
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2. Experimental sections 2.1. Raw materials Extracted titanium tailing was obtained from Panzhihua Steel Co. Ltd (Sichuan Province, China). Glass waste was gathered from broken glass (glass bottles, discarded window glasses, and glassware, etc). The chemical composition of extracted titanium tailing and glass waste were examined by using an X-ray fluorescence spectrometer (XRF-1800, Shimadzu Limited), as shown in Table 1. Sodium carbonate (Na2CO3, 99.8%), diboron trioxide (B2O3, 98%) and ethyl alcohol (CH3CH2OH, 99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd, China.
2.2. Preparation of glass-ceramic foams Extracted titanium tailing and broken glass were crushed by crushing machine and sieved by 180 mesh screen, respectively. And then stoichiometric amount of
extracted titanium tailing and glass waste (2:8) with the addition of 2 wt% Na2CO3 and 2 wt% B2O3 were mixed by wet ball milling with ethyl alcohol as milling media. Subsequently, the mixed raw materials were pressed at 20 MPa to form cylindrical bodies with a diameter of 20 mm and a thickness of 5 mm. The samples were heated with a heating rate of 5 °C min1 from room temperature to 400 °C, then these samples were kept at 400 °C for 20 min. After the furnace temperature was stable, the samples were heated up to 600 °C with a heating rate of 5 °C min1. Then the heating rate rose to 10 °C min1 from 600 to 780 °C. It follows an isothermal process at a temperature of 780 °C for 30 min to complete the foaming process. After the sintering process finished, the furnace was cooled to room temperature with a cooling rate of 10 °C min1 (Fig. 1). In order to investigate the effects of sintering temperature, sintering time, Na2CO3 content and B2O3 content on the average pore diameter, apparent density and compressive strength of the glass-ceramic foams. The sintering temperatures were varied from 760 °C to 790 °C with an interval of 10 °C, the sintering time was varied from 10 min to 40 min with an interval of 10 min, the content of Na2CO3 was varied from 0 to 3 wt% with an interval of 1 wt% and content of B2O3 was varied from 0 to 3 wt% with an interval of 1 wt%.
Fig. 3. Cross-sectional view SEM images and product photos of glass-ceramic foams prepared at different sintering temperatures (a and b) 760 °C, (c and d) 770 °C, (e and f) 780 °C, (g and h) 790 °C (time: 30 min, Na2CO3 content: 2 wt%, B2O3 content: 2 wt%).
C. Xi et al. / Construction and Building Materials 190 (2018) 896–909 2.3. Characterization of glass-ceramic foams The thermal effect curve of the extracted titanium tailing, glass waste, and additives mixture was measured by using a differential scanning calorimeter (DSC, Diamond, PerkinElmer instruments). The morphologies of the glass-ceramic foams were observed on a scanning electron microscope (SEM, Hitachi-1500, Japan). The diameters of every pore appeared in the SEM images are measured by Nano Measurer 1.2. The statistical data of average pore diameter was obtained by a survey of three SEM images of glass-ceramic foams. The apparent density (q) of these samples were calculated from the measured weight divided by the volume. The powder density (q0) was measured by pycnometry. The total porosity (P) was calculated according to Eq. (1):
P ¼ ð1 q=q0 Þ 100%
ð1Þ
The compressive strength of these samples was measured by using an electronic universal testing machine (DNS500, Changchun research institute for mechanical science co. Ltd, China) with a cross-head speed of 1 mm min1. The thermal conductivity of the glass-ceramic foams was determined on a thermal conductivity analyzer (DRX-RL, Instrument and meter Co., Xiangtan, China). The apparent density, compressive strength, thermal conductivity, and porosity were obtained by a survey of five sets of data for each sample. The thermal expansion coefficient of the sample was obtained on a dilatometer (Netzsch DIL 402PC). The crystal structure of the glass-ceramic foams was characterized by X-ray diffractometer (XRD, Rigaku, Dmax-2550 diffractometer using Cu Ka radiation).
3. Results and discussion 3.1. Effect of sintering temperature on the morphology and properties of the glass-ceramic foams From the literature [23], the sintering temperature plays an important role in the formation of glass-ceramic foams. Fig. 2
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shows the differential scanning calorimetric curve of the extracted titanium tailing, waste glass and additives mixture. A small endothermic peak appears at about 721 °C, and it reaches a maximum value at about 803 °C with a range of 80 °C. As is well know, the formation of the liquid phase, the decomposition of Na2CO3 and the crystallization of the ceramic phase are all endothermic reactions. Thus the sintering temperatures should be in the range of 721 °C and 803 °C. To investigate the effect of sintering temperature on the morphology and properties of the glass-ceramic foams, the samples are sintered at different temperatures from 760 to 790 °C, which are named TE-760, TE-770, TE-780, and TE-790. Fig. 3 shows the corresponding cross-sectional view SEM images and product photos. From the micro-morphologies, the average pore diameters of samples are 0.47 ± 0.35, 0.72 ± 0.47, 1.60 ± 0.86 and 1.18 ± 0.98 mm at the sintering temperatures of 760, 770, 780 and 790 °C, respectively (Fig. 4 and Table 2), which means that the average pore diameters increase with the increasing of sintering temperatures from 760 to 780 °C while the pore size decreases when the sintering temperature further increases to 790 °C. The product photos show the same trend and the uneven distribution of pore size is obviously observed on the surface of the sample sintered at 790 °C. Besides, the wall thicknesses of pores decrease as the temperatures rise. This phenomenon could be explained as follows: with the increase in temperatures, the viscosity of raw material mixtures decreases and the bubbles are more easy to expand [24]. Then the pore size increase accordingly. However, when the sintering temperature rises too high, the viscosity of mixtures is too low that the pore structure is destroyed and a
Fig. 4. Pore size distribution of glass-ceramic foams prepared at different sintering temperatures (a) 760 °C, (b) 770 °C, (c) 780 °C, (d) 790 °C (time: 30 min, Na2CO3 content: 2 wt%, B2O3 content: 2 wt%).
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Table 2 The structure and properties of glass-ceramic foams prepared at different sintering temperatures. Sample
Pore size distribution (mm)
Apparent density (g cm3)
Compressive strength (MPa)
Thermal conductivity (W m1 °C1)
Porosity (%)
TE-760 TE-770 TE-780 TE-790
0.47 ± 0.35 0.72 ± 0.47 1.60 ± 0.86 1.18 ± 0.98
0.54 ± 0.03 0.44 ± 0.03 0.30 ± 0.01 0.40 ± 0.02
3.3 ± 0.2 1.3 ± 0.1 1.0 ± 0.1 1.0 ± 0.1
0.150 ± 0.003 0.112 ± 0.004 0.060 ± 0.002 0.093 ± 0.002
78.4 ± 1.2 82.4 ± 1.2 88.0 ± 0.4 84.0 ± 0.8
large number of connected pores are generated. Then the gas is easy to escape from connected pores, resulting in small and uneven pore sizes. Fig. 5a depicts the apparent density and compressive strength change curves of glass-ceramic foams at different temperatures (760 °C–790 °C). From Fig. 5a, the apparent density of glass-ceramic foams is 0.54 ± 0.03, 0.44 ± 0.03, 0.30 ± 0.01, 0.40 ± 0.02 g cm3 at sintering temperatures of 760, 770, 780 and 790 °C, respectively. And the corresponding compressive strength of these samples is 3.3 ± 0.2, 1.3 ± 0.1, 1.0 ± 0.1 and 1.0 ± 0.1 MPa. The compressive strength of these samples could satisfy the requirement of a commercial standard (0.4–6.0 MPa) [25]. The apparent density decreases with the increasing of sintering temperatures (from 760 to 780 °C), then increases when the sintering temperature further increases to 790 °C. The apparent density has the opposite variation trend with the pore sizes as the sintering temperatures changes. The apparent density is mainly depended on the size and number of pores in the glass-ceramic foams. The compressive strength of samples has a sharp decline with the sintering temperatures increasing from 760 to 780 °C and the compressive strength keeps almost unchanged when the sintering temperatures increase from 780 to 790 °C. It could be explained as follows: at low sintering temperatures (760–780 °C), the pore sizes increase as the temperature increased. Then both the apparent density and compressive strength decrease accordingly. At high sintering temperatures (780–790 °C), the compressive strength and apparent density have the opposite variation trend mainly because the sample sintered at 790 °C has a large number of connected pores and the sizes of pores are uneven [26–28]. Through the analyses above, the glass–ceramic foams sintered at 780 °C have a low apparent density of 0.30 ± 0.01 g cm3, which are better than those reported in articles concerned [29–31]. From Fig. 5b, the thermal conductivity of the sample has a positive correlation with the apparent density since the thermal conductivity of a sample with more pores would be even lower [32]. The sample sintered
in the optimism condition has a low thermal conductivity value of 0.060 W m1 °C1. The porosity of the sample shows a reverse trend to that of the apparent density. And the sample sintered in the optimism condition has a high porosity of 88.0%. The high porosity value is significantly higher than those of previously reported glass-ceramic foams [2,18,21]. The thermal expansion coefficient of the sample is in a range of 2.02–8.55 106 mm1 °C1 from 30 to 300 °C (Fig. 6). It has the same order of magnitude as some literature [33,34]. Above all, the glass-ceramic foam sintered in the optimism condition has an available compressive strength, a low thermal conductivity, a high porosity, and a low thermal expansion coefficient, which could be used in construction and building industry.
Fig. 6. The thermal expansion coefficient of the sample sintered in the optimism condition versus temperature.
Fig. 5. (a) Apparent density and compressive strength, (b) thermal conductivity and porosity of glass-ceramic foams prepared at different sintering temperatures (time: 30 min, Na2CO3 content: 2 wt%, B2O3 content: 2 wt%).
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3.2. Effect of sintering time on the morphology and properties of the glass-ceramic foams Fig. 7 shows the SEM images of glass–ceramic foams prepared at different sintering time (10–40 min), which are named TI-10, TI-20, TI-30, and TI-40. According to statistics (Fig. 8 and Table 3), the average pore diameters of samples are 0.39 ± 0.31, 0.87 ± 0.77, 1.60 ± 0.86 and 0.76 ± 0.56 mm at the sintering time of 10, 20, 30 and 40 min, respectively. That is to say, the average pore diameters gradually increase as the sintering time increasing from 10 min to 30 min. While the pore sizes decrease as the sintering time further increasing to 40 min. In addition, It can also be seen that the upper half are more porous than the lower half from the photos of all the four glass-ceramic foams. This phenomenon could be
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explained as follows: during the heating process, CO2 bubbles are generated in the matrix when the sintering temperature exceeds the decomposition temperature of Na2CO3. With the sintering time increased, the number of CO2 bubbles gradually increase and these bubbles are easier to merge together to form large bubbles. Then the large bubbles rise from bottom to top. However, if the sintering time is too long, the large bubbles will escape from the upper surface of the glass-ceramic foams, resulting in a sintering densification process. Fig. 9a shows the apparent density and compressive strength change curves of glass-ceramic foams at different sintering time. The apparent density of the four glass-ceramic foams is 0.61 ± 0.03, 0.49 ± 0.02, 0.30 ± 0.01 and 0.41 ± 0.02 g cm3, respectively. Similarly to the result obtained by the effect of temperature
Fig. 7. Cross-sectional view SEM images and product photos of glass-ceramic foams prepared at different sintering times (a and b) 10 min, (c and d) 20 min, (e and f) 30 min, (g and h) 40 min (temperature: 780 °C, Na2CO3 content: 2 wt%, B2O3 content: 2 wt%).
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Fig. 8. Pore size distribution of glass-ceramic foams prepared at different sintering time (a) 10 min, (b) 20 min, (c) 30 min, (d) 40 min (temperature: 780 °C, Na2CO3 content: 2 wt%, B2O3 content: 2 wt%). Table 3 The structure and properties of glass-ceramic foams prepared at different sintering times. Sample
Pore size distribution (mm)
Apparent density (g cm3)
Compressive strength (MPa)
Thermal conductivity (W m1 °C1)
Porosity (%)
TI-10 TI-20 TI-30 TI-40
0.39 ± 0.31 0.87 ± 0.77 1.60 ± 0.86 0.76 ± 0.56
0.61 ± 0.03 0.49 ± 0.02 0.30 ± 0.01 0.41 ± 0.02
6.4 ± 0.4 1.3 ± 0.2 1.0 ± 0.1 0.9 ± 0.2
0.166 ± 0.002 0.134 ± 0.003 0.060 ± 0.002 0.093 ± 0.003
75.6 ± 1.2 80.4 ± 0.8 88.0 ± 0.4 83.6 ± 0.8
Fig. 9. (a) Apparent density and compressive strength, (b) thermal conductivity and porosity of glass-ceramic foams prepared at different sintering time (temperature: 780 °C, Na2CO3 content: 2 wt%, B2O3 content: 2 wt%).
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on the micromorphology, the apparent density has the opposite variation trend with the pore sizes as the sintering time changes. The pore sizes decrease firstly and then increase with the time. While the apparent density increases firstly and then decreases. Both the peaks appear at sintering time of 30 min. The compressive strength of the four samples is 6.4 ± 0.4, 1.3 ± 0.2, 1.0 ± 0.1 and 0.9 ± 0.2 MPa, respectively. The compressive strength of sample TI-30 is weaker than those of samples TI-10 and TI-20 because of its low apparent density. However, the compressive strength of sample TI-40 is weaker than that of sample TI-30, although the sample TI-40 has a relatively higher apparent density. This phenomenon could be explained by cell coalescence. With the increas-
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ing of sintering time from 30 min to 40 min, smaller pores begin to coalesce into large pores and the pore structure becomes seriously uneven. At the same time, the pore walls will become thicker, resulting in an enhanced probability of forming critical flaws and pore opening. Thus the compressive strength does not increase with the increasing of apparent density. This part of the experiment illustrates that the sintering time needs to be 30 min in order to reach a low apparent density. But both the parameters of apparent density and compressive strength will weaken when the sintering time is too long. Combined with the thermal conductivity and porosity in Fig. 9b, the sample sintered at 30 min has good comprehensive performances.
Fig. 10. Cross-sectional view SEM images and product photos of glass-ceramic foams prepared at different Na2CO3 contents (a and b) 0, (c and d) 1 wt%, (e and f) 2 wt%, (g and h) 3 wt% (temperature: 780 °C, time: 30 min, B2O3 content: 2 wt%).
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3.3. Effect of Na2CO3 at different contents on the morphology and properties of the glass-ceramic foams We know that the pores are mainly caused by CO2 bubbles which are generated by the decomposition of Na2CO3. Thus the
content of Na2CO3 plays an important role in the formation of glass-ceramic foams. Fig. 10 shows the SEM images of glassceramic foams prepared at different Na2CO3 contents (0–3 wt%), which are named NA-0, NA-1, NA-2, and NA-3, respectively. Obviously, the average pore diameters of samples increase with the
Fig. 11. Pore size distribution of glass-ceramic foams prepared at different Na2CO3 contents (a) 0, (b) 1 wt%, (c) 2 wt%, (d) 3 wt% (temperature: 780 °C, time: 30 min, B2O3 content: 2 wt%).
Fig. 12. (a) Apparent density and compressive strength, (b) thermal conductivity and porosity of glass-ceramic foams prepared at different Na2CO3 contents (temperature: 780 °C, time: 30 min, B2O3 content: 2 wt%).
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increasing of Na2CO3 contents. The average pore diameters of samples are 0.44 ± 0.32, 0.50 ± 0.41, 1.60 ± 0.86 and 1.68 ± 0.93 mm for these samples, as obtained in Fig. 11. The porous structure still appears in the sample without addition of Na2CO3. It might be because the raw matrix contains the carbon element and the
carbon element is easily oxidized in air. It is noted that the porous structures of samples NA-2 and NA-3 have no significant changes, indicating that the gas capacity has reached saturation when the Na2CO3 content is 2 wt%.
Table 4 The structure and properties of glass-ceramic foams prepared at different Na2CO3 contents. Sample
Pore size distribution (mm)
Apparent density (g cm3)
Compressive strength (MPa)
Thermal conductivity (W m1 °C1)
Porosity (%)
NA-0 NA-1 NA-2 NA-3
0.44 ± 0.32 0.50 ± 0.41 1.60 ± 0.86 1.68 ± 0.93
0.55 ± 0.02 0.35 ± 0.02 0.30 ± 0.01 0.30 ± 0.01
5.9 ± 0.2 1.4 ± 0.2 1.0 ± 0.1 1.0 ± 0.1
0.153 ± 0.003 0.089 ± 0.004 0.060 ± 0.002 0.064 ± 0.003
78.0 ± 0.8 86.0 ± 0.8 88.0 ± 0.4 88.0 ± 0.4
Fig. 13. Cross-sectional view SEM images and product photos of glass-ceramic foams prepared at different B2O3 contents (a and b) 0, (c and d) 1 wt%, (e and f) 2 wt%, (g and h) 3 wt% (temperature: 780 °C, time: 30 min, Na2CO3 content: 2 wt%).
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Fig. 12a shows the apparent density and compressive strength change curves of glass-ceramic foams prepared at different Na2CO3 contents. As shown in Table 4, the apparent density of samples NA-0, NA-1, NA-2 and NA-3 is 0.55 ± 0.02, 0.35 ± 0.02, 0.30 ± 0.01 and 0.30 ± 0.01 g cm3, respectively. and the compressive strength of the four samples is 5.9 ± 0.2, 1.4 ± 0.2, 1.0 ± 0.1 and 1.0 ± 0.1 MPa. Through the analysis of section 3.1 and 3.2, we know that Na2CO3 will be completely decomposed to CO2 and Na2O under the condition of the sintering temperature of 780 °C for 30 min. It is also confirmed that the apparent density and compressive strength of the sample directly relate to its pore number and pore size. As the increasing of Na2CO3 contents, the CO2 gas bubbles increase, leading to a increased porosity. However, this increasing trend is not unlimited. The maximum porosity depends on the gas capacity of the raw matrix. This part of the experiment demonstrates that the sample has the minimum apparent density and available strength when the content of Na2CO3 is at 2 wt%. It’s not necessary to continue
increasing the content of Na2CO3 since the sample also has the minimum thermal conductivity and the maximum porosity (Fig. 12b). 3.4. Effect of B2O3 at different contents on the morphology and properties of the glass-ceramic foams During the formation of glass-ceramic foams, the viscosity of the raw matrix has great influence at high temperature. If the matrix has a low viscosity, it has a good fluidity and promotes the formation of gas bubbles. B2O3 could be used as a fluxing agent to lower the softening temperature and increase the liquidity. Fig. 13 shows the SEM images of glass-ceramic foams prepared at different B2O3 contents (0–3 wt%), which are named B-0, B-1, B-2, and B-3, respectively. According to the statistical data (Fig. 14 and Table 5), the average pore diameters of the four samples are 0.31 ± 0.27, 0.36 ± 0.29, 1.60 ± 0.86 and 0.46 ± 0.40 mm. The porosity increases with the increasing of B2O3 contents until
Fig. 14. Pore size distribution of glass-ceramic foams prepared at different B2O3 contents (a) 0, (b) 1 wt%, (c) 2 wt%, (d) 3 wt% (temperature: 780 °C, time: 30 min, Na2CO3 content: 2 wt%).
Table 5 The structure and properties of glass-ceramic foams prepared at different B2O3 contents. Sample
Pore size distribution (mm)
Apparent density (g cm3)
Compressive strength (MPa)
Thermal conductivity (W m1 °C1)
Porosity (%)
B-0 B-1 B-2 B-3
0.31 ± 0.27 0.36 ± 0.29 1.60 ± 0.86 0.46 ± 0.40
0.87 ± 0.04 0.60 ± 0.06 0.30 ± 0.01 0.83 ± 0.02
12.5 ± 0.6 4.5 ± 0.2 1.0 ± 0.1 9.3 ± 0.5
0.263 ± 0.003 0.160 ± 0.003 0.060 ± 0.002 0.270 ± 0.003
65.2 ± 1.6 76.0 ± 2.4 88.0 ± 0.4 66.8 ± 0.8
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Fig. 15. (a) Apparent density and compressive strength, (b) thermal conductivity and porosity of glass-ceramic foams prepared at different B2O3 contents (temperature: 780 °C, time: 30 min, Na2CO3 content: 2 wt%).
the B2O3 content reaches 2 wt%. Then a turning point appears when the B2O3 content further increases to 3 wt%. This phenomenon could be explained as follows: when a small number of B2O3 as fluxing agent is added in the raw matrix, the viscosity is reduced and big gas bubbles tend to be formed [35]. However, when a large number of B2O3 is added, the excessive of B2O3 will be converted to [BO4] tetrahedron. And the formed
[BO4] tetrahedron will increases the rigidity of the raw matrix, resulting in a high viscosity [36]. Fig. 15a shows the apparent density and compressive strength change curves of glass-ceramic foams prepared at different B2O3 contents correlated with Fig. 13. The apparent density and compressive strength are 0.87 ± 0.04, 0.60 ± 0.06, 0.30 ± 0.01 and 0.83 ± 0.02 g cm3 and 12.5 ± 0.6, 4.5 ± 0.2, 1.0 ± 0.1 and 9.3 ± 0.5 MPa, respectively, for samples B-0, B-1, B-2, B-3. Both the apparent density and compressive strength has presented concave curves as the B2O3 contents increased. And the peaks appear at sample B-2 with the content of 2 wt%. It could be explained as follows: first, the content of B2O3 fluxing agent affects the fluidity and viscosity of the raw matrix. And then the viscosity of sample affects significantly the size and number of gas bubbles, which could further affect the apparent density of glass-ceramic foams. Generally, the compressive strength is roughly proportional to the apparent density. Besides, it also can be found that the fluxing agent has a bigger impact on apparent density and compressive strength than the foaming agent at a content range of 0–3 wt%. In addition, the sample B-2 has a relatively low thermal conductivity and high porosity, which shows a good comprehensive performance than others (Fig. 15b). 3.5. The formation mechanism of glass-ceramic foams
Fig. 16. XRD patterns of sample sintered in the optimism condition (temperature: 780 °C, time: 30 min, Na2CO3 content: 2 wt%, B2O3 content: 2 wt%).
Through the front chapter analysis, the formation process of glass-ceramic foams can be described as follows: When the sintering temperature reaches a certain value, the alkali metal ions in Na2O will break silicon-oxygen bonds. And the main compositions (SiO2, CaO and Na2O) of glass phase begin to soften. The B2O3 could
Fig. 17. The formation mechanism of glass-ceramic foams.
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be used as a fluxing agent. A small content of B2O3 contributes to lowering the viscosity of the raw matrix. While excessive B2O3 can lead to the formation of [BO4] tetrahedron, which may increase the viscosity. When the sintering temperature exceeds the decomposition temperature of Na2CO3, it generates a lot of CO2 gas bubbles with the smaller size. And these smaller gas bubbles tend to form large one if the content of Na2CO3 is enough. At the same time, diopside CaMg(SiO3)2 is generated near the TiO2. Because TiO2 could be used as a nucleating agent, which effectively improve the crystallization and promote the formation of the ceramic phase. As the crystallization reaction progress, some metal ions (Fe3+, Al3+) can be dissolved in the diopside to form (Mg,Fe,A l,Ti)(Ca,Mg,Fe)(Si,Al)2O6. Then both the glass phase and ceramic phase are formed in the mixture. Once the mixture has cooled, massive micropores are generated in the matrix and the glassceramic foams are synthesized (Fig. 16). And the ceramic phase in the glass-ceramic foam is helpful to improve the compressive strength. However, if the sintering temperature is too high or the sintering time is too long, the viscosity of mixture will be dramatically decreased that a lot of connected pores are generated. Then the large bubbles rise from bottom to top and the gas is easy to escape from connected pores, resulting in side effects. The corresponding forming diagram of glass-ceramic foams is given in Fig. 17. 4. Conclusions In summary, glass-ceramic foams have been synthesized using extracted titanium tailing and glass waste as raw materials by a simple powder foaming method. It is shown that the pore structure plays an important role in determining the apparent density and compressive strength of the glass-ceramic foams. The sample with a large pore size and uniform pore distribution has a low density and a high strength. The pore structure can be controlled by changing the sintering temperature, sintering time, Na2CO3 content and B2O3 content. The combined effects of sintering temperature, sintering time and Na2CO3 content determine the pore size, number, and distribution. And the content of B2O3 contributes to lowering the viscosity of the raw matrix. The glass-ceramic foams have a relatively lower apparent density of 0.30 ± 0.01 g cm3 and an available compressive strength of 1.0 ± 0.1 MPa, a thermal conductivity of 0.060 ± 0.002 W m1 °C1, a high porosity of 88.0% and an average thermal expansion coefficient of 5.27 106 m m1 °C1, synthesized at sintering temperature of 780 °C for 30 min with 2 wt% Na2CO3 content and 2 wt% B2O3 content. Author contributions C. Xi completed the extraction procedure and undertook the sintering process. J. Xu supervised the experimental component. Y. Peng complete the DSC while W. Yang conducted the SEM study. Y. Li complete the pore size distribution and P. Li the physical measurement component. Q. Zhen conceived the project and co-wrote the first draft with F. Zheng. S. Bashir completed the SEM analysis and second draft with J. L. Liu who also checked standard deviations and Fig. 17. Conflict of Interest There are no conflicts of interest. Acknowledgments This work is supported by the National Nature Science Foundation of China (No. 51472156, 51672170, 51604202, 51708022), the
Shanghai Excellent Technology Leader Program (No. 17XD1424700), the Science and Technology Commission of Shanghai (No. 17010500600), the Science and technology project of Guangdong Province (No. 2013B090600025), the State Key Laboratory of Refractories and Metallurgy, China (No. 2016QN09), the Petroleum Research Fund of the American Chemical Society (53827-UR10) and the Robert Welch Foundation (Departmental Grant, AC-0006).
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Preparation of glass-ceramic foams using extracted titanium tailing and glass waste as raw materials Cuiping Xi a, Feng Zheng a,⇑, Jiahe Xu a, Weiguang Yang a, Yuqing Peng a, Yang Li b, Peng Li c, Qiang Zhen a,⇑, Sajid Bashir d, Jingbo Louise Liu d a
Research Center of Nano Science and Technology, School of Materials Science and Engineering, Shanghai University, Shanghai 200444, PR China The State Key Laboratory for Refractories and Metallurgy, School of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, PR China School of Iron and Steel, Soochow University, Suzhou 215021, PR China d Department of Chemistry, Texas A&M University-Kingsville, MSC 161, 700 University Boulevard, Kingsville, TX 78363, United States b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Glass-ceramic foams are prepared
using extracted titanium tailing as raw materials. The pore size, number and distribution of the foams could be controllable prepared. The optimized condition for preparing low density glass-ceramic foams is obtained. The formation mechanism of glassceramic foams is reasonably given.
a r t i c l e
i n f o
Article history: Received 24 April 2018 Received in revised form 20 September 2018 Accepted 24 September 2018
Keywords: Glass-ceramic foams Extracted titanium tailing Glass waste Apparent density Compressive strength
a b s t r a c t Extracted titanium tailing, a large amount of solid waste produced from Panzhihua in China, not only occupies a lot of lands, but also seriously pollutes the environment. Glass-ceramic foams could be synthesized using extracted titanium tailing as a raw material, mixed with glass waste, foaming agent Na2CO3 and fluxing agent B2O3 by a simple powder foaming method. The effects of preparing conditions such as sintering temperature (760–790 °C), sintering time (10–40 min), Na2CO3 content (0–3 wt%) and B2O3 content (0–3 wt%) on the micromorphology have been systematically investigated by scanning electron microscopy. The crystallinity, compressive strength, thermal conductivity, and thermal expansion coefficient are investigated by X-ray diffraction, electronic universal testing machine, heat flow method and thermal dilatometer. The pore size, percent, and distribution of the glass-ceramic foams can be controllably prepared by adjusting the preparing conditions. And the pore structure plays an important role in determining the apparent density, compressive strength, thermal conductivity, and thermal expansion coefficient. The glass-ceramic foam has a low apparent density of 0.30 ± 0.01 g cm3, an available compressive strength of 1.0 ± 0.1 MPa, a thermal conductivity of 0.060 ± 0.002 W m1 °C 1, a high porosity of 88.0% and an average thermal expansion coefficient of 5.27 106 m m1 °C1 under the optimal preparation parameters, indicating that the glass-ceramic foam could be widely used in construction and building industry. Ó 2018 Elsevier Ltd. All rights reserved.
⇑ Corresponding authors. E-mail addresses: [email protected] (F. Zheng), [email protected] (Q. Zhen). https://doi.org/10.1016/j.conbuildmat.2018.09.170 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
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In this paper, glass-ceramic foams have been prepared by using extracted titanium tailing and glass waste as raw materials. We demonstrate that the sintering temperature, sintering time, Na2CO3 content and B2O3 content have great impacts on the micromorphology and apparent density. The effects of pore size, number, and distribution on the compressive strength of the glass-ceramic foams have been systematically investigated. And the formation mechanism of the glass-ceramic foams prepared by extracted titanium tailing has also been discussed. Preparation of glass-ceramic foams using extracted titanium tailing and glass waste as raw materials are developed in order to minimize the environmental issues and effective utilization of resources. The glass-ceramic foam sintered in the optimism condition has a compressive strength of 1.0 ± 0.1 MPa and a low thermal conductivity value of 0.060 ± 0.002 W m1 °C1 and an average thermal expansion coefficient of 5.27 10-6 m m1 °C1, which could be widely used as roofs, walls, floors, ceilings, fireplaces, and grills.
1. Introduction In the Panxi area of southwestern China, a large amount of high titanium blast furnace slag (HTBFS, 250 million tons per year) [1] is generated in the process of steel-making and iron-making using vanadium-titanium magnetic iron ore that belongs only to Panzhihua. These piled HTBFS not only occupies a lot of lands, but also seriously pollutes the environment. The HTBFS contains 20 wt%–27 wt% TiO2 that will be a great loss if it cannot be fully utilized [2]. To address this concern, many efforts have been made to develop the extraction technology of titanium from HTBFS [3– 5]. Wang et al. [6] and Zhang et al. [7] investigated the enrichment conditions of perovskite phase (CaTiO3) from HTBFS, respectively. Sui et al. [8] prepared nano-TiO2 photocatalysts from Ti-bearing slag. And this photocatalysts showed a good photocatalysis activity for the degradation of Rhodamine B. Li et al. [9] synthesized potassium hexatitanate (K2Ti6O13) whiskers from Ti-bearing electric arc furnace molten slag. And this K2Ti6O13 whiskers had a excellent thermal stability which could not decompose up to 1430 °C. Although the above-mentioned works make contributions to the enrichment and extraction of titanium (Ti) element from Tibearing furnace slag, the large amount of extracted titanium tailing has not been processed effectively. Most of the residue is used for low-value products, such as filling and paving material. There are also a small number of studies prepared high value-added products. Li et al. [10] synthesized NaA zeolite and sodalite using the residue as the raw material. However, the requirement of extracted titanium tailing is limited. Through the process of titanium extraction from HTBFS, the residues are mainly silicon dioxide (SiO2), calcium oxide (CaO), magnesium oxide (MgO), aluminum oxide (Al2O3) and titanium dioxide (TiO2) in the extracted titanium tailing [11]. The main elemental composition of the extracted titanium tailing is similar to the components of the ceramic phase [12]. And the SiO2 and CaO are also the main components of glass phase [13]. Thus the extracted titanium tailing can be used as a raw material for the manufacture of glass-ceramic foams. Glass-ceramic foams are mixtures of glass phase and the ceramic phase with a large number of micro pores [14]. It is a high value-added product, which not only has advantages of both glass phase and ceramic phase, but also some unique properties, such as low density and dielectric loss, high strength and mechanical property, good erosion resistance and thermal-shock resistance, excellent heat insulation and controllable thermal expansion [15–17]. Thus the glass-ceramic foams can be widely used in the fields of building insulation, acoustic insulating and environmental chemistry [18,19]. Many researchers have got to start preparing glass-ceramic foams from various solid wastes. Zhou et al. [20] prepared ceramic foams using fly ash by powder foaming method. The apparent density could be as low as 0.32 g cm3. Li et al. [21] fabricated glass-ceramic foams by using coal gangue as the raw material. The apparent density and flexible strength varied between 0.59 and 0.68 g cm3 and 4.5– 6.0 MPa, respectively. Tang et al. [22] prepared glass-ceramic foams using municipal solid waste. Their products had a high strength (2.5 MPa) with a low density (0.3 g cm3). Although the glass-ceramic foams have been prepared by various solid wastes. Few kinds of literature reported glass-ceramic foams prepared by extracted titanium tailing. The optimal preparation parameters and formation mechanism of glass-ceramic foams were not clear.
Fig. 1. The sintering process versus sintering time.
Fig. 2. Differential scanning calorimetric curve of the extracted titanium tailing, glass waste and additives mixture.
Table 1 The chemical composition of extracted titanium tailing and glass waste (wt%).
Extracted titanium tailing. Glass waste.
SiO2
CaO
Na2O
MgO
TiO2
Al2O3
Fe2O3
C
others
21.3 70.0
35.2 12.7
0.5 11.5
9.5 3.9
11.0 /
8.9 0.9
2.5 0.4
9.6 /
1.5 0.6
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2. Experimental sections 2.1. Raw materials Extracted titanium tailing was obtained from Panzhihua Steel Co. Ltd (Sichuan Province, China). Glass waste was gathered from broken glass (glass bottles, discarded window glasses, and glassware, etc). The chemical composition of extracted titanium tailing and glass waste were examined by using an X-ray fluorescence spectrometer (XRF-1800, Shimadzu Limited), as shown in Table 1. Sodium carbonate (Na2CO3, 99.8%), diboron trioxide (B2O3, 98%) and ethyl alcohol (CH3CH2OH, 99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd, China.
2.2. Preparation of glass-ceramic foams Extracted titanium tailing and broken glass were crushed by crushing machine and sieved by 180 mesh screen, respectively. And then stoichiometric amount of
extracted titanium tailing and glass waste (2:8) with the addition of 2 wt% Na2CO3 and 2 wt% B2O3 were mixed by wet ball milling with ethyl alcohol as milling media. Subsequently, the mixed raw materials were pressed at 20 MPa to form cylindrical bodies with a diameter of 20 mm and a thickness of 5 mm. The samples were heated with a heating rate of 5 °C min1 from room temperature to 400 °C, then these samples were kept at 400 °C for 20 min. After the furnace temperature was stable, the samples were heated up to 600 °C with a heating rate of 5 °C min1. Then the heating rate rose to 10 °C min1 from 600 to 780 °C. It follows an isothermal process at a temperature of 780 °C for 30 min to complete the foaming process. After the sintering process finished, the furnace was cooled to room temperature with a cooling rate of 10 °C min1 (Fig. 1). In order to investigate the effects of sintering temperature, sintering time, Na2CO3 content and B2O3 content on the average pore diameter, apparent density and compressive strength of the glass-ceramic foams. The sintering temperatures were varied from 760 °C to 790 °C with an interval of 10 °C, the sintering time was varied from 10 min to 40 min with an interval of 10 min, the content of Na2CO3 was varied from 0 to 3 wt% with an interval of 1 wt% and content of B2O3 was varied from 0 to 3 wt% with an interval of 1 wt%.
Fig. 3. Cross-sectional view SEM images and product photos of glass-ceramic foams prepared at different sintering temperatures (a and b) 760 °C, (c and d) 770 °C, (e and f) 780 °C, (g and h) 790 °C (time: 30 min, Na2CO3 content: 2 wt%, B2O3 content: 2 wt%).
C. Xi et al. / Construction and Building Materials 190 (2018) 896–909 2.3. Characterization of glass-ceramic foams The thermal effect curve of the extracted titanium tailing, glass waste, and additives mixture was measured by using a differential scanning calorimeter (DSC, Diamond, PerkinElmer instruments). The morphologies of the glass-ceramic foams were observed on a scanning electron microscope (SEM, Hitachi-1500, Japan). The diameters of every pore appeared in the SEM images are measured by Nano Measurer 1.2. The statistical data of average pore diameter was obtained by a survey of three SEM images of glass-ceramic foams. The apparent density (q) of these samples were calculated from the measured weight divided by the volume. The powder density (q0) was measured by pycnometry. The total porosity (P) was calculated according to Eq. (1):
P ¼ ð1 q=q0 Þ 100%
ð1Þ
The compressive strength of these samples was measured by using an electronic universal testing machine (DNS500, Changchun research institute for mechanical science co. Ltd, China) with a cross-head speed of 1 mm min1. The thermal conductivity of the glass-ceramic foams was determined on a thermal conductivity analyzer (DRX-RL, Instrument and meter Co., Xiangtan, China). The apparent density, compressive strength, thermal conductivity, and porosity were obtained by a survey of five sets of data for each sample. The thermal expansion coefficient of the sample was obtained on a dilatometer (Netzsch DIL 402PC). The crystal structure of the glass-ceramic foams was characterized by X-ray diffractometer (XRD, Rigaku, Dmax-2550 diffractometer using Cu Ka radiation).
3. Results and discussion 3.1. Effect of sintering temperature on the morphology and properties of the glass-ceramic foams From the literature [23], the sintering temperature plays an important role in the formation of glass-ceramic foams. Fig. 2
899
shows the differential scanning calorimetric curve of the extracted titanium tailing, waste glass and additives mixture. A small endothermic peak appears at about 721 °C, and it reaches a maximum value at about 803 °C with a range of 80 °C. As is well know, the formation of the liquid phase, the decomposition of Na2CO3 and the crystallization of the ceramic phase are all endothermic reactions. Thus the sintering temperatures should be in the range of 721 °C and 803 °C. To investigate the effect of sintering temperature on the morphology and properties of the glass-ceramic foams, the samples are sintered at different temperatures from 760 to 790 °C, which are named TE-760, TE-770, TE-780, and TE-790. Fig. 3 shows the corresponding cross-sectional view SEM images and product photos. From the micro-morphologies, the average pore diameters of samples are 0.47 ± 0.35, 0.72 ± 0.47, 1.60 ± 0.86 and 1.18 ± 0.98 mm at the sintering temperatures of 760, 770, 780 and 790 °C, respectively (Fig. 4 and Table 2), which means that the average pore diameters increase with the increasing of sintering temperatures from 760 to 780 °C while the pore size decreases when the sintering temperature further increases to 790 °C. The product photos show the same trend and the uneven distribution of pore size is obviously observed on the surface of the sample sintered at 790 °C. Besides, the wall thicknesses of pores decrease as the temperatures rise. This phenomenon could be explained as follows: with the increase in temperatures, the viscosity of raw material mixtures decreases and the bubbles are more easy to expand [24]. Then the pore size increase accordingly. However, when the sintering temperature rises too high, the viscosity of mixtures is too low that the pore structure is destroyed and a
Fig. 4. Pore size distribution of glass-ceramic foams prepared at different sintering temperatures (a) 760 °C, (b) 770 °C, (c) 780 °C, (d) 790 °C (time: 30 min, Na2CO3 content: 2 wt%, B2O3 content: 2 wt%).
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Table 2 The structure and properties of glass-ceramic foams prepared at different sintering temperatures. Sample
Pore size distribution (mm)
Apparent density (g cm3)
Compressive strength (MPa)
Thermal conductivity (W m1 °C1)
Porosity (%)
TE-760 TE-770 TE-780 TE-790
0.47 ± 0.35 0.72 ± 0.47 1.60 ± 0.86 1.18 ± 0.98
0.54 ± 0.03 0.44 ± 0.03 0.30 ± 0.01 0.40 ± 0.02
3.3 ± 0.2 1.3 ± 0.1 1.0 ± 0.1 1.0 ± 0.1
0.150 ± 0.003 0.112 ± 0.004 0.060 ± 0.002 0.093 ± 0.002
78.4 ± 1.2 82.4 ± 1.2 88.0 ± 0.4 84.0 ± 0.8
large number of connected pores are generated. Then the gas is easy to escape from connected pores, resulting in small and uneven pore sizes. Fig. 5a depicts the apparent density and compressive strength change curves of glass-ceramic foams at different temperatures (760 °C–790 °C). From Fig. 5a, the apparent density of glass-ceramic foams is 0.54 ± 0.03, 0.44 ± 0.03, 0.30 ± 0.01, 0.40 ± 0.02 g cm3 at sintering temperatures of 760, 770, 780 and 790 °C, respectively. And the corresponding compressive strength of these samples is 3.3 ± 0.2, 1.3 ± 0.1, 1.0 ± 0.1 and 1.0 ± 0.1 MPa. The compressive strength of these samples could satisfy the requirement of a commercial standard (0.4–6.0 MPa) [25]. The apparent density decreases with the increasing of sintering temperatures (from 760 to 780 °C), then increases when the sintering temperature further increases to 790 °C. The apparent density has the opposite variation trend with the pore sizes as the sintering temperatures changes. The apparent density is mainly depended on the size and number of pores in the glass-ceramic foams. The compressive strength of samples has a sharp decline with the sintering temperatures increasing from 760 to 780 °C and the compressive strength keeps almost unchanged when the sintering temperatures increase from 780 to 790 °C. It could be explained as follows: at low sintering temperatures (760–780 °C), the pore sizes increase as the temperature increased. Then both the apparent density and compressive strength decrease accordingly. At high sintering temperatures (780–790 °C), the compressive strength and apparent density have the opposite variation trend mainly because the sample sintered at 790 °C has a large number of connected pores and the sizes of pores are uneven [26–28]. Through the analyses above, the glass–ceramic foams sintered at 780 °C have a low apparent density of 0.30 ± 0.01 g cm3, which are better than those reported in articles concerned [29–31]. From Fig. 5b, the thermal conductivity of the sample has a positive correlation with the apparent density since the thermal conductivity of a sample with more pores would be even lower [32]. The sample sintered
in the optimism condition has a low thermal conductivity value of 0.060 W m1 °C1. The porosity of the sample shows a reverse trend to that of the apparent density. And the sample sintered in the optimism condition has a high porosity of 88.0%. The high porosity value is significantly higher than those of previously reported glass-ceramic foams [2,18,21]. The thermal expansion coefficient of the sample is in a range of 2.02–8.55 106 mm1 °C1 from 30 to 300 °C (Fig. 6). It has the same order of magnitude as some literature [33,34]. Above all, the glass-ceramic foam sintered in the optimism condition has an available compressive strength, a low thermal conductivity, a high porosity, and a low thermal expansion coefficient, which could be used in construction and building industry.
Fig. 6. The thermal expansion coefficient of the sample sintered in the optimism condition versus temperature.
Fig. 5. (a) Apparent density and compressive strength, (b) thermal conductivity and porosity of glass-ceramic foams prepared at different sintering temperatures (time: 30 min, Na2CO3 content: 2 wt%, B2O3 content: 2 wt%).
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3.2. Effect of sintering time on the morphology and properties of the glass-ceramic foams Fig. 7 shows the SEM images of glass–ceramic foams prepared at different sintering time (10–40 min), which are named TI-10, TI-20, TI-30, and TI-40. According to statistics (Fig. 8 and Table 3), the average pore diameters of samples are 0.39 ± 0.31, 0.87 ± 0.77, 1.60 ± 0.86 and 0.76 ± 0.56 mm at the sintering time of 10, 20, 30 and 40 min, respectively. That is to say, the average pore diameters gradually increase as the sintering time increasing from 10 min to 30 min. While the pore sizes decrease as the sintering time further increasing to 40 min. In addition, It can also be seen that the upper half are more porous than the lower half from the photos of all the four glass-ceramic foams. This phenomenon could be
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explained as follows: during the heating process, CO2 bubbles are generated in the matrix when the sintering temperature exceeds the decomposition temperature of Na2CO3. With the sintering time increased, the number of CO2 bubbles gradually increase and these bubbles are easier to merge together to form large bubbles. Then the large bubbles rise from bottom to top. However, if the sintering time is too long, the large bubbles will escape from the upper surface of the glass-ceramic foams, resulting in a sintering densification process. Fig. 9a shows the apparent density and compressive strength change curves of glass-ceramic foams at different sintering time. The apparent density of the four glass-ceramic foams is 0.61 ± 0.03, 0.49 ± 0.02, 0.30 ± 0.01 and 0.41 ± 0.02 g cm3, respectively. Similarly to the result obtained by the effect of temperature
Fig. 7. Cross-sectional view SEM images and product photos of glass-ceramic foams prepared at different sintering times (a and b) 10 min, (c and d) 20 min, (e and f) 30 min, (g and h) 40 min (temperature: 780 °C, Na2CO3 content: 2 wt%, B2O3 content: 2 wt%).
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Fig. 8. Pore size distribution of glass-ceramic foams prepared at different sintering time (a) 10 min, (b) 20 min, (c) 30 min, (d) 40 min (temperature: 780 °C, Na2CO3 content: 2 wt%, B2O3 content: 2 wt%). Table 3 The structure and properties of glass-ceramic foams prepared at different sintering times. Sample
Pore size distribution (mm)
Apparent density (g cm3)
Compressive strength (MPa)
Thermal conductivity (W m1 °C1)
Porosity (%)
TI-10 TI-20 TI-30 TI-40
0.39 ± 0.31 0.87 ± 0.77 1.60 ± 0.86 0.76 ± 0.56
0.61 ± 0.03 0.49 ± 0.02 0.30 ± 0.01 0.41 ± 0.02
6.4 ± 0.4 1.3 ± 0.2 1.0 ± 0.1 0.9 ± 0.2
0.166 ± 0.002 0.134 ± 0.003 0.060 ± 0.002 0.093 ± 0.003
75.6 ± 1.2 80.4 ± 0.8 88.0 ± 0.4 83.6 ± 0.8
Fig. 9. (a) Apparent density and compressive strength, (b) thermal conductivity and porosity of glass-ceramic foams prepared at different sintering time (temperature: 780 °C, Na2CO3 content: 2 wt%, B2O3 content: 2 wt%).
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on the micromorphology, the apparent density has the opposite variation trend with the pore sizes as the sintering time changes. The pore sizes decrease firstly and then increase with the time. While the apparent density increases firstly and then decreases. Both the peaks appear at sintering time of 30 min. The compressive strength of the four samples is 6.4 ± 0.4, 1.3 ± 0.2, 1.0 ± 0.1 and 0.9 ± 0.2 MPa, respectively. The compressive strength of sample TI-30 is weaker than those of samples TI-10 and TI-20 because of its low apparent density. However, the compressive strength of sample TI-40 is weaker than that of sample TI-30, although the sample TI-40 has a relatively higher apparent density. This phenomenon could be explained by cell coalescence. With the increas-
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ing of sintering time from 30 min to 40 min, smaller pores begin to coalesce into large pores and the pore structure becomes seriously uneven. At the same time, the pore walls will become thicker, resulting in an enhanced probability of forming critical flaws and pore opening. Thus the compressive strength does not increase with the increasing of apparent density. This part of the experiment illustrates that the sintering time needs to be 30 min in order to reach a low apparent density. But both the parameters of apparent density and compressive strength will weaken when the sintering time is too long. Combined with the thermal conductivity and porosity in Fig. 9b, the sample sintered at 30 min has good comprehensive performances.
Fig. 10. Cross-sectional view SEM images and product photos of glass-ceramic foams prepared at different Na2CO3 contents (a and b) 0, (c and d) 1 wt%, (e and f) 2 wt%, (g and h) 3 wt% (temperature: 780 °C, time: 30 min, B2O3 content: 2 wt%).
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3.3. Effect of Na2CO3 at different contents on the morphology and properties of the glass-ceramic foams We know that the pores are mainly caused by CO2 bubbles which are generated by the decomposition of Na2CO3. Thus the
content of Na2CO3 plays an important role in the formation of glass-ceramic foams. Fig. 10 shows the SEM images of glassceramic foams prepared at different Na2CO3 contents (0–3 wt%), which are named NA-0, NA-1, NA-2, and NA-3, respectively. Obviously, the average pore diameters of samples increase with the
Fig. 11. Pore size distribution of glass-ceramic foams prepared at different Na2CO3 contents (a) 0, (b) 1 wt%, (c) 2 wt%, (d) 3 wt% (temperature: 780 °C, time: 30 min, B2O3 content: 2 wt%).
Fig. 12. (a) Apparent density and compressive strength, (b) thermal conductivity and porosity of glass-ceramic foams prepared at different Na2CO3 contents (temperature: 780 °C, time: 30 min, B2O3 content: 2 wt%).
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increasing of Na2CO3 contents. The average pore diameters of samples are 0.44 ± 0.32, 0.50 ± 0.41, 1.60 ± 0.86 and 1.68 ± 0.93 mm for these samples, as obtained in Fig. 11. The porous structure still appears in the sample without addition of Na2CO3. It might be because the raw matrix contains the carbon element and the
carbon element is easily oxidized in air. It is noted that the porous structures of samples NA-2 and NA-3 have no significant changes, indicating that the gas capacity has reached saturation when the Na2CO3 content is 2 wt%.
Table 4 The structure and properties of glass-ceramic foams prepared at different Na2CO3 contents. Sample
Pore size distribution (mm)
Apparent density (g cm3)
Compressive strength (MPa)
Thermal conductivity (W m1 °C1)
Porosity (%)
NA-0 NA-1 NA-2 NA-3
0.44 ± 0.32 0.50 ± 0.41 1.60 ± 0.86 1.68 ± 0.93
0.55 ± 0.02 0.35 ± 0.02 0.30 ± 0.01 0.30 ± 0.01
5.9 ± 0.2 1.4 ± 0.2 1.0 ± 0.1 1.0 ± 0.1
0.153 ± 0.003 0.089 ± 0.004 0.060 ± 0.002 0.064 ± 0.003
78.0 ± 0.8 86.0 ± 0.8 88.0 ± 0.4 88.0 ± 0.4
Fig. 13. Cross-sectional view SEM images and product photos of glass-ceramic foams prepared at different B2O3 contents (a and b) 0, (c and d) 1 wt%, (e and f) 2 wt%, (g and h) 3 wt% (temperature: 780 °C, time: 30 min, Na2CO3 content: 2 wt%).
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Fig. 12a shows the apparent density and compressive strength change curves of glass-ceramic foams prepared at different Na2CO3 contents. As shown in Table 4, the apparent density of samples NA-0, NA-1, NA-2 and NA-3 is 0.55 ± 0.02, 0.35 ± 0.02, 0.30 ± 0.01 and 0.30 ± 0.01 g cm3, respectively. and the compressive strength of the four samples is 5.9 ± 0.2, 1.4 ± 0.2, 1.0 ± 0.1 and 1.0 ± 0.1 MPa. Through the analysis of section 3.1 and 3.2, we know that Na2CO3 will be completely decomposed to CO2 and Na2O under the condition of the sintering temperature of 780 °C for 30 min. It is also confirmed that the apparent density and compressive strength of the sample directly relate to its pore number and pore size. As the increasing of Na2CO3 contents, the CO2 gas bubbles increase, leading to a increased porosity. However, this increasing trend is not unlimited. The maximum porosity depends on the gas capacity of the raw matrix. This part of the experiment demonstrates that the sample has the minimum apparent density and available strength when the content of Na2CO3 is at 2 wt%. It’s not necessary to continue
increasing the content of Na2CO3 since the sample also has the minimum thermal conductivity and the maximum porosity (Fig. 12b). 3.4. Effect of B2O3 at different contents on the morphology and properties of the glass-ceramic foams During the formation of glass-ceramic foams, the viscosity of the raw matrix has great influence at high temperature. If the matrix has a low viscosity, it has a good fluidity and promotes the formation of gas bubbles. B2O3 could be used as a fluxing agent to lower the softening temperature and increase the liquidity. Fig. 13 shows the SEM images of glass-ceramic foams prepared at different B2O3 contents (0–3 wt%), which are named B-0, B-1, B-2, and B-3, respectively. According to the statistical data (Fig. 14 and Table 5), the average pore diameters of the four samples are 0.31 ± 0.27, 0.36 ± 0.29, 1.60 ± 0.86 and 0.46 ± 0.40 mm. The porosity increases with the increasing of B2O3 contents until
Fig. 14. Pore size distribution of glass-ceramic foams prepared at different B2O3 contents (a) 0, (b) 1 wt%, (c) 2 wt%, (d) 3 wt% (temperature: 780 °C, time: 30 min, Na2CO3 content: 2 wt%).
Table 5 The structure and properties of glass-ceramic foams prepared at different B2O3 contents. Sample
Pore size distribution (mm)
Apparent density (g cm3)
Compressive strength (MPa)
Thermal conductivity (W m1 °C1)
Porosity (%)
B-0 B-1 B-2 B-3
0.31 ± 0.27 0.36 ± 0.29 1.60 ± 0.86 0.46 ± 0.40
0.87 ± 0.04 0.60 ± 0.06 0.30 ± 0.01 0.83 ± 0.02
12.5 ± 0.6 4.5 ± 0.2 1.0 ± 0.1 9.3 ± 0.5
0.263 ± 0.003 0.160 ± 0.003 0.060 ± 0.002 0.270 ± 0.003
65.2 ± 1.6 76.0 ± 2.4 88.0 ± 0.4 66.8 ± 0.8
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Fig. 15. (a) Apparent density and compressive strength, (b) thermal conductivity and porosity of glass-ceramic foams prepared at different B2O3 contents (temperature: 780 °C, time: 30 min, Na2CO3 content: 2 wt%).
the B2O3 content reaches 2 wt%. Then a turning point appears when the B2O3 content further increases to 3 wt%. This phenomenon could be explained as follows: when a small number of B2O3 as fluxing agent is added in the raw matrix, the viscosity is reduced and big gas bubbles tend to be formed [35]. However, when a large number of B2O3 is added, the excessive of B2O3 will be converted to [BO4] tetrahedron. And the formed
[BO4] tetrahedron will increases the rigidity of the raw matrix, resulting in a high viscosity [36]. Fig. 15a shows the apparent density and compressive strength change curves of glass-ceramic foams prepared at different B2O3 contents correlated with Fig. 13. The apparent density and compressive strength are 0.87 ± 0.04, 0.60 ± 0.06, 0.30 ± 0.01 and 0.83 ± 0.02 g cm3 and 12.5 ± 0.6, 4.5 ± 0.2, 1.0 ± 0.1 and 9.3 ± 0.5 MPa, respectively, for samples B-0, B-1, B-2, B-3. Both the apparent density and compressive strength has presented concave curves as the B2O3 contents increased. And the peaks appear at sample B-2 with the content of 2 wt%. It could be explained as follows: first, the content of B2O3 fluxing agent affects the fluidity and viscosity of the raw matrix. And then the viscosity of sample affects significantly the size and number of gas bubbles, which could further affect the apparent density of glass-ceramic foams. Generally, the compressive strength is roughly proportional to the apparent density. Besides, it also can be found that the fluxing agent has a bigger impact on apparent density and compressive strength than the foaming agent at a content range of 0–3 wt%. In addition, the sample B-2 has a relatively low thermal conductivity and high porosity, which shows a good comprehensive performance than others (Fig. 15b). 3.5. The formation mechanism of glass-ceramic foams
Fig. 16. XRD patterns of sample sintered in the optimism condition (temperature: 780 °C, time: 30 min, Na2CO3 content: 2 wt%, B2O3 content: 2 wt%).
Through the front chapter analysis, the formation process of glass-ceramic foams can be described as follows: When the sintering temperature reaches a certain value, the alkali metal ions in Na2O will break silicon-oxygen bonds. And the main compositions (SiO2, CaO and Na2O) of glass phase begin to soften. The B2O3 could
Fig. 17. The formation mechanism of glass-ceramic foams.
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be used as a fluxing agent. A small content of B2O3 contributes to lowering the viscosity of the raw matrix. While excessive B2O3 can lead to the formation of [BO4] tetrahedron, which may increase the viscosity. When the sintering temperature exceeds the decomposition temperature of Na2CO3, it generates a lot of CO2 gas bubbles with the smaller size. And these smaller gas bubbles tend to form large one if the content of Na2CO3 is enough. At the same time, diopside CaMg(SiO3)2 is generated near the TiO2. Because TiO2 could be used as a nucleating agent, which effectively improve the crystallization and promote the formation of the ceramic phase. As the crystallization reaction progress, some metal ions (Fe3+, Al3+) can be dissolved in the diopside to form (Mg,Fe,A l,Ti)(Ca,Mg,Fe)(Si,Al)2O6. Then both the glass phase and ceramic phase are formed in the mixture. Once the mixture has cooled, massive micropores are generated in the matrix and the glassceramic foams are synthesized (Fig. 16). And the ceramic phase in the glass-ceramic foam is helpful to improve the compressive strength. However, if the sintering temperature is too high or the sintering time is too long, the viscosity of mixture will be dramatically decreased that a lot of connected pores are generated. Then the large bubbles rise from bottom to top and the gas is easy to escape from connected pores, resulting in side effects. The corresponding forming diagram of glass-ceramic foams is given in Fig. 17. 4. Conclusions In summary, glass-ceramic foams have been synthesized using extracted titanium tailing and glass waste as raw materials by a simple powder foaming method. It is shown that the pore structure plays an important role in determining the apparent density and compressive strength of the glass-ceramic foams. The sample with a large pore size and uniform pore distribution has a low density and a high strength. The pore structure can be controlled by changing the sintering temperature, sintering time, Na2CO3 content and B2O3 content. The combined effects of sintering temperature, sintering time and Na2CO3 content determine the pore size, number, and distribution. And the content of B2O3 contributes to lowering the viscosity of the raw matrix. The glass-ceramic foams have a relatively lower apparent density of 0.30 ± 0.01 g cm3 and an available compressive strength of 1.0 ± 0.1 MPa, a thermal conductivity of 0.060 ± 0.002 W m1 °C1, a high porosity of 88.0% and an average thermal expansion coefficient of 5.27 106 m m1 °C1, synthesized at sintering temperature of 780 °C for 30 min with 2 wt% Na2CO3 content and 2 wt% B2O3 content. Author contributions C. Xi completed the extraction procedure and undertook the sintering process. J. Xu supervised the experimental component. Y. Peng complete the DSC while W. Yang conducted the SEM study. Y. Li complete the pore size distribution and P. Li the physical measurement component. Q. Zhen conceived the project and co-wrote the first draft with F. Zheng. S. Bashir completed the SEM analysis and second draft with J. L. Liu who also checked standard deviations and Fig. 17. Conflict of Interest There are no conflicts of interest. Acknowledgments This work is supported by the National Nature Science Foundation of China (No. 51472156, 51672170, 51604202, 51708022), the
Shanghai Excellent Technology Leader Program (No. 17XD1424700), the Science and Technology Commission of Shanghai (No. 17010500600), the Science and technology project of Guangdong Province (No. 2013B090600025), the State Key Laboratory of Refractories and Metallurgy, China (No. 2016QN09), the Petroleum Research Fund of the American Chemical Society (53827-UR10) and the Robert Welch Foundation (Departmental Grant, AC-0006).
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