CHENG Jiangguo, ZHANG Jie, XIE Fei, TUO Biyang, ZHANG Yusong

(1. College of Materials and Metallurgy, Guizhou University, Guiyang 550025, China; 2. Mining College, Guizhou University, Guiyang 550025, China; 3. Guizhou Key Laboratory of Comprehensive Utilization of Nonmetallic Mineral Resources, Guiyang 550025, China; 4.National & Local Joint Laboratory of Engineering for Effective Utilization of Regional Mineral Resources from Karst Areas, Guiyang 550025, China)

Abstract: A polarizing microscope, X-ray diffraction (XRD), fourier transform infrared spectrometer (FTIR), scanning electron microscope and energy dispersive spectrometer (SEM-EDS), X-ray photoelectron spectroscopy (XPS), and micro computed tomography (Micro CT) were used to investigate the relation between the structure and properties of the composite. Meanwhile, the physical properties, mechanical properties and strength mechanism were researched. The experimental results show that the structure and morphology of coated phosphogypsum remain intact in the composite, which shows good compatibility and forms a clear interface layer of transition zone between the coated phosphogypsum and the matrix, conforming to the structure of particle reinforced inorganic composites. The emulsion coated phosphogypsum has a certain strengthening effect on the aluminous rock mineral polymer composite. The compressive strength of the composite can reach 16.5 MPa when the amount of coated phosphogypsum is 40%, and the apparent density is 1.75 g·cm-3, which is significantly lower than that of common concrete; the thermal stability of the composite is also improved to a certain extent. Some certain chemical reactions occur in the process of forming the matrix of aluminous rock mineral polymer materials, with a structure of three-dimensional network. The research will provide a new way for the comprehensive utilization of phosphogypsum and low-grade aluminous rock.

Key words: composite; organic emulsion coated phosphogypsum; aluminous rock; sodium silicate; structure and performance

1 Introduction

According to the principle of coating modification, coating one or more layers of organic or inorganic materials on the periphery of raw materials can improve the performance of raw materials, prolong the service life, and even prepare new materials[1]. The research mainly focused on the surface coating treatment of powders, such as polymer fillers[2,3]and electrode materials of lithium-ion batteries[4,5].

Phosphogypsum (abbreviated as PG), as a by-product of wet process phosphoric acid industry, is mainly composed of calcium sulfate dihydrate (CaSO4·2H2O). There are many disadvantages in resource utilization of PG, especially in building materials[6-8], such as poor chemical stability, poor water resistance, easy to absorb moisture, mildew and frost, which greatly limits its application. The organic emulsion was used as coating agent to modify the PG with coating and granulating processing, and the organic emulsion coated PG particles were prepared by re-spray coating in previous work[9]. Compared with the original PG powder, the acid resistance, alkali resistance and water resistance of the coated PG particles enhanced obviously, and the chemical stability improved as well. Organic coating modification also inhibits the migration and precipitation of sulfate ions in PG, and the coated PG particle also have a certain mechanical strength, which provides a guarantee of performance for its application in new inorganic mineral polymer composites which can be prepared at room temperature and atmospheric pressure.

Low-grade aluminous rock that is almost no resource utilization is a solid waste discharged from bauxite mine, which is composed of aluminum hydroxides, kaolinite and other clay minerals. It has been reported that the author’s research group had prepared mineral polymer material with using it[10]. In this paper, the organic emulsion coated phosphogypsum particles prepared in former experiments are used as bone fillers of the aluminous rock mineral polymer materials that has been researched successfully to prepare new inorganic mineral composite. Means of polarizing microscope, X-ray diffraction (XRD), fourier transform infrared spectrometer (FTIR), scanning electron microscope and energy dispersive spectrometer (SEM-EDS), X-ray photoelectron spectroscopy (XPS), micro computed tomography (Micro CT) are used to research the structure and components of the composite. Furthermore, the relation between the structure and properties of the composite is investigated by testing the physical and mechanical properties, and strength mechanism are researched. The results are going to provide theoretical basis for the efficient utilization of PG and low-grade aluminous rock.

2 Experimental

2.1 Materials

There are three main materials used in the experiment, such as organic emulsion coated phosphogypsum (abbreviated as E-PG), low-grade aluminous rock (abbreviated as AR) and sodium silicate. The E-PG is prepared in accordance with the method reported in literature[9], by using the organic silicone acrylic emulsion and PG from a slag storage of a phosphate mine enterprise in Guizhou province of China. Three kinds of size fraction of E-PG were used in the experiment: the large (2-3 mm), the medium (1-2 mm) and the small (＜1 mm). The appearance of E-PG is shown in Fig.1; bulk density is shown in Fig.2.

Fig.1 Appearance of the E-PG

Fig.2 Bulk density of the E-PG

The AR was obtained from the surrounding rock yard discharged of a bauxite mine in Guizhou province of China. The raw material was broken into small pieces by using manual tools, and then crushed with laboratory jaw crusher, finally crushed and ground by using the experimental prototype until all of them pass the 60-target sieve, dried and sealed for storage. Its chemical element composition is shown in Table 1.

Table 1 Chemical composition of the low-grade aluminous rock

Sodium silicate was produced by a refractory factory in Zhejiang province of China, with the model of SP38 and the density of 38.5Be° at 20 ℃, the silicon dioxide content of 27.3%, the sodium oxide content of 8.54%, the solid content of 56.66%, and the modulus of 3.30. The original sodium silicate was colorless, transparent and viscous liquid. After adjusting the modulus with sodium hydroxide, it was used as the activator in preparing mineral polymer materials. In addition, sodium hydroxide, calcium oxide, aluminum tri-polyphosphate (as the curing agent), calcined kaolin, water reducer, 425#common cement were used in the experiments.

2.2 Preparation of E-PG reinforced AR mineral polymer (E-PG/AR) composite

Firstly, the adjustment of the sodium silicate modulus should be taken by adding the sodium hydroxide. Based on the basic parameters of sodium silicate, the mass (or amount of substance) of sodium hydroxide added to adjust the known mass of sodium silicate to the specified modulus was calculated according to the formula (1). Then the sodium hydroxide was added into the sodium silicate with stirring continuously. This process was a violent exothermic process, and it can accelerated the dissolution rate of sodium hydroxide. After the sodium hydroxide has been dissolved completely, stirred the mixture for 10 minutes and aged for 24 hours.

where,Wis the mass of sodium hydroxide to be added into 100 g raw sodium silicate.

Secondly, the basic composition formula of E-PG/AR composite was designed based on the previous research of aluminous rock mineral polymer materials, which is shown in Table 2. Then, the sodium silicate with adjusted modulus was added into the solid materials, which were thoroughly mixed in a tub, and the mixture was quickly stirred for 3 minutes, then poured into the triple 40 mm×40 mm×160 mm die, and naturally leveled to slightly exceed the edge of the mold by 0.5 mm. Cut vertically along the cross section direction of the mold to eliminate the surface bubbles and waited for the surface to be naturally flat and solidified. After the composite samples were solidified with the mold for 48 hours, demolded and put them into a plastic bag or wrapped with plastic preservation film in sealing for 14 days curing at room temperature, then removed the plastic bag or film and put them into a shelter of shady corner for another 14 days curing. According to the content of E-PG (wt%) added in the composite, record the E-PG/AR composite as C00, C10, C20, C30, and C40, respectively.

Table 2 Basic formulation of E-PG/AR composites

Table 3 Main mineral composition of E-PG/AR mineral polymer composites

2.3 Characterization and test methods of the E-PG/AR composite

The structure and composition of the E-PG/AR composites were investigated by the FTIR manufactured by Bruker Co.( Germany) with using KBr method and X’pert powder type XRD manufactured by Panalytical B.V. (Netherlands) with copper node and PSD length of 3.35 (2θ) at 25°. The OLYMPUS CX21P polarizing microscope manufactured by Olympus Corp. (Japan) was used to analysis the mineralogical characteristics of the E-PG/AR composite. HITACHIS-3400N SEM-EDAX manufactured by Hitachi Co. (Japan) was used to characterize the surface morphology and elemental composition of the E-PG/AR composite, with a voltage of 20 kV and a resolution of 125.9 eV. The microstructure of the interface was analyzed by using a Kα+ XPS manufactured by Thermo Fisher (USA) with non-monochromatic Al-Kα radiation (1486.6 eV) at a power of 150 W. The TG of TG209F1LIbrahin type manufactured by Netzsch (Germany) was used to characterize the thermal stability. The mechanical properties of the E-PG/AR composites were tested by YAW-300B microcomputer controlled bending and compression testing machine. The other physical and chemical properties were tested by gravimetric analysis.

3 Results and discussion

3.1 Surface morphological and mineralogical characteristics of E-PG/AR composite

Fig.3 shows the polarizing microscope photographs of the E-PG/AR composite with 40% content of E-PG. It turns out that the structure of E-PG is well preserved and the boundary is clear in the composite. There is an obvious transition zone between the matrix of composite and E-PG margin, which can be observed from a particular color different from the other region of E-PG and AR matrix in the photos. It indicates that a new interface layer is formed between the two structure units, which makes the E-PG and the AR matrix bonded firmly, and ensures the final properties of the composite. Meanwhile, it can also be seen from the figure that the AR matrix surrounding the E-PG presents an obvious network structure, which could be inferred existing as a three-dimensional structure in the composite. The three-dimensional network newly should be the conclusion mentioned in previous research, which formed by the de-polymerization and re-condensation of the raw materials contained silicon and aluminum components with the alkali activators, but there is no relevant substantive evidence in former literatures.

Fig.3 Microscope photos of the E-PG/AR composite (The left is single polarized light (-) and the right is orthogonal polarized light (+))

Fig.4 and Fig.5 show the SEM-EDAX analysis images and spectra of the E-PG/AR composite. From the Fig.4(a), due to the relatively low hardness and loose texture of E-PG, it is easy to fall off from the AR matrix of the composite when it subjects to friction and shear. Nevertheless, it shows that there are also some E-PG remains at the interface in Fig.4(b), and it indicates that the interface bonding strength between the E-PG and the AR matrix is high. Moreover, we can see that the three dimensional network structure film formed by emulsion is clearly visible inside the E-PG, and the PG crystal is well compatible with the adhesive film and closely integrated with each other in Figs.4(c) and 4(d). The organic adhesive film on the surface of E-PG will not have an adverse effect on the compatibility between the E-PG and AR matrix, but will improve the properties of the composite. From the SEM-EDAX spectra in Fig.5, it turns out that the elements of carbon and silicon are detected inside of the E-PG, which is from the resource of silicone acrylic emulsion; the elements of silicon and aluminum appeared in other areas is mainly of the AR matrix.

Fig.4 SEM images of E-PG/AR composite ((a) and (b) are the traces left on the surface of AR matrix after the the E-PG was stripped, (c) is the junction area of the E-PG and AR matrix, and (d) is the inside structure of the E-PG)

Fig.5 SEM-EDAX spectra of the E-PG/AR mineral polymer composite

3.2 Composition and structure of the E-PG/ AR mineral polymer composite

3.2.1 FTIR analysis of the E-PG/AR mineral polymer composite

Fig.6 shows the infrared spectra of the E-PG/AR composite and the pure AR matrix. It can be seen from the spectra that the flexural vibration peak of silicon oxygen bond at the 471 cm-1appears in both of the E-PG/AR composite and pure AR matrix. There is a broad absorption band from 900 cm-1to 1 300 cm-1, which is the characteristic band of the hydrogen free single bond, such as carbon oxygen bond, silicon oxygen bond, phosphorus oxygen single and double bond, sulfur oxygen double bond, and so on. However, the scale of absorption band of the E-PG/AR composite is bigger than the pure AR matrix. It is mainly because of the addition of the E-PG in composite and the reaction between the AR and the activator system of sodium silicate. Moreover, there are two medium intensity peaks of carbon oxygen double bond from ester group at 1 622 cm-1and 1 730 cm-1and a weak peak of hydroxyl groups at 2 965 cm-1newly appearing in E-PG/AR composite. There are also some new associated hydroxyl and hydroxyl located in the free hydroxyl group on the surface of amorphous silica and in the lattice of zeolite at many different peak position above 3 000 cm-1. The changes of the FTIR spectra indicate that when the E-PG is added into the AR matrix, the organic emulsion components of can transfer into the composite and certain chemical interactions between the E-PG and composite matrix may be occurred, which can promote the compatibility between the E-PG and matrix, and improve the interfacial bonding.

Fig.6 FTIR spectra of the E-PG/AR composite

3.2.2 XRD analysis of the E-PG/AR mineral polymer composite

The XRD results of the E-PG/AR composite with different E-PG contents are shown in Fig.7 and Table 3. It turns out that there are obvious differences in the spectrum of pure AR mineral polymer materials, E-PG/AR mineral polymer composites and interface area. There is a new component of gismondine appeared in both of E-PG/AR composite and pure AR mineral polymer, which is different from the original mineral components of the kaolinite and diaspore in the AR resource. In addition, the content of gismondine at the interface area is more than that in the E-PG/AR composite and the AR mineral polymer. The gismondine may be formed by the depolymerization-polycondensation reaction between the AR and the activator system of sodium silicate and sodium hydroxide, and the difference in content of the gismondine indicates that the gismondine phase is easy to diffuse to the interface of E-PG in the composites.

Fig.7 XRD patterns of the E-PG/AR mineral polymer composite

Fig.8 shows the result of Micro CT scanning and structural reconstruction of the composite with 40% of E-PG. The scanning data was calculated and changed color by specific graphic processing software. From the figure, it turns out that the shape and structure of E-PG in the composite remain intact with obvious boundary contour, the organic film network formed by emulsion is also occurred, and the dispersion of the E-PG in the matrix is uniform with no obvious agglomeration and settlement. The E-PG wrapped by the matrix shows different cross-section morphology in different sections, and it indicates that a three-dimensional structure formed between the E-PG and the AR matrix.

Fig.8 Micro CT scanning and reconstruction of the E-PG/AR composite ((a) is the sample for CT scanning; (b) and (c) are the reconstruction diagrams, and the white represents the phosphogypsum, the brown represents the matrix, the purple represents the organic film, and the red represents the siliceous)

3.3 Interface area analysis of E-PG/AR composite

Fig.9 shows the XPS scan spectra of three areas at the interface, inside of E-PG and AR matrix in the composite. It turns out that there are C (1s), O (1s), Ca (2p), Si (2p), Si (2s), Al (2p), and Al (2s) binding energy peaks in the spectra of all three scanning area[11]. Moreover, there are S (2p) (168.5 eV) corresponding to sodium sulphate and S (2s) (229 eV) binding energy peaks in the interior of E-PG and the interface, but these two peaks do not appear in the interior of AR matrix. It indicates that the sulfate ion in the PG was sealed up after coating, and did not migrate from the E-PG to the matrix during the preparation of the composite. Furthermore, it turns out that the peak shape of Si (2p) and Al (2p) is flat inside of the E-PG, indicating that there may be a few amount of reaction products containing silicon and aluminum in the AR matrix infiltrated into the E-PG except for the very small amount of silicon in quartz and aluminum contained in PG itself; and there is no binding energy peak of sulfur element outside the E-PG.

Fig.9 XPS spectra of the internal and external environment of coated particles in the composite

Fig.10 shows the XPS elemental spectra of O 1s (figure (a)) and Ca2p (figure (b)) based on Fig.9. It shows that the binding energy of O (1s) and Ca (2p) increased gradually from the AR matrix to the inside of E-PG, and it indicates that the environment of oxygen and calcium atoms changed significantly. In the AR matrix, oxygen and calcium atoms are changed from the original oxide forms of silicon dioxide, aluminum oxide and calcium oxide to a new, stable and lower binding energy of Si (Al)-O-Ca tetrahedral bridging oxygen forms formed by reacting with the alkali excitation[12]. In addition, the oxygen and calcium atoms keep of the original forms of partial oxide and CaSO4at the interface and inside of the E-PG.

Fig.10 XPS elemental spectra ((a) is spectra of Ca2p; (b) is spectra of O1s)

3.4 Physical properties of the E-PG/AR composite

Fig.11 shows the results of water absorption of E-PG/AR composite prepared with different contents of E-PG. It turns out that the water absorption rate increases slowly with the content of E-PG increasing; the water absorption rate reaches the highest value when the content of E-PG is 30%; after that, the water content begins to rise, and the water absorption rate begins to decline. The water absorption rate of the composite with 40% E-PG is 6.04% under natural conditions. The reason may be that: with the E-PG content increasing, because the E-PG size is much larger than that of the AR powder of matrix, many pores are formed in the composite, so the water molecules can be absorbed in the pores, which causes the water absorption rate increasing. However, because the surface of E-PG is wrapped with organic film, which can enhance the hydrophobicity of the composite, it is difficult to enter into the interior of E-PG, therefore, the water absorption decreases at a high content of E-PG.

Fig.11 Water absorption of the E-PG/AR composite

Fig.12 shows the thermal analysis result of the E-PG/AR composite with 40% of E-PG. The figure turns out that the weight loss curve of the composite has an obvious stable period from 200 ℃ to 400 ℃, and then the mass reduction is the same as that of the AR matrix. The weight curve of the AR matrix material tends to be stable from about 690 ℃. Furthermore, the mass curve of the composite is always above the AR matrix, indicating that the decomposition process of AR is delaying due to the addition of E-PG and making the thermal stability of the composite improved.

Fig.12 Thermal stability of the E-PG/AR composite

Fig.13 shows the pore size distribution and adsorption desorption result of the E-PG/AR composite by the gas adsorption method. It turns out that the micropores are mainly concentrated between 5-55 nm, leading to that the adsorption capacity is very small under low adsorption pressure. The specific surface area and total pore volume are 7.1547 m2·g-1and 0.03469 cm3·g-1, respectively.

Fig.13 The micorpore size distribution of the E-PG/AR composite

Fig.14 shows the result of the apparent density of the E-PG/AR composite with different content of E-PG. It shows that the apparent density of the composite added 20% E-PG is lower than that with no E-PG, and the composite with E-PG of large size has the lowest density. There is little change on the density of the composite with different content of E-PG (according to a grade ratio of 1:1:1), but it also shows a slow decreasing trend with the increase of E-PG content, and the density of composite with mixed size E-PG is slightly higher than that with single particle size under the same E-PG content. When the E-PG content is 40%, the apparent density of the composite is 1.75 g·cm-3. It may be because that the density of the large E-PG is correspondingly high, and it is easy to form the bridging structure in the composite, which reduces the density of the structure, leading to the decrease of the density of the composite; when the sizes of large, medium and small form the gradation, the small particles can fill the pores formed by the large particles, making the structure of the material become dense, so the density of the composite is slightly higher than that with single size E-PG.

Fig.14 Apparent density of the E-PG/AR composite ((a) is the density of E-PG, and (b) is the density of composite)

Fig.15 shows the mechanical properties of the composite prepared with different sizes and content of E-PG. It can be found from Fig.15(a) that the compressive strength of the composite prepared with small size of E-PG is the highest, while the bending strength with large size is the highest, and the composites prepared with medium size respondingly have medium performance. Therefore, the particle matching of E-PG needs to be design according to the requirements of practical production and application. From Fig.15(b), we can see that the compressive and bending strength of the E-PG/AR composite can reach 16.5 MPa and 4.0 MPa respectively, which is prepared with 40% E-PG of the particle grade ratio of 1:1:1 at 1.5 modulus and solid-liquid ratio of 2.5, and the performance is reaching to the level of C15 concrete.

Fig.15 Mechanical properties of the E-PG/AR composite ((a) is the composite with different E-PG sizes; (b) is the composite with different contents of E-PG )

Fig.16 shows the crack propagation, destruction and cross section of the E-PG/AR composite under the compressive stress and shear stress. It can be seen from the figures (a) and (b) that the tip direction of the crack caused by the external stress was always changed and circumvented when it encountered the large E-PG particle in front of it in the process of propagation of the material, then it continued to propagate after bypassing the E-PG particle. With the continuously changing the direction and bypassing the particles, finally the crack stopped in front of the E-PG particle. According to the elementary knowledge of the composite, we know that the energy will be consumed continuously in the process of the direction changing and bypassing of the crack propagation, and because of the crack deflection, the crack propagation path in the matrix is lengthened, so it plays a toughening role by consuming more fracture energy.

Fig.16 Destruction with stress of the E-PG/AR composite ((a) is the crack propagation under the shear stress; (c) is the crack propagation under the compressive stress; (b) and (d) are the cross sections after material fracture along the crack)

Moreover, we can also see from the Fig.16(a) that the damage of the E-PG/AR composite shows a certain degree of ductile fracture under the shear stress, which is characterized by interface debonding and the E-PG parting from the matrix, and a certain degree of plastic deformation under the continuous compressive stress, which is characterized by the compression deformation and a large number of cracks and dislocations produced in the fracture surface. It shows that the interface bonding is not firmly and E-PG particles in the composite can bear the load and restrain the deformation of the matrix to a certain degree, and the greater the ability of particles to prevent the dislocation movement of the matrix, the better the reinforcement effect. Under the action of external stress, the dislocation slip in the matrix is blocked at the interface, and the stress concentration occurs on the E-PG particles, causing the crackles appearance. In addition, the interface also has an effect of transferring the external stress to the reinforced particles as a bridge in the composite; it can prevent the crack growth, interrupt the material damage and slow down the stress concentration. So the strengthen mechanism of the E-PG/AR composite prepared in the experiment is according with the crack deflection mechanism and dislocation theory of particle reinforced composites.

4 Conclusions

A new composite is prepared at room temperature and atmospheric pressure by using the E-PG, low-grade AR and sodium silicate as main raw materials. The conclusions are shown as follows:

a) There is some certain chemical reaction between the AR and the system of sodium silicate and sodium hydroxide which formed the products of gismondine tending to transfer to the interface of the composite，and other aluminosilicate, Si-O-Si, causing an increasing trend of the binding energy of Ca (2p) and O (1s) from the matrix to the E-PG.

b) The E-PG has good compatibility with the AR matrix, and its structure is well preserved in the composite; the AR reacts with the system of sodium silicate and sodium hydroxide, forms a three-dimensional network structure; interface layer of transition zone is formed between the E-PG and AR.

c) The E-PG has a certain strengthening effect on the AR mineral polymer material, and the content of E-PG can be up to 40% in the AR mineral composite; on this condition, the compressive strength of the E-PG/AR composite can reach 16.5 MPa, reaching the standard of C15 concrete. The strengthen mechanism of the E-PG/AR composite is according with the crack deflection mechanism and dislocation theory of particle reinforced composites.

d) The E-PG/AR composite contains a large number of pores produced in the preparation process, and the micro pores are mainly concentrated between 5 nm-55 nm. The apparent density of the E-PG/AR composite is 1.75 g·cm-3, which is significantly lower than that of traditional materials as ordinary clay brick and common concrete, and even within the density of foamed concrete.

e) It provides a strong support for the large-scale comprehensive utilization of phosphogypsum and aluminous rock. The E-PG/AR composite can be used for base material of basement parking floor and large outdoor and indoor squares.