Morphological Evolution of Low-Grade Silica Fume at Elevated Temperature

TG-DTA analysis

Figure 1 shows TG-DTA curves of low-grade silica fume from room temperature to 1,200 °C in air at the heating rate of 10 °C/min. The reaction process can be divided into three stages depending on temperature, i. e. 0–300 °C, 300–600 °C and 600–1,200 °C. In the first stage, the weight loss is up to 13 % and a big endothermic peak appears in the corresponding DTA curve, indicating the dehydration reaction of free water and adsorbed molecular water in the sample occurs. When temperature is increased to 300–600 °C, a continuous weight loss of 7 % is shown in TG curves. This can be ascribed to the reaction of carbon oxidation and other reactions. With temperature further increasing, TG curve becomes stable while the corresponding DTA curve appears a big exothermic peak at about 750 °C, indicating the reaction relating to the change of material structure takes place.

Figure 1:

TG-DTA of low-grade silica fume from room temperature to 1,200 °C at the heating rate of 10 °C/min in air.

Phase and microstructure development

The photos of silica fume before and after heat treatment at different temperature are shown in Figure 2. It can be seen the appearance of the sample does not change obviously until 700 °C (as shown in figure b, c and d). When treated at 800 °C, the sample begins to shrink as shown in Figure 2(e). When the thermal treatment temperature is reached to 900 °C, the sample shrinks a lot to be compact as shown in Figure 2(f).

Figure 2:

Photographs of low-grade silica fume before and after heat treated at different temperature for 180 min (a) untreated; (b) 300 °C; (c) 600 °C; (d) 700 °C; (e) 800 °C; (f) 900 °C.

The phase evolution of sample at different temperature is characterized by the XRD patterns as shown in Figure 3. It can be seen that amorphous silica is the main phase before thermal treatment, which is in agreement with the reported result [13]. When treated at 300 °C, 600 °C and 700 °C for 180 min respectively, no obvious change is observed compared with the sample before thermal treatment, indicating that there is no effect on the phase of silica fume at the temperature below 700 °C. When treated at 800 °C, the crystalline phase appears. With temperature further increasing to 900 °C, crystalline phase, i. e. cristobalite dominates. Therefore it can be concluded that the formation of crystal phase is the main reason of the appearance of the big exothermic peak at about 750 °C in DTA curve. In view of the effect of the impurities in the raw material, carbon and hydroxyl substance tends to be oxidized during the thermal treatment. Other impurities, i. e. CaO, Fe₂O₃, MgO and Al₂O₃ etc. do not affect the microstructure of silica fume until 1,100 °C [13]. Therefore the effect of impurities on the thermal treatment is not taken into consideration in this work.

Figure 3:

XRD patterns of low-grade silica fume heat treated at different temperatures for 180 min.

The microstructure evolution of sample at different temperature is further investigated using SEM and TEM techniques. Figure 4 is the microstructure of silica fume before thermal treatment. It can be seen that silica fume is agglomerated spherical particles with the particle size of 300 nm or so. When silica fume is treated at 300 °C for 180 min, the microstructure does not change much and is similar to that before thermal treatment (Figure 5(a)). TEM analysis also indicates that the spherical shape remains while the edges of some tiny particles are merged to each other (Figure 5(b) and (c)). With temperature increasing to 600 °C, more tiny particles are adhered to each other while the sample remains the spherical shape with the size of about 300 nm (Figures 5(d)–(f)). The selected area electron diffraction (SAED) result inserted in Figure 5(f) indicates the sample is still an amorphous structure [14]. With temperature further increasing to 700 °C, the microstructure is similar to that treated at 300 °C and 600 °C (Figure 5(g)–(i)), demonstrating that there is no effect on the phase of silica fume below 700 °C. This is in agreement with the XRD results. Figure 5(j) is the SEM image of silica fume heated at 800 °C. More silica fume particles merge together and exhibit a more compacter structure. TEM analysis indicates that spherical particles begin to change into irregular forms (Figure 5(k)). At the same time, the appearance of disordered diffraction spots and the concentric circle of SAED inserted in Figure 5(l) indicate that the sample contains both crystalline and amorphous structures [15]. When the temperature increases to 900 °C, TEM images show that the spherical microstructure of sample disappears completely and is replaced by irregular forms. SAED image inserted in Figure 5(o) indicates the crystalline phase is formed. Combing with the XRD results, it can be concluded that amorphous SiO₂ is transformed into the crystalline phase from 800 °C.

Figure 4:

SEM image of low-grade silica fume before heat treated.

Figure 5:

Microstructure of low-grade silica fume before and after heat treated at deferent temperature (a–c) 300 °C; (d–f) 600 °C; (g–i) 700 °C; (j–l) 800 °C; (m–o) 900 °C.

Structure development

FTIR analysis is applied to reveal the structural transformation of silicon fume before and after heat treatment as shown in Figure 6. For silica fume before thermal treatment, peaks at 469 cm⁻¹, 802 cm⁻¹ and 1,106 cm⁻¹ show the strongest absorbance bands, which are the characteristic of nano SiO₂ powder. The band at 469 cm⁻¹ is the bending frequency of O-Si-O (δ(O-Si-O)). The band centered at 802 cm⁻¹ is due to symmetric stretching of Si-O-Si (υ_s(Si-O-Si)) and the broad band at 1,106 cm⁻¹ is attributed to asymmetric stretching frequency of Si-O-Si (υ_as(Si-O-Si)) [16]. Besides these, the small band at around 1,617 cm⁻¹ also appears which is connected with O-H bending vibration of adsorbed water molecules. The broad peak at 3,435 cm⁻¹ is attributed to the asymmetric stretching frequency of -OH groups and water on the surface which is hydrogened-boned to each other and silanols [17]. Magnification of band at 3,600–3,800 cm⁻¹ exhibits some small while significant peaks (Figure 6(a)), which is corresponding to asymmetric stretching frequency of -OH groups (3,630 cm⁻¹), mutually bonded Si-OH species (3,660 cm⁻¹) and isolated SiO-H vibrations (3,730 cm⁻¹) respectively [18]. The possibly reason is that the silica fume reacts with the water during storage or the heat-treatment conditions [19].

Figure 6:

FT-IR spectra of low-grade silica fume before and after heat treated at deferent temperature (a); (b) magnification of bands around 3,600 cm⁻¹ to 3,800 cm⁻¹ for untreated silica fume.

When temperature is increased to 300 °C, the intensity of the absorption peaks at 3,435 cm⁻¹ and 1,617 cm⁻¹ decreases, indicating the dehydration reaction occurs. Thus the two neighboring silanols condensed into a siloxane bridge [20]. Because of the dehydration, the sample loses weight as shown in TG-DTA curves at around 300 °C. Further heated to 800 °C, the broad peak at 1,106 cm⁻¹ attributing to the asymmetric stretching frequency of Si-O-Si is shifted to 1,116 cm⁻¹. Thorpe [21], Galeener [22] and Almeida [23] have pointed out that the shift of Si-O-Si mode can be explained by following equation:

where ω is related to the Si-O-Si asymmetric stretching frequency, θ stands for the intertetrahedral bond angle, and k represents the Si-O stretching force constant. m_O and m_Si are the mass of the oxygen and silicon atoms respectively. From Eq. (1), it can be seen that the influence factors of transformation of frequency are bond angle θ and stretching force constant k. When θ increases, vibrational frequency will be larger. On the other hand, an increase in the Si-O bond length leads to a decrease of k which ultimately results in a decrease of asymmetric stretching frequency ω [24]. In the experiment, silica fume is in the form of amorphous structure, which is generally considered to be disordered tetrahedron of [SiO₄]. Mozzlt and Warren [25] roughly estimated θ of the Si-O-Si bond following the distribution curve V(θ), and it varies in the range of 120° to 180° with a maximum of about 144°. In our experiment, the bond angle θ of the cristobalite crystal formed is about 146.8° [26]. The increase of θ may cause an increase of the asymmetric stretching frequency ω.

Raman spectra of silica fume before and after treated at different temperature are further conducted and the results are shown in Figure 7. In view of the sample before treated, the sharp band at 292 cm⁻¹ can be assigned to the modes of [SiO₄] and oxygen vibrations [27]. The sharp bands at 505 cm⁻¹ and 603 cm⁻¹, known as defect modes, have been associated to breathing vibrations of oxygen atoms in four- and three-membered rings, respectively [19]. Especially in view of the band at 505 cm⁻¹, the strong intensity indicates a fairly more defects in the silica fume. The defects may be caused by broken bands, metallic impurities and vacancies. Bates [28] pointed out that the defects modes of 505 cm⁻¹ and 603 cm⁻¹ belong to a kind of defect with one broken Si-O bond which can be represented by -Si-O…Si-. The small band at 987 cm⁻¹ indicates the formation of Si-OH, which may be caused by connecting with water resulting in the formation of Si-OH and even adjacent hydroxyl groups Si-OH•OH-Si. The small signals obtained at 797 cm⁻¹ are attributed to the network Si-O-Si symmetric stretching vibrating mode [29].

Figure 7:

Raman spectra of silica fume before and after heat treated at different temperature.

For a better comparison, spectra have been arbitrarily normalized to the 292 cm⁻¹ intensity peak. When the sample is treated at different temperature, the band of 603 cm⁻¹ disappears. The intensity of the sharp band at 505 cm⁻¹ decreases and red-shifts to a higher band of 515 cm⁻¹ as the treatment temperature increases. It has been pointed out that the two bands are associated with the defects. Therefore the decrease and red-shift of the two bands indicated a densification of the defects, i. e. the three-membered ring (SiO)₃ reconstructed into a larger and stable structure with the increasing treatment temperature [30, 31].

From above experiments, a hypothetical planar model is proposed for the reaction process of low-grade silica fume at elevated temperature (Figure 8). Silica fume formed randomly with disordered [SiO₄] tetrahedron and the central atom was silicon. The Si-O rings may consist of three-, four-, five-, six- or more fold silicon as the amorphous structure. Six- to eight-fold Si-O rings are verified to be the most stable because of the lower energy [32]. In our work, some defects such as Si-O and Si≡ exist in the raw material (Figure 8(a)). When exposed to moist air, some broken bonds tend to absorb water to form the bands of Si-OH or Si-OH•OH-Si (Figure 8(b)). At the same time, free water enters into the inner part of silica fume as the loose structure. When temperature is increased to 600 °C, the dehydration reaction occurs accompanying with the reconstruction of several broken bonds (Figure 8(c)). From 800 °C, the reaction of reconstruction of the residual broken bonds begins (Figure 8(d)). The defects of Si-O and Si≡ rebuild and the three- or four-fold Si-O rings connects to form a larger and stable ring. Therefore the disordered structure changes to be ordered and the loose structure of silica fume became compact accompanying with a huge shrinkage in volume.

Figure 8:

Schematic diagram of structure evolution of low-grade silica fume at elevated temperature. (a) Before contacting with the moist air; (b) after contacting with the moist air at room temperature; (c) treated at low temperature (≤700 °C); (d) treated at high temperature (≥800 °C).