J. Cent. South Univ. Technol. (2009) 16: 0881-0886

DOI: 10.1007/s11771-009-0146-08

Thermal-mechanical properties of short carbon fiber reinforced

geopolymer matrix composites subjected to thermal load

LIN Tie-song(林铁松), JIA De-chang(贾德昌), HE Pei-gang(何培刚), WANG Mei-rong(王美荣)

(Institute for Advanced Ceramics, Harbin Institute of Technology, Harbin 150001, China)

Key words:

short carbon fiber; α-Al2O3; thermal-mechanical properties; geopolymer; thermal load；

1 Introduction

In recent years, geopolymer materials have attracted much more attention due to their low density, low cost, low curing/hardening temperatures, environmentally friendly nature and excellent thermal stability at high temperatures[1-4]. They are considered as the potential substitute materials for ordinary Portland cement (OPC) and organic polymer materials and are suitable for high temperature applications[1, 5].

Fiber reinforced geopolymer matrix composites can effectively improve the mechanical performances and obtain graceful failure to meet construction applications and prevent catastrophic failure during service. Over the past years, various types fabrics or short fibers have been used successfully to strengthen and toughen geopolymer materials and the strength, and fracture behavior and fatigue resistance of fiber reinforced geopolymer matrix composites have been examined[1-2, 6-10]. These open the window to a variety of applications where metals or polymer composites are used such as engine exhaust systems, offshore drilling platforms, pressure vessel liners and cabin interior materials for aircraft or ship.

Due to their inorganic network structure, geopolymer matrix composites can survive a serious fire. After a proper reconstruction they can continue their service life for many years[11]. Therefore, it is useful to study the properties of geopolymer matrix composites not only in usual conditions but also in thermal load conditions. Some researches for different geopolymer matrixes after thermal load were done. BARBODSA and MACKENZIE[12] studied the effects of heat treatment temperature on phase transition of Na- and K-based metakaolin geopolymers. Similarly, KONG et al[13] and ZUDA et al[14-15] analyzed the effects of heat treatment up to 1 200 ℃ on the compressive and flexural strength of geopolymer with different fillers. On the other hand, DUXSONA et al[16-18] reported the evolution of geopolymer during thermal exposure, including thermal shrinkage, crystallization and mechanical strength at elevated temperatures. However, studies on the effects of thermal load on thermal-mechanical properties and fracture behavior of fiber reinforced geopolymer matrix composites have not been conducted yet.

Therefore, in this work, short carbon fiber preform reinforced geopolymer composites containing different contents of α-Al₂O₃ particle fillers were fabricated, and the effects of thermal load and the content of α-Al₂O₃ particle filler on thermal-mechanical properties and fracture behavior up to 1 200 ℃ were studied.

2 Experimental

Short carbon fibers used for preparing preforms had an average length of 7 mm. They were first separated by an ultrasonic vibrator in ethanol, and then filtered by a wire sieve to form sheet-like short carbon fiber preforms (see Fig.1) with a thickness in the range of 0.15-0.2 mm.

Fig.1 Optical image of as-prepared sheet-like short carbon fiber preform

The compositions of the mixture for sample preparation are listed in Table 1. A typical processing route for geopolymer resin is as follows: potassium silicate solution was made by dissolving silica sol (40%, in mass fraction) into KOH solution with a magnetic stirrer. Kaolin powder (4.08 μm in average diameter) was calcined at 800 ℃ for 2 h in order to obtain metakaolin powder. The metakaolin powder was added to the potassium silicate solution and mixed for 30 min with a high-shear mixer to form a geopolymer resin. Whereafter, α-Al₂O₃ particle was added to geopolymer resin and mixed homogeneously.

The as-prepared preforms were impregnated with geopolymer resin in a plastic container and put together one by one to get a stack consisting of 30 layer preforms. In order to avoid the formation of small pores between the sheet preforms during the lamination, the sheet performs were put in a vacuum-bag at 80 ℃ for 48 h, followed by curing at 120 ℃ for 24 h. The volume fraction of short carbon fibers in the as-prepared

Table 1 Compositions of C_f(α-Al₂O₃)/geopolymer composites

composites was 3.5%.

The heating of the samples to the predetermined temperature was carried out at a rate of 5 ℃/min in a vacuum atmosphere; then the specimens were left at that temperature for a period of 2 h; and finally they were slowly cooled. The chosen heat treatment temperatures were 400, 600, 800, 1 000 and 1 200 ℃, respectively.

The constitution phases of the obtained composites were determined by X-ray diffractometry (XRD). The thermal shrinkage of the geopolymers subjected to different temperatures was obtained from the volume of the geopolymers before and after heat treatment. Flexural strength measurements were conducted on specimens  (4 mm×3 mm×36 mm) using a three-point bend fixture on an Instron-500 testing machine with a span length of 30 mm at a crosshead speed of 0.5 mm/min. All flexural bars were machined with the tensile surface perpendicular to the direction of lamination. Load—displacement curves were recorded. Fracture work was calculated by the area between load curves and X-axis in the load—displacement curves till the load drops to 90% of the maximum. Six specimens were tested under each test condition. Fracture surfaces of the composites were observed by scanning electron microscopy (SEM).

3 Results and discussion 3.1 Thermal-physical evolution

Typical XRD traces of C_f(α-Al₂O₃)/geopolymer composites after heat treatment at different temperatures are shown in Fig.2. Composites C7A0 (Fig.2(a)) show a characteristic amorphous hump 2θ of about 28? and a sharp major reflection of the quartz in the original kaolinite in the temperature range of 25-800 ℃. The diffraction lines corresponding to leucite (KAlSi₂O₆) are found at 1 000 ℃ and become sharper at 1 200 ℃ as the broad amorphous background feature disappears, which indicates that the amorphous geopolymer matrix is crystallized and leucite is formed at 1 000 ℃. However, for composites C7A3 (Fig.2(b)), the leucite phase is not found until 1 200 ℃. The onset crystalline temperature of composites C7A3 is obviously higher than that of composites C7A0. Therefore, the addition of α-Al₂O₃ particle filler into geopolymers clearly increases the onset crystalline temperature. The distribution of α-Al₂O₃ particles in geopolymer matrix probably generates “inserted effect” in amorphous structure of geopolymer and reduces the viscosity of geopolymer at high temperatures, which maybe results in the retardance of onset crystalline temperature.

Fig.3 shows the thermal shrinkage of the composites after heat treatment at different temperatures. It can be observed that the overall thermal shrinkage of specimens is greatly influenced by heat treatment temperature. With the increase of heat treatment temperature, the shrinkage in the direction perpendicular to the lamination of composites (Fig.3(a)) gradually increases until 1 000 ℃. Beyond 1 000 ℃, the composites exhibit a larger degree of shrinkage. However, in the direction parallel to the lamination (Fig.3(b)), the composites show an expansion behavior in the temperature range of 400-1 000 ℃. At 1 200 ℃, a larger degree of shrinkage is the same to that perpendicular to the lamination of the composites. Furthermore, the volume change of the composites gradually decreases with the increase of the content of α-Al₂O₃ particles at the same temperature. This indicates that the addition of α-Al₂O₃ is helpful to keep volume of the composites stable.

Fig.2 XRD patterns of composites C7A0 (a) and C7A3 (b) after heat treatment at different temperatures

Geopolymer matrix after curing retains about 15% water, including free water and hydration water[5]. The free water is lost at low temperatures. However, the hydration water is either bounded tightly or less able to diffuse to the surface, and continues to evolve gradually until about 500 ℃ and beyond[5]. The water evolution will undoubtedly result in the decrease of mass. In addition, the microstructure of geopolymer matrix is of nanoparticulate features and pores with size of     5-10 nm[19]. The densification and crystallization of geopolymer matrix also result in reduction and disappearance of nanopores.

The addition of α-Al₂O₃ particle into the composites indirectly reduces the content of water and nanopores due to the fact that α-Al₂O₃ has no water and nanopores. Furthermore, the thermal resistance of α-Al₂O₃ is far larger than that of geopolymer matrix. Therefore, the increase in content of α-Al₂O₃ particles (composites C7A3 and C7A5) clearly reduces the shrinkage and keeps the volume stable at high temperatures, especially in the temperature range of 800-1 200 ℃. In addition, the onset crystalline temperature of geopolymer matrix increases by almost 200 ℃ due to α-Al₂O₃ filler, as shown in Fig.2. This also favors to reduce the shrinkage and keep the composites stable at high temperatures. The expansion behavior in the direction parallel to the lamination of the composites is probably due to the fact that the shrinkage of matrix results in debonding between short carbon fiber preforms and matrix because the interlaminar shear strength of the composites is relatively low. However, the densification and crystallization of the composites at 1 200 ℃ lead to the larger degree of shrinkage.

Fig.3 Thermal shrinkage of C_f(α-Al₂O₃)/geopolymer com- posites perpendicular (a) and parallel (b) to lamination at different heat treatment temperatures

3.2 Mechanical properties and fracture behavior

Table 2 shows the mechanical properties of the as-prepared C_f(α-Al₂O₃)/geopolymer composites after heat treatment at different temperatures. The flexural strength of the composites decreases with the increase of heat treatment temperature until 600-800 ℃, as shown in Fig.4(a). Geopolymer network consists of SiO₄ and AlO₄ tetrahedral linked alternately by sharing all the oxygens. And positive ions (K⁺, H₃O⁺) should be present in the framework cavities to balance the negative charge of Al³⁺ in IV-fold coordination[20]. The balance will be destroyed once the free water and hydration water are gradually lost as the composites are subjected to the high heat treatment temperatures. And the degree of polycondensation of the geopolymer matrix is lowered, which will weaken the strength of the composites. In addition, the thermal loads lead to the appearance of macrocracks as the composites are subjected to the high heat treatment temperatures. These cracks negatively affect the mechanical properties of the composites in quite a remarkable way. The hydration water of geopolymer matrix is lost at about 600 ℃. At the same time, the flexural strength of the composites shows the minimum in the temperature range of 600-800 ℃. The strength increases at 1 000 ℃. This is probably due to the fact that the geopolymer matrix is sintered at 1 000 ℃ and the content of nanopores is reduced. The sharp decrease of strength at 1 200 ℃ is likely due to the clear shrinkage (as shown in Fig.3) that destroys the structure of short fiber preforms and weakens the interface of fiber/matrix. The fracture work of the composites shows almost a similar trend as the flexural strength, as demonstrated in Fig.4(b).

The addition of α-Al₂O₃ particle into geopolymer matrix cannot improve the mechanical properties of composites at room temperature, but the mechanical properties of the composites subjected to high temperatures can be improved to a certain extent, because the addition of α-Al₂O₃ particles effectively reduces the shrinkage and keeps the volume of the composites stable at high temperatures.

The typical load—displacement curves for the geopolymer composites are given in Fig.5. The composites reinforced with short carbon fiber preform show a noncatastrophic fracture behavior. It can be seen that the composites extend elastically at the beginning of the test. Beyond the elastic limit, the applied load produces plastic deformation until the maximum load reaches, and then the load drops with the increase of displacement and forms a long tail due to the fiber debonding and pulling-out. Although the ultimate strength of the composites reduces after heat treatment in comparison with that of the composites at room temperature, the head displacement prior to final fracture increases at 400, 800 and 1 000 ℃. This indicates that the interfacial structure between fiber and matrix is ideal and suitable for the fiber pulling-out mechanism.

The fracture behavior of the investigated composites

Table 2 Mechanical properties of as-prepared C_f(α-Al₂O₃)/geopolymer composites at different heat treatment temperatures

Fig.4 Flexural strength (a) and fracture work (b) of C_f(α-Al₂O₃)/geopolymer composites after heat treatment at different temperatures

can be demonstrated clearly from SEM images by observing the fracture surface of the composites, as shown in Fig.6. A lot of pulling-out fibers are found on the fracture surface. Because the strength of fiber/matrix is relatively weak as shown from the clean surface of the pulling-out fibers, and the strength of carbon fibers is far higher than that of geopolymer matrix. The pulling-out length is long to ensure substantially effective toughening effect from carbon fibers and prevent catastrophic fracture of the composites. In addition, a lot of matrix cracks (as shown in Fig.6(b)) distribute in the

Fig.5 Load—displacement curves of composites C7A3 after heat treatment at different temperatures

Fig.6 SEM images of composites C7A3 after heat treatment at 1 000 ℃ for 2 h: (a) Parallel to fracture surface; (b) Perpen- dicular to fracture surface

geopolymer matrix on the side of the composite beam near the fracture surface. These cracks are generated because lots of fibers are pulled out from the geopolymer matrix in different directions during testing to failure. And they are helpful to obtain more facture work and make the composites show the pseudoplasticity behavior.

4 Conclusions

(1) From room temperature (25 ℃) to 1 000 ℃, the thermal shrinkage in the direction perpendicular to the lamination of the composites gradually increases. However, in the direction parallel to the lamination of the composites an expansion behavior appears. Beyond    1 000 ℃, in the two directions the composites exhibit a larger degree of shrinkage due to the densification and crystallization of composites.

(2) The flexural strength and fracture work of the composites show the minimum in the temperature range of 600-800 ℃ due to the loss of the hydration water of geopolymer matrix.

(3) Leucite phase forms at high heat treatment temperatures. The addition of α-Al₂O₃ particles into the composites clearly increases the onset crystalline temperature of geopolymer matrix and reduces the volume change of the composites at high temperatures, which improves the mechanical properties of the composites subjected to high temperatures to a certain extent.

(4) The main toughening mechanisms of the composites subjected to high temperatures are attributed to fiber pulling-out.

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(Edited by CHEN Wei-ping)

Foundation item: Project supported by the Science Fund for Distinguished Young Scholars of Heilongjiang Province, China; Project supported by the Program for Excellent Team in Harbin Institute of Technology.

Received date: 2009-03-13; Accepted date: 2009-05-20

Corresponding author: JIA De-chang, Professor, PhD; Tel: +86-451-86418792; E-mail: dcjia@hit.edu.cn