## 3 Results and Discussion
### Physical Parameters of the Composites  
Ceramic composites showed a similar weight loss and bulk density as pristine ceramic (Figure 3a,b). No obvious difference in weight loss was observed when the firing temperature was increased from 1200 \({}^{\circ}\)C to 1350 \({}^{\circ}\)C (Figure 3a), which indicates that the weight loss mainly occurred at a temperature lower than 1200 \({}^{\circ}\)C. The main weight loss came from the dihydroxylation of kaolin as shown as follows [24; 29; 35].  
\[2\text{SiO}_{2}\cdot\text{Al}_{2}\text{O}_{3}\cdot\text{2H}_{2}\text{O}\left( \text{Kaolin}\right)\to 2\text{SiO}_{2}\cdot\text{Al}_{2}\text{O}_{3}\left( \text{Metakaolin}\right)+2\text{H}_{2}\text{O} \tag{7}\]  
Meanwhile, the bulk density of the composites increased slightly from 2.02 g/cm\({}^{3}\) for Composite-1200 to 2.32 g/cm\({}^{3}\) for Composite-1350 (Figure 3b). This increase may be attributed to the reduced volume from the shrinkage of the specimen.  
The open porosity of the composites reduced greatly from 24.19% for Composite-1200 to 5.39% for Composite-1350 (Figure 3c). The porosity between carbon fiber and ceramic, together with the un-densification of the ceramic, are the two main reasons for the formation of the open porosity. As the temperature increased, feldspar started to melt, and transformed to a glassy phase around 1100 \({}^{\circ}\)C,  
Figure 3: Physical properties of composites fired at different temperatures. (**a**) Weight loss, (**b**) bulk density, (**c**) open porosity, and (**d**) water absorption.  
which enabled it to permeate into the microstructure, leading to the decreased open porosity [22, 24]. Water absorption is related to open porosity, as shown in Figure 3d. Water absorption of the ceramic composites decreased from 12.45% to 2.29% as the firing temperature was increased from 1200 \({}^{\circ}\)C to 1350 \({}^{\circ}\)C. The open porosity of pristine ceramic reduced from 15.3% to 2.75% after sintering from 1200 \({}^{\circ}\)C to 1350 \({}^{\circ}\)C. Meanwhile, the water absorption of pristine ceramic also decreased from 7.1% to 1.2%. The reduction in open porosity and water absorption of the composites is attributed to the densification of the ceramic. Whereas, compared with pristine ceramic, the composites exhibited increased open porosity and higher water absorption, which is due to the presence of a gap between the ceramic and carbon fibers in the composites. Work is on-going to optimize the mechanical properties of the carbon fiber/ceramic composites.  
The obtained carbon fiber/ceramic composites were white in color after sintering, as shown in Figure 4a. The sintering process led to shrinkage of the composites and the linear shrinkage of the composites increased with an increase in the firing temperature (Figure 4b). The linear shrinkage values were 2.23% for Composite-1200 and 6.97% for Composite-1350. The linear shrinkage of all of the composites showed the same trend as that of pristine ceramic.  
A three point flexural test was carried out to evaluate the flexural strength of the obtained carbon fiber/ceramic composites. Figure 4c shows the flexural strength of the carbon/fiber composites and pristine ceramic, which were prepared at different temperatures. The flexural strength of both the ceramic composites and pristine ceramic increased as the firing temperature increased, which may be caused by the reduced porosity (Figure 3c). The flexural strength of the ceramic composite reached 28.88 MPa at 1350 \({}^{\circ}\)C. Compared with the ceramic composites, the pristine ceramic without carbon fibers showed a higher flexural strength of 38.25 MPa at 1350 \({}^{\circ}\)C. It is suggested that the gaps between the carbon fiber and ceramic resulted in the lower flexural strength of the ceramic composite.