# Effect of Fly Ash on the Properties of Ceramics Prepared from Steel Slag
## 3 Results and Discussion  
### Physical and Mechanical Properties  
The variations in sintering shrinkage, water absorption, and flexural strength of the samples with firing temperature are shown in Figure 2. The sintering shrinkage of the samples is illustrated in Figure 2a. The sintering shrinkage of samples 1-4 at 1110 \({}^{\circ}\)C is very low, which indicates that the densification process has not started at 1110 \({}^{\circ}\)C. At 1120 \({}^{\circ}\)C, the sintering shrinkage is about 2%, and increases with an increase in the sintering temperature, indicating that the sample has begun to become dense. When the temperature reaches 1130 \({}^{\circ}\)C, the sintering shrinkage increases rapidly, reaching 8% for samples 1-3 and about 5% for sample 4. At 1135 \({}^{\circ}\)C, the sintering shrinkage of sample 4 reaches about 8%, which indicates that the addition of fly ash significantly affects the densification process of the sample. The sintering shrinkage increased rapidly from 1120 \({}^{\circ}\)C to 1130 \({}^{\circ}\)C for samples 1-3, and from 1120 \({}^{\circ}\)C to 1135 \({}^{\circ}\)C for sample 4. Above 1130 \({}^{\circ}\)C and 1135 \({}^{\circ}\)C, the sintering shrinkage remains constant for samples 1-3 and sample 4, respectively. This was due to a large amount of liquid phase in the two temperature ranges, which promoted the sintering reaction and filled the gap between the grains. Compared with samples 1-4, the densification process of sample 5 begins at 1130 \({}^{\circ}\)C, and the rapid shrinkage is caused by a large amount of liquid phase produced at the temperature range of 1130 \({}^{\circ}\)C to 1150 \({}^{\circ}\)C; the sintering shrinkage slightly changes at higher temperatures. Water absorption is another important factor for the progress of densification. The variation in the water absorption of the samples is inversely proportional to the variation in sintering shrinkage, as shown in Figure 2b. The water absorption is very low (close to 0) for samples 1 and 2 at 1130 \({}^{\circ}\)C, for samples 3 and 4 at 1135 \({}^{\circ}\)C, and for sample 5 at 1160 \({}^{\circ}\)C. Beyond the respective temperatures of 1130 \({}^{\circ}\)C, 1135 \({}^{\circ}\)C, and 1160 \({}^{\circ}\)C, the water absorption remains generally unchanged for samples 1 and 2, samples 3 and 4, and sample 5, respectively. According to the appearances of the samples, it was inferred that sintering deformation occurs when the sintering temperature is 1135 \({}^{\circ}\)C for samples 1 and 2, while the sintering deformation temperatures of samples 3, 4, and 5 are 1140 \({}^{\circ}\)C, 1150 \({}^{\circ}\)C, and 1170 \({}^{\circ}\)C, respectively. Combined with the analysis in Figure 2, the sintering range for samples 1 and 2 can be determined as 1130-1135 \({}^{\circ}\)C, for sample 3 as 1135-1145 \({}^{\circ}\)C, for sample 4 as 1135-1150 \({}^{\circ}\)C, and for sample 5 as 1160-1170 \({}^{\circ}\)C. The optimum sintering temperature is 1130 \({}^{\circ}\)C for samples 1 and 2, 1135 \({}^{\circ}\)C for sample 3, 1145 \({}^{\circ}\)C for sample 4, and 1160 \({}^{\circ}\)C for sample 5. According to the analysis in Figure 2, the physical properties of the samples at the optimum sintering temperature are shown in Table 3. With an increase in the fly ash addition amount, the optimum sintering temperature increased from 1130 \({}^{\circ}\)C to 1160 \({}^{\circ}\)C. The sintering shrinkage and water absorption of samples 1-5 at the optimum sintering temperatures are similar, with the water absorption below 0.5%, and the sintering shrinkage at about 8%. The difference is that the flexural strength decreases with an increase in the fly ash addition amount. The national standard of Chinese architectural ceramics stipulates that the water absorption of an ordinary ceramic tile should be less than 0.5%, and the lowest flexural strength should be 35 MPa.