# Effect of Fly Ash on the Properties of Ceramics Prepared from Steel Slag
## 3 Results and Discussion
### SEM Analysis  
SEM analysis was performed on the fractured surfaces of the sintered samples. The microscopic morphologies of the fractures of samples 1-5 at the optimum sintering temperature are shown in Figure 5. Two main features could be observed. First, pores are observed on the surface of all the samples. These pores are closed pores, that is, the pores are not connected to each other. The presence of closed pores played an important role in the low water absorption of the sintered samples [24]. This is consistent with the result of low water absorption of all of the samples, as shown in Table 3. All of the pores are nearly round, but have different sizes. Most of the pores in sample 1 have a diameter of 20-60 \(\upmu\)m. The pores increased with the increase in fly ash content. The pore diameter of sample 5 was about 100 \(\upmu\)m, and some even exceeded 110 \(\upmu\)m. The presence of pores can affect the mechanical properties of the sample, which is an important reason for the decrease in flexural strength [25]. Second, Figure 5f-j show the presence of many crystal in the five sintered samples. Large massive quartz particles can be clearly seen in sample 1. With an increase in the amount of fly ash, the quartz particles become smaller, and the amount is significantly reduced, which is consistent with the results of XRD. Moreover, all of the samples had rod-shaped crystals of similar sizes, with a length of 5-10 \(\upmu\)m and a width of 2-3 \(\upmu\)m. They were arranged in a crisscross arrangement in the sample, and were surrounded by the glass phase, imparting certain strength to the sintered sample. Figure 5: _Cont._  
The ceramics were sintered via the liquid phase sintering process. During the sintering process, the glass phase is generated, which is also an important factor affecting the performance [26]. On one hand, CaO and FeO are present in all of the samples, and their contents remain roughly the same. The Ca-O bonds in CaO are easily broken at low temperatures to produce Ca\({}^{2+}\) ions, which diffuse into the liquid phase and break the Si-O bonds. Therefore, the degree of glass network connection in the liquid phase decreases, and the number of broken bonds increases, resulting in an increase in the liquid phase and a decrease in liquid viscosity, which promotes the precipitation of crystals during the sintering process [27]. The presence of FeO also promotes the production of liquid phases, as well as the crystallization process [28]. On the other hand, in the presence of enough alkali metal ions such as Na\({}^{+}\) and K\({}^{+}\) or alkaline-earth metal ions such as Mg\({}^{2+}\) and Ca\({}^{2+}\), the aluminium element will first exist as a tetrahedral structure [AlO4]. Since this structure is similar to the silicon tetrahedron [SiO4], Al atoms can substitute for Si atoms to form Al-Si-O compounds as the Al\({}_{2}\)O\({}_{3}\) content in the batch increases. The Al-Si-O compounds lead to an increase in liquid viscosity [29]. The surplus negative charge of [AlO4] will occur when the alumina concentration is increased. According to the crystalchemical formula and the principle of electricalneutrality, the monovalent and divalent cations can be employed to maintain an aluminium-oxygen tetrahedral balance, resulting in the formation of [AlO4]\({}_{2}\)Mg, [AlO4]\({}_{2}\)Ca, [AlO4]K,  
Figure 5: SEM micrographs of samples at the optimum sintering temperature: (**a**,**f**) sample 1; (**b**,**g**) sample 2; (**c**,**h**) sample 3; (**d**,**i**), sample 4; and (**e**,**j**) sample 5.  
and [AlO4]Na complexes [30,31]. The increase in fly ash addition amount gradually increases the Al2O3 content. When the content of Al2O3 is excessive, the Ca2+ and Na+ ions are not sufficient to fully compensate the [AlO4] entities, and the Al3+ ions will exist as an octahedral structure [AlO6][32].