**2.6 SiO2- CaO system**

SiO2 can react with CaO to form CaO·SiO2 (CS), 3CaO·2SiO2 (C3S2), 2CaO·SiO2 (C2S) and 3CaO·SiO2(C3S) in roasting process. The reactions are shown in table 6, and the relationships between △G0 of the reactions of SiO2 with CaO and temperature are shown in figure 7.


Table 6. The *GT* of SiO2-CaO system( *G A BT <sup>T</sup>* , J/mol)

When CaO is insufficient, redundant Al2O3 may promote the newly generated high calciumto-aluminum ratio (CaO to Al2O3 mole ratio) calcium aluminates to transform into lower calcium-to-aluminum ratio calcium aluminates. The reactions of the equations are presented

**200 400 600 800 1000 1200 1400 1600 1800**

T,K

Figure 6 shows that, Gibbs free energy of the reaction of Al2O3-calcium aluminates system are negative at 400~1700K, and all the reactions automatically proceed to generate the corresponding low calcium-to-aluminum ratio calcium aluminates; Except for the reaction

becomes more negative. Comparing figure 4 with figure 5, it can be found that Al2O3 reacts

SiO2 can react with CaO to form CaO·SiO2 (CS), 3CaO·2SiO2 (C3S2), 2CaO·SiO2 (C2S) and 3CaO·SiO2(C3S) in roasting process. The reactions are shown in table 6, and the relationships between △G0 of the reactions of SiO2 with CaO and temperature are shown in figure 7.

CaO+SiO2 = CaO·SiO2(pseud-wollastonite) -83453.0 -3.4 298~1817 CaO+SiO2 = CaO·SiO2(wollastonite) -89822.9 -0.3 298~1817

3CaO+SiO2 = 3CaO·SiO2 -111011.9 -11.3 298~1800 2CaO+SiO2 = 2CaO·SiO2(β) -125875.1 -6.7 298~2403 2CaO+SiO2 = 2CaO·SiO2(γ) -137890.1 3.7 298~1100

*G A BT <sup>T</sup>* , J/mol)

**2**

**+Al**

**2**

**O3**

**O3**

**=(12/5)CaO·Al**

**2**

**O3**

**CaO·Al**

**(1/5)3CaO·Al**

**(1/2)3CaO·Al2**

**(4/3)3CaO·Al** **2**

**=(12/7)CaO·2Al2**

**O3**

**2**

**O3**

**O3**

**+Al**

**+Al**

**+Al**

**2**

**O3**

**2**

**2**

**O3**

**=(1/3)12CaO·7Al**

**O3**

**O3** **=CaO·2Al**

**=(3/5)CaO·2Al**

**=(3/2)CaO·Al2**

**2**

*GT* of reactions Al2O3-calcium aluminates system and

*GT* of the rest reactions decreases with the rise of temperature and

Reactions A, J/mol B, J/K.mol Temperature, K


**O3**

*GT* of reactions of Al2O3-calcium aluminates system and

**2**

**O3**

**2**

**O3**

**O3**

**(1/5)12CaO·7Al**

**(1/7)12CaO·7Al2**

**2**

**O3**

**+Al**

**O3 +Al2 O3**

**2**

**O3**

in table 5:

The relationships between

Fig. 6. Relationships between

with CaO easily to generate C12A7.

temperature

of Al2O3-C12A7, the

**2.6 SiO2- CaO system** 

temperature (T) are shown in figure 6.

**-100**

2 2

*GT* of SiO2-CaO system(

3 1 CaO+SiO =( ) 3CaO 2SiO

2 2

Table 6. The

**-80**

**-60**

△G,kJ.Mol-1

*T G* 

/(KJ·Mol-1)

**-40**

**-20**

**0**

Fig. 7. Relationships between *GT* and temperature

Figure7 shows that, SiO2 reacts with CaO to form γ-C2S when temperature below 1100K, but β-C2S comes into being when the temperature above 1100K. At normal roasting temperature, the thermodynamic order of forming calcium silicate is C2S, C3S, C3S2, CS.

Figure 5 ~ figure 7 show that, CaO reacts with SiO2 and Al2O3 firstly to form C2S, and then C12A7. Therefore, it is less likely to form aluminium silicates in roasting process.
