**2.7 SiO2- calcium aluminates system**

In the CaO-Al2O3 system, if there exists some SiO2, the newly formed calcium aluminates are likely to react with SiO2 to transform to calcium silicates and Al2O3 because SiO2 is more acidity than that of Al2O3. The reaction equations are presented in table 7, the relationships between *GT* and temperature are shown in figure 8.

Figure 8 shows that, the *GT* of all the reactions increases with the temperature increases; the reaction (3CA2+SiO2=C3S+6Al2O3) can not happen when the roasting temperature is above 900K , i.e., the lowest calcium-to-aluminum ratio calcium aluminates cannot transform to the highest calcium-to-silicon ratio (CaO to SiO2 molecular ratio) calcium silicate; when the temperature is above 1500K, the *GT* of reaction(3CA+ SiO2=C3S+3Al2O3) is also more than zero; but the other calcium aluminates all can react with SiO2 to generate calcium silicates at 800~1700K. The thermodynamic sequence of calcium aluminates reaction with SiO2 is firstly C3A, and then C12A7, CA, CA2.

Thermodynamics of Reactions Among

**-120**

excess, C2F can react with Fe2O3 to form CF.

Fig. 8. Relationships between

**2.8 CaO- Fe2O3 system** 

figure 9.

Table 8. The

**-110**

(1/6)**12CaO·7Al**

(1/8)**12CaO·7Al**

(2)**CaO·Al**

(3/2)**CaO·2Al**

**CaO·2Al**

**2**

*GT* of Fe2O3-CaO system(

**O3**

**2**

**+SiO2**

**+SiO2**

**+SiO2**

**2**

**+SiO2**

**=3CaO·SiO2**

**O3**

**2**

(3/2)**CaO·Al**

**2**

**O3**

**2**

**O3**

**(3)CaO·2Al**

> **O3**

> > **O3**

**+SiO2**

**+SiO2**

**(2/3)3CaO·Al**

**=2CaO·SiO2**

(1/12)**12CaO·7Al**

**+SiO2**

**=2CaO·SiO2**

**=(1/2)3CaO·2SiO2**

**(3)CaO·Al**

**+6Al**

**2**

**O3**

**2**

**=CaO·SiO2**

> **O3**

> > **+7/6Al**

**=(1/2)3CaO·2SiO2**

**(1/2)3CaO·Al**

**2**

**O3**

**+SiO2**

**+2Al**

**2**

**O3**

**+SiO2**

**2**

**3CaO·Al**

**2**

**O3**

**O3**

**(1/3)3CaO·Al**

**2**

**2**

**O3**

**+2Al**

**=(1/2)3CaO·2SiO2**

**+3Al**

**2**

**O3**

> **O3**

**+7/8Al**

**+SiO2**

**CaO·Al**

**2**

**O3**

**(1/4)12CaO·7Al**

**2**

**2**

**O3**

**O3**

**+(3/2)Al**

**+SiO2**

**2**

**O3**

**=3CaO·SiO2**

> **+SiO2**

**=2CaO·SiO2**

**+7/4Al**

**2**

**O3**

**2**

**O3**

**+7/12Al**

**=3CaO·SiO2**

**=CaO·SiO2**

**=CaO·SiO2**

**2**

**O3** **+Al**

**2**

**+Al**

**2**

**+1/3Al**

**+1/2Al**

**2**

**2**

**O3**

**O3**

**O3**

**O3**

**+4Al**

**2**

**O3**

**=CaO·SiO2**

**2**

**+SiO2**

**=2CaO·SiO2**

**O3**

**+SiO2**

**+SiO2**

**=(1/2)3CaO·2SiO2**

**+2/3Al**

**2**

*GT* and temperature in SiO2-calcium aluminates system

**O3**

**2**

**(2)CaO·2Al**

**2**

**O3**

**+3Al**

**2**

**O3**

**+SiO2**

**O3**

**-100**

**-90**

**-80**

**-70**

△

*T G* 

G,kJ.Mol

/(KJ·Mol-1)


**-60**

**-50**

**-40**

**-30**

**-20**

**-10**

**0**

Al2O3, CaO, SiO2 and Fe2O3 During Roasting Processes 833

**=3CaO·SiO2**

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

T,K

Fe2O3 can react with CaO to form CaO·Fe2O3(CF) and 2CaO·Fe2O3(C2F). When Fe2O3 is used up, the newly formed C2F can react with Fe2O3 to form CF. The reaction equations are shown in table 8, and the relationships between △G0 and temperature are shown in

Figure 9 shows that, Fe2O3 reacts with CaO much easily to form C2F; CF is not from the reaction of C2F and Fe2O3, but from the directly reaction of Fe2O3 with CaO. When Fe2O3 is

Reactions A, J/mol B, J/K.mol Temperature, K CaO+Fe2O3=CaO·Fe2O3 -19179.9 -11.1 298~1489 2CaO+Fe2O3=2CaO·Fe2O3 -40866.7 -9.3 298~1723 2CaO·Fe2O3+Fe2O3=(2)CaO·Fe2O3 2340.8 -12.6 298~1489

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


Table 7. The *GT* of the reactions SiO2 with calcium aluminates( *G A BT <sup>T</sup>* , J/mol)

Reactions A, J/mol B,

(3)CaO·2Al2O3 +SiO2=3CaO·SiO2+6Al2O3 -69807.8 70.8 298~1800 (3)CaO·Al2O3 +SiO2=3CaO·SiO2+3Al2O3 -62678.8 42.6 298~1800

(2)CaO·2Al2O3+SiO2 =2CaO·SiO2+4Al2O3 -98418.8 48.1 298~1710

CaO·2Al2O3+SiO2= CaO·SiO2+2Al2O3 -76146.6 27.1 298~1817 CaO·Al2O3+SiO2 =CaO·SiO2+Al2O3 -73770.2 17.7 298~1817

(2)CaO·Al2O3+SiO2 =2CaO·SiO2+2Al2O3 -93666.1 29.2 298~1710

3CaO·Al2O3+SiO2= 3CaO·SiO2+Al2O3 -100774.6 16.9 298~1808

*GT* of the reactions SiO2 with calcium aluminates(

23 2 2 23

23 2 2 23

23 2 2 23

23 2 2 23

23 2 2 23

23 2 2 23

23 2 2 23

23 2 2 23

23 2 2 23

( )CaO 2Al O SiO ( )3CaO 2SiO 3Al O

3 13 ( )CaO Al O SiO ( )3CaO 2SiO Al O 2 22

1 7 ( )12CaO 7Al O SiO CaO SiO Al O 12 12

<sup>1</sup> 1 7 ( )12CaO 7Al O SiO ( )3CaO 2SiO Al O 8 2 8

1 7 ( )12CaO 7Al O SiO 2CaO SiO Al O 6 6

1 1 ( )3CaO Al O SiO CaO SiO Al O 3 3

1 11 ( )3CaO Al O SiO ( )3CaO 2SiO Al O 2 22

2 2 ( )3CaO Al O SiO 2CaO SiO Al O 3 3

Table 7. The

1 7 ( )12CaO 7Al O SiO 3CaO SiO Al O 4 4

3 1

2 2

J/K.mol










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

Temperature, K

Fig. 8. Relationships between *GT* and temperature in SiO2-calcium aluminates system
