Advanced Process Control and Simulation for Chemical Engineers

182 Advanced Process Control and Simulation for Chemical Engineers

The melamine aggregates were been saturated surface dry. Therefore, melamine

aggregates immerse in water at approximately 21

o

C for 24 hr and removing surface

moisture by warm air bopping.

FIGURE 2 The SEM photographs of materials: (a) melamine aggregates and (b) silica fume.

13.2.4 Aluminum Powder

Aluminum powder was selected as an agent to produce hydrogen gas (air entrainment)

in the cement. This type of lightweight concrete is then called aerated concrete. The

following are possible chemical reactions of aluminum with water:

2Al + 6H

2

O 2Al(OH)

3

+ 3H

2

(1)

2Al + 4H

2

O 2AlO(OH) + 3H

2

(2)

2Al + 3H

2

O Al

2

O

3

+ 3H (3)

The fi rst reaction forms the aluminum hydroxide bayerite (Al(OH)

3

) and hydro-

gen, the second reaction forms the aluminum hydroxide boehmite (AlO(OH)) and hy-

drogen, and the third reaction forms aluminum oxide and hydrogen. All these reactions

are thermodynamically favorable from room temperature past the melting point of

aluminum (660°C). All are also highly exothermic. From room temperature to 280°C,

Al(OH)

3

is the most stable product, while from 280–480°C, AlO(OH) is most stable.

Above 480°C, Al

2

O

3

is the most stable product (3) [16, 17]. The following equation

illustrates the combined effect of hydrolysis and hydration on tricalcium silicat.

CaO.SiO

2

+ water xCaO.ySiO

2

(aq.) + Ca(OH)

2

(4)

In considering the hydration of Portland cement it is demonstrate that the more basic

calcium silicates are hydrolyzed to less basic silicates with the formation of calcium

hydroxide or ‘slaked lime’ as a byproduct. It is this lime which reacts with the alumi-

num powder to form hydrogen in the making of aerated concrete from Portland cement

[18]:

2Al + 3Ca(OH)

2

+ 6H

2

O 3CaO.Al

2

O

3

.6H

2

O+ 3H

2

(5)