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 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)

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