Civil Engineering Reference
In-Depth Information
4) Reaction of metal oxides with ammonium phosphates,
magnesium acid phosphates, aluminum acid phosphates,
and other metal phosphates.
Phosphate binders contain hydroxy-groups and undergo the following
condensation-polymerization reaction on dehydration.
O
O
O
O
||
||
||
||
Eq. (14)
HO
−
P
−
OH
+
HO
−
P
−
OH
→
HO
−
P
−
O
−
P
−
OH
+
H
O
2
|
|
|
|
O
O
O
O
|
|
|
|
The function of the phosphoric acid is to provide the phosphate linkages, to
act as a dispensing medium to promote cohesion and to serve as a wetting
liquid to increase adhesion.
There is, with acid phosphate reaction cements, a relationship with
bond strength and the rate of reaction. The rate of reaction of oxides with
phosphoric acid depends on the basicity of the oxide. The reaction between
aluminum oxide and phosphoric acid is slow enough to permit gradual
development of the phosphate bonds. The zinc oxide reaction is so rapid that
the structure develops no cohesion.
A few specific examples of phosphate cement systems are listed
below.
1)
Mixes comprising MgO, dolomite, and ammonium phos-
phate solutions set quickly at ambient temperatures and
are useful as repair materials.
2)
A hard solid is formed when phosphoric acid reacts with
a metal oxide (e.g., Al
2
O
3
) at 20-200°C. Acid phos-
phates of the type Al(H
2
PO
4
)
3
are formed initially;
subsequent heating produces the AlPO
4
bonding phase.
3)
Aluminum acid phosphate [Al(H
2
PO
4
)
3
], reacts with
MgO to form crystalline MgHPO
4
•3H
2
O as well as
AlH
3
(PO
4
)
2
gel. If P/(Al + Mg) > 1, problems may result
owing to the formation of water soluble Mg(H
2
PO
4
)
2
.
4)
A sodium hexametaphosphate (Na
6
P
6
O
18
) solution pre-
pared at 25°C is mixed with magnesite powder and used
as a mortar for bonding magnesite brick. This cement
gives high strength mortars when used with fireclay
aggregate. The cement is cured for 24 hr at 120°C.
