Civil Engineering Reference
In-Depth Information
6.0
MAGNESIA-PHOSPHATE CEMENTS
There are several commercial rapid-setting cements used for the
repair of airport runways and other concrete infrastructure where rapid
setting and strength gain are mandated. These include magnesia-phosphate
cements.
Several basic oxides will react with phosphoric acid or acid
phosphates at ordinary temperature forming cohesive masses, setting and
giving high compressive strengths. Magnesia-phosphate cements are quick
setting cements based on magnesium ammonium phosphate. These ce-
ments consist primarily of magnesia, ammonium phosphate, and sodium
tripolyphosphate. Magnesium ammonium phosphate hexahydrate and
magnesium phosphate tetrahydrate have been identified as hydration prod-
ucts.
[24]
The initial hydration product, Mg(NH
4
)
2
-H
2
(PO)
4
•4H
2
O
(tetrahydrate), converts to MgNH
4
-PO
4
•6H
2
O (hexahydrate). In the pres-
ence of colloidal silica, more tetrahydrate forms. There is some uncertainty
about the reaction sequence, but it appears the hexahydrate formation is
responsible for strength.
[25]
Abdelrazig and co-workers studied the hydration (at 22°C) of MgO
(75 g) and monoammonium phosphate (56 g) a composition ratio not
dissimilar to commercial phosphate cements.
[26]
Mortars (Systems 1 and 2)
and pastes (System 4) were investigated. The mortars (containing quartz
sand) were prepared at water/solid ratios of 0.62 and 0.125; the pastes had
water/solid ratios of 0.125. Thermograms (DTA) of mortar and paste are
shown in Fig. 28a. The hydration time is one week.
A small endotherm at 51°C is due to the dehydration of
NaNH
4
HPO
4
•4H
2
O. The dehydration of the hexahydrate into the monohy-
drate (MgNH
4
PO
4
•H
2
O) is shown by the well-developed endotherm at
114°C. The small endotherms at 198°C and 232°C are attributed to the
unreacted monoammonium phosphate and the dehydration of the monohy-
drate respectively. The sharp endotherm at 575°C is due to the
β
polymorphic transformation of quartz. The exotherms at 614°C and 812°C
are due to the crystallization of Mg
2
P
2
O
7
and the formation of Mg
3
(PO
4
)
2
respectively.
The compressive strength of mortar continues to develop for
several weeks. If colloidal particles of hydrate form around nuclei of
hexahydrate, it may account for continuous strength development.
α
-
