Page 403 - Handbook of Thermal Analysis of Construction Materials

Section 7.0 - Magnesia Phosphate Cement Binders 381

5) Aluminum chlorophosphate hydrate decomposes on heat-
ing to form AlPO . It can be used with the addition of MgO
4
in formulations designed to set at ambient temperatures.

7.0 MAGNESIA PHOSPHATE CEMENT

BINDERS

These cements are based on the chemical reaction of solid basic
magnesia powder and an aqueous acidic ammonium phosphate solu-
tion. [63][64] The acid-base reactions that occur on mixing lead to the
formation of insoluble magnesium ammonium phosphate hydrates, the
primary binding material in the hardened product. Commercial products
generally contain inert fillers and a set retarder to facilitate control of the
exothermic setting process.
The principal reaction product in the system based on magnesia and
ammonium dihydrogen orthophosphate is magnesium ammonium phos-
phate hexahydrate (struvite):

Eq. (15) MgO + NH H PO + 5H O → MgNH PO •6H
4 2 4 2 4 4

Hall, et al., report that MgNH PO •H O (dittmorite) and Mg(NH )
4 4 2 4 2
(HPO ) •4H O (schertelite) can form when the water content is too low to
2
4 2
ensure complete reaction of NH H PO . [63] Abdelrazig [64] reports the
4 2 4
presence of schertelite as a primary intermediate reaction product:
Eq. (16) MgO + 2NH H PO + 3H O → Mg(NH ) (HPO ) •4H O
4 2 4 2 4 2 4 2 2
Eq. (17) Mg(NH ) (HPO ) •4H O + MgO + 7H O → MgNH PO •6H O
4 2
2
4
4
2
2
4 2
The formation of magnesium ammonium phosphate monohydrate,
MgNH PO •H O at low water contents was reported by Popovics, et al. [65]
4 4 2
The use of excess water in magnesia-phosphate cement systems has been
found to lead to increased porosity and strength reduction.
7.1 Mechanical Properties
Compressive strength, flexural strength, and the elastic modulus of
magnesia-phosphate cement products are affected by composition and
water content. The compressive strength, at different times, for seven
phosphate-cement concretes is plotted in Fig. 8. [66]