Types of Cement

Magnesium Phosphate Cement

Magnesium phosphate cements are formed by the reaction of magnesium oxide with a soluble phosphate, such as ammonium phosphate, either the mono or dibasic salt; or an agricultural fertilizer solution known as 10-34-0 (NPK designation) can also be used.

By virtue of their rapid set and very high early strength properties magnesia cements have found utility as rapid patching mortar for road and aircraft run-ways, which can typically be re-opened after about 45 minutes.

It has very good adhesion to a wide variety of aggregates and substrates. This cement system has good water and freeze thaw resistance. Commercial magnesium phosphate cements typically reach a compressive strength of about 2900 psi after 1 hour, with an ultimate strength of 8000 psi.

The reaction mechanism is thought to be an acid-base reaction between the MgO and the acid phosphate. This results in an initial gel formation followed by the crystallization of this gel into an insoluble phosphate, mainly magnesium ammonium phosphate hexahydrate, NH₄MgPO₄.6H₂O. The magnesium oxide used in this system is a fairly unreactive MgO, either hard or dead-burnt, and is used in conjunction with a set retarder, typically either borax or boric acid, to afford a workable set time.