Effect of Water Content on the Structure and Mechanical Properties of Magnesia‐Phosphate Cement Mortar

Magnesia‐phosphate cement mortars, based on magnesia and ammonium dihydrogen orthophosphate, were prepared with water contents (measured as a weight fraction of the dry material) in the range 5% to 12%. It was shown by XRD that the major reaction product was magnesium ammonium phosphate hexahydrate (MgNH 4 PO 4 6H 2 O), also known as the mineral struvite. Evidence was also found to suggest the presence of an amorphous or poorly crystalline phase with a lower degree of hydration in materials prepared with water contents less than 8%. For higher water contents, the excess water gradually evaporated during aging to leave an interconnected fine porous structure which exhibited poor mechanical properties. An increase in the water content from 5% to 12% also led to a significant increase in the crystallite size, from around 0.5 µm to 10 µm.