STRENGTH DEVELOPMENT IN PHOSPHATE-BONDED CALCIUM ALUMINATE CEMENTS

The chemical reaction at room temperature between calcium aluminate cement (CAC) and NH4H2PO4-based fertilizer solution causes the formation of ammonium calcium pyrophosphate (AmCPP) as an amorphous cementitious phase. This phase was responsible for the development of strength in rapid-setting phosphate-bonded cement (PBC) specimens. Hydrothermal treatment of PBC at 200-degrees-C led to phase transformations of AmCPP and CAC into crystalline hydroxyapatite as the major phase and anorthite as the minor one. The moderate growth of these interlocked crystals in the interspaces of amorphous phases played an important role in creating a dense microstructure, thereby conferring a very high strength to the PBC. In contrast, an excessive crystal growth, which caused the formation of porous microstructure, resulted in a reduction in strength.