Aluminosilicate phosphate cements - a critical review

Phosphate cements are formed by the reaction at an environmental temperature between a phosphate solution and a solid precursor. The precursors can be divided into two categories: One contains mono- or divalent cations that are dissolved and form cation-phosphate precipitates acting as a binder. The other category is based on aluminosilicates, and in these cements, covalent Al-O-P and Si-O-P bonds are formed. There is however not yet agreement on the structure and properties of these cements in the limited literature on this last category. Compressive strengths between 40 and 146 MPa were reported, but these high values were only obtained by one group. The mentioned mechanical strengths in literature render the materials promising candidates for construction purposes. More important is that phosphate cements have a neutral pH after hardening. This allows the use of low-cost glass fibers as reinforcement. The production of Textile Reinforced Cements is thus one of the important future applications. Moreover, there is some disagreement on the molecular structure formed. Some state that Al is present in VI-fold coordination. Others state that berlinite-like structures are formed. Thus, Al is in IV-fold coordination. Crystalline berlinite is however only proven to be present after thermal treatment to at least 250 degrees C. The formation of stable crystalline phases at high temperature makes the material also suited for use as a refractory. A drawback is the scarcity of phosphate minerals to produce phosphoric acid. For this reason, the use of phosphate cement should be limited to niche applications. As phosphates can be fast setting cements, one of these applications might be in additive manufacturing.