Development of novel aluminosilicate phosphate cement for structural applications

In this study, the prospects of using acidic activation of metakaolinite (MK) as the main binder in cement production were investigated. The setting reaction of metakaolinite/phosphate cement (MKPC) was investigated by means of isothermal calorimetry (TAM Air), differential scanning calorimetry (DSC), thermogravimetry (TGA), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The reaction of metakaolinite powder with the aqueous phosphoric acid solution led to the formation of a phospho-aluminosilicate cement.
The reaction kinetics were studied by varying the Si/P molar ratio from 0.52 to 2.94 and by keeping the quantities of metakaolinite and water in the system constant. The results revealed that the optimum stoichiometric value for the reaction of metakaolinite with the phosphoric acid was close to 1.1 Si/P molar ratio. The compressive strength was found to be strongly dependent on the PO2.5 concentration. In fact, the decrease in the Si/P molar ratio led to an increase in strength up to a maximum around a Si/P ratio of 1.1. Thereafter, by further decreasing the Si/P molar ratio, a decrease in strength occurred again due to the excess of phosphates and even self-degradation of the specimens after initial hardening. The investigated MKPC mortar achieved strengths up to 70MPa in compression after 7 days, which render it a promising candidate for construction purposes.