EFFECT OF CARBONISATION ON THE MECHANICAL PROPERTIES AND HYDRATION OF MAGNESIUM AMMONIUM PHOSPHATE CEMENT

The flexural strength and compressive strength of a magnesium ammonium phosphate cement and a magnesium ammonium phosphate cement-fly ash composite binder before and after carbonisation were compared, and the effects of carbonisation on its hydration and microstructure were studied.The results show that carbonisation can transform the amorphous phase and dittmarite into crystalline struvite, and refine the pore size of more than 1000 nm, so as to improve the strength of the magnesium ammonium phosphate cement.Fly ash inhibits the hydration of magnesium oxide in the magnesium ammonium phosphate cement, thus greatly increasing the porosity, resulting in the reduction of strength; however, carbonisation curing promotes the hydration of magnesium oxide in the magnesium ammonium phosphate cement-fly ash composite binder, and increases the content of the crystalline struvite.Carbonisation reduces the porosity by refining the pore structure, and consequently improves the flexural and compressive strength of the magnesium ammonium phosphate cement-fly ash composite binder.