The invention relates to a geopolymeric cement or binder comprising an amorphous vitreous matrix consisting of a poly(sialate-disiloxo)-type geopolymeric compound, having approximation formula (Na, K, Ca)(--Si--O--Al--O--Si--O--Si--O) or (Na, K, Ca)-PSDS. The inventive cement consists of a mixture of different varieties of polysialates in which the atomic ratio Si:Al varies between 2 and 5.5, the average of the Si:Al atomic ratio values as measured with the electronic microprobe being close to between 2.8 and 3. The remaining components of the geopolymeric cement or binder, such as mellilite particles, aluminosilicate particles and quartz particles, are not used in said Si:Al atomic ratio calculation. The geopolymeric structure of type (K, Ca)-Poly(sialate-disiloxo) (K, Ca)-PSDS is between 50% and 60% more mechanically resistant than that of type (K, Ca)-Poly(sialate-siloxo) (K, Ca)--PSS of the prior art.