Abstract: Geopolymeric cement based on aluminosilicate fly ashes of class F, which, contrary to the prior art, are harmless to use and harden at ambient temperature, favouring their use in common applications in the construction and civil engineering fields. This harmlessness is achieved thanks to a mixture containing: 10 to 15 parts by weight of a non corrosive alkali metal silicate solution in which the M2O:SiO2 molar ratio is less than 0.78, preferably less than 0.69 and the SiO2:M2O ratio greater than 1.28, preferably greater than 1.45, M denoting Na or K; added to this are 10 to 20 parts by weight of water and 5 to 15 parts by weight of blast furnace slag having a specific surface area less than 400 m2/kg preferably less than 380 m2/kg and 50 to 100 parts by weight of class F aluminosilicate fly ash.