A direct 3D printing ink formulation, characterized in that it includes: - a geopolymeric formulation in the fluid state including as ingredients: at least one inorganic compound selected from a first group consisting of a compound containing aluminum associated with at least one compound containing silicon, a silicon-aluminate compound, a compound containing polysilicates, polysiloxane, polysilicon-aluminates or polysialates; at least one alkaline activator and water; or at least one inorganic compound selected from a second group consisting of an inorganic oxide, preferably of a divalent or trivalent metal; and an acidic phosphate; said ingredients being present in such proportions so as to undergo a geopolymerization reaction and produce at the end of the reaction a solid geopolymer; wherein the formulation, before and during at least a part of the geopolymerization reaction, when three-dimensional chemical bonds are not yet formed, forms a non-newtonian, viscoelastic, gel reversible fluid that, when in the fluid state, has a starting flow-threshold limit or yield stress that is greater than or equal to 20 Pa and that, when in the fluid state and at a low shear rate, namely less than or equal to 0.1 1/s, has a viscosity of at least an order of magnitude higher than the viscosity that the same formulation in the fluid state has at a high shear rate, namely equal to or higher than 100 1/ s, measured according to the method mentioned in the description; said formulation comprises metakaolin, a substance that is a source of polysilicates, preferably fly ash, a soluble alkaline polysilicate and an alkaline solution, the formulation having an atomic ratio Si:Al of between 1:1 and 35:1, including the extremes of this range; a (Na2O, K2O)/SiO2 molar ratio of between 0.20 and 0.28; a SiO2/Al2O3 molar ratio of between 3.5 and 4.5; a H2O/(Na2O, K2O) molar ratio of between 10 and 25; and a (Na2O, K2O)/Al2O3 molar ratio of between 0.80 and 1.20; also including within said ranges the extremes of each range; and characterized in that it further includes - at least one geopolymerization reaction retarding agent, preferably selected from a group consisting in: lignosulfonates, sugars; even more preferably consisting of sodium lignosulfonate, saccarose or glucose; the retarding agent being present within the formulation in an amount comprised between 0.1 and 5% by weight on the total weight. - a geopolymeric formulation in the fluid state including as ingredients: at least one inorganic compound selected from a first group consisting of a compound containing aluminum associated with at least one compound containing silicon, a silicon-aluminate compound, a compound containing polysilicates, polysiloxane, polysilicon-aluminates or polysialates; at least one alkaline activator and water; or at least one inorganic compound selected from a second group consisting of an inorganic oxide, preferably of a divalent or trivalent metal; and an acidic phosphate; said ingredients being present in such proportions so as to undergo a geopolymerization reaction and produce at the end of the reaction a solid geopolymer; wherein the formulation, before and during at least a part of the geopolymerization reaction, when three-dimensional chemical bonds are not yet formed, forms a non-newtonian, viscoelastic, gel reversible fluid that, when in the fluid state, has a starting flow-threshold limit or yield stress that is greater than or equal to 20 Pa and that, when in the fluid state and at a low shear rate, namely less than or equal to 0.1 1/s, has a viscosity of at least an order of magnitude higher than the viscosity that the same formulation in the fluid state has at a high shear rate, namely equal to or higher than 100 1/ s, measured according to the method mentioned in the description; said formulation comprises metakaolin, a substance that is a source of polysilicates, preferably fly ash, a soluble alkaline polysilicate and an alkaline solution, the formulation having an atomic ratio Si:Al of between 1:1 and 35:1, including the extremes of this range; a (Na2O, K2O)/SiO2 molar ratio of between 0.20 and 0.28; a SiO2/Al2O3 molar ratio of between 3.5 and 4.5; a H2O/(Na2O, K2O) molar ratio of between 10 and 25; and a (Na2O, K2O)/Al2O3 molar ratio of between 0.80 and 1.20; also including within said ranges the extremes of each range; and characterized in that it further includes - at least one geopolymerization reaction retarding agent, preferably selected from a group consisting in: lignosulfonates, sugars; even more preferably consisting of sodium lignosulfonate, saccarose or glucose; the retarding agent being present within the formulation in an amount comprised between 0.1 and 5% by weight on the total weight.