A flame retardant product that contains an aqueous solution that includes a hydroxylated carbon-based component, an intumescent agent suitable for at least partially or even completely dehydrating the hydroxylated carbon-based component in such a way as to generate, under the effect of heat, a carbonaceous foam, and a thixotropic agent, selected to have a viscosity that increases as a result of water being added in the absence of shear. The solution has a sufficient concentration of the hydroxylated carbon-based component and the intumescent agent so that an addition of water to said solution increases the viscosity thereof in the absence of shear. The content by weight of the hydroxylated carbon-based component is greater than 10%.