A method providing a self-dispensing additive for buffering a projectile trapping medium and passivating spent projectiles trapped therein. The additive is a buffering compound formed as blocks of low-density foamed-concrete that self-dispenses the additive when contacted by the fired projectiles. The blocks contain dry components that may include one or more of low-solubility phosphate compounds, low-solubility aluminum compounds, iron compounds, sulfate compounds, and calcium carbonate mixed with a cementing material, water, and an aqueous-based foam in substantially stoichiometric amounts. The aqueous-based foam is added in a quantity sufficient to adjust the density of the block to neutral buoyancy in the projectile-trapping medium. The additive chemically stabilizes the medium while also passivating projectiles, in particular heavy-metal projectiles, trapped in the medium.