An additive for buffering a projectile trapping medium and spent projectiles trapped therein is a buffering compound formed as a low density foamed concrete block that will self-dispense via fragmentation or pulverization when subjected to incoming fire. The block combines at least one dry component selected from the group consisting of low solubility phosphate compounds, low solubility aluminum compounds, iron compounds, sulfate compounds, and calcium carbonate 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 resulting block to be non-buoyant without sinking in the projectile trapping medium. The additive may be employed in a projectile trapping medium to chemically stabilize the medium and environmentally stabilize projectiles trapped therein.