The disclosed process for making cast vegetable/mineral structural products having flame retardant properties utilize a major volume portion of ligneus plant fragments such as soft and hardwoods, sugarcane, cereal and fiber plant stalks, and a minor volume proportion of a mineral binder deposit comprised of magnesium or calcium oxyphosphates and inert filler particles. Fragments having thicknesses ranging from 0.3 mm to 8 mm including chips, shavings, strips, strands, fibre bundles, slivers, fibres and peeled and sawn veneer sheets, have applied to their surfaces an aqueous solution of ammonium polyphosphate or soluble acid phosphate salt supplying from 0.15 to 0.40 parts of P2O5 as phosphate ion per part of fragments by weight, and particulate cement solids comprised of MgO or CaO or Mg(OH)2 or Ca(OH)2 or MgCO3 or CaCO3 ranging from 0.25 to 1.0 part per part of fragment, and from 0.01 to 0.80 parts of inert filler particles and the mixture is molded and held under predetermined compaction pressure until the product has rigidified, in about 10 minutes' time. The process is practically immune to cement poisoning sugars and polyphenolics which were found to be detrimental to other cement mixes.