Abstract: The specification discloses a method of producing composite building materials from a mixture of plant fibers bonded with portland cement. Plant fibers, cement and soluble silicates in certain proportions are mixed and heated under pressure for a short period to get physically stable product that can be cured under atmospheric conditions to full strength. The plant fibers may initially be pretreated with an aqueous solution containing dichromate or permanganate ion prior to adding the cement to negate the adverse effects of set inhibiting water-soluble compounds in the fiber. Other chemicals may be added to modify the reaction and improve the product.