Abstract: A glass fiber reinforced cementitious product and treated glass fibers for use in the reinforcement of cementitious materials wherein the reinforcement is in the form of bundles of glass fibers, the bundles including an alkali impervious impregnant therein to protect the glass fibers from alkaline materials liberated during age hardening of cementitious products. The use of impregnated bundles of glass fibers prevents or substantially minimizes loss in strength with aging of glass fiber-reinforced cementitious products as is characteristic of prior art products.