This paper describes an innovative use of plastic bottle waste as cement-substitution within composite materials for preventing chemical attacks or repairing various reinforced concrete structures. Various weight fractions of cement varying from 2.5% to 7.5% were substituted by the same weight of polyethylene terephthalate (PET). The specimens were tested in flexure and compressive strength and for chemical resistance to acid, basic solutions at 5% and for chloride ion permeability. From this study, it was found that the PET-modified mortars exposed to aggressive environments showed better resistance to chemical attack and higher resistance to chloride ion penetration than unmodified one without substantially affecting the mechanical strength in tap water. The addition of PET to the modified mortars, means reducing the penetration of aggressive agents. So, the mass loss of composites exposed to hydrochloric and acetic acid solutions is lower than those of unmodified mortar. The basic solutions are harmless for composite materials. The formations which appear such as different calcium salts were determined by X-ray diffraction and FT-IR.