Modified polyethylene terephthalate polymers (MPETs), as a novel dispersing agent, were successfully prepared from polyethylene terephthalate (PET (flakes of empty bottles in order to recycle waste from one particularly abundance packaging material. Different techniques were used to characterize the MPETs samples, including attenuated total reflectance Fourier transform infrared (ATR-FTIR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), scanning electron microscope (SEM), dynamic light scattering (DLS) and Brunauer-Emmett-Teller (BET) surface area. The fluidity and mini-slump retention of fresh cement paste were tested to evaluate the dispersion capability of the MPETs. The effect of MPETs on the setting time and compressive strength of the cement paste were studied. The adsorptive behavior of the MPETs dispersions was examined using total organic carbon (TOC) and zeta potential to interpret the interaction of the MPETs with cement. The results show that the MPETs can be adsorbed on the cement particles and improve the flowability, setting time and the compressive strength of cement paste. Adding value by generating a cheap and effective dispersing agent from recycling waste polymers is a great approach toward eco-friendly waste management.