Abstract: The synthetic aggregate for use in concrete is a composite material of recycled plastic having a filler encapsulated in the plastic. The synthetic aggregate includes between 30% and 50% recycled shredded plastic, the balance being filler. The plastic may be linear low-density polyethylene (LLDPE). The filler can include red sand, fly ash and quarry fines. The synthetic aggregate is best used to make concrete with a water-to-cement ratio of at least 0.5. A method of making the synthetic aggregate includes the steps of mixing plastic with filler to form a homogenized mixture, compressing the homogenized mixture in a mold, melting the plastic in the homogenized mixture to form a composite sheet or slab, and shredding the composite sheet or slab to form either coarse or fine aggregates for use in making concrete.