The potential of using a synthetic lightweight aggregate (SLA) in concrete is examined. The aggregates are manufactured through thermal processing using plastic to encapsulate and bind fly ash particles. Nominal maximum-size aggregates of 9.5 mm were produced with fly ash contents of 0%, 35%, and 80% by total mass of the aggregate. The plastic used was high-density polyethylene, and the fly ash had a carbon content of 12%. An expanded clay lightweight aggregate and a normal-weight aggregate were used as comparison. Gradation, specific gravity, and absorption capacity are reported for the aggregates. Five batches of concrete were made with the different coarse aggregate types. Volume fractions of cement, water, fine aggregates, and coarse aggregates were kept the same for comparative purposes. Mechanical properties of the concrete determined included density, compressive strength, elastic modulus, splitting tensile strength, fracture toughness, and fracture energy. Salt-scaling resistance, a concrete durability property, was also examined. Compressive and tensile strengths were lower for the synthetic aggregates than for the normal-weight and expanded clay lightweight aggregates; however, comparable fracture properties were obtained. Relatively low compressive modulus of elasticity was found for concretes with the SLAs, although high ductility was also obtained. As fly ash content of the SLAs increased, all properties of the concrete were improved. Excellent salt-scaling resistance was obtained with the SLA containing 80% fly ash.