Abstract: A lightweight insulative, structural concrete is produced from a cement mix containing lightweight aggregates having precisely defined physical and chemical properties. These aggregates are very low in density and very high in amorphous SiO.sub.2 content. The concrete product produced using the mix of this invention has a unique combination of high structural strength (up to 3400 psi, 28-day compressive), low density (50 to 110 pounds per cubic foot) and high thermal resistance (R) values (up to 3.16/inch). Such a product is just as strong, one half the weight, and 36 times more insulative than ordinary hard rock concrete.