Vermiculite Grade C - 3 is a medium particle size material meeting ASTM C516 Type 1, Grade 3 specifications. The material is widely used as a component in growing media for professional growers. The swimming pool industry uses large quantities of this high-quality vermiculite mixed with Portland cement to make a lightweight concrete underlayment for vinyl pool liners. The unique properties of insulation and cushioning lengthens liner life and performance.