free bitumen

例句：

1. 1. Free bitumen is a valuable resource that can be used for various purposes.
   自由沥青是一种宝贵的资源，可用于各种用途。
2. 2. The extraction of free bitumen from the earth is a complex process.
   从地球中提取自由沥青是一个复杂的过程。
3. 3. Free bitumen has been used in construction materials to improve their durability and strength.
   自由沥青被用于建筑材料中，以增强其耐用性和强度。