Waterproof Cement Used for Roads
Puzzle 4 of Aquarium Easy Pack
Question is: Waterproof Cement Used for Roads
and the answer is: ASPHALT
Word Meaning:
ASPHALT — A sticky, black to brown and highly viscous liquid or semi-solid, composed almost entirely of bitumen with small mineral particles, that is present in most crude petroleums and in some natural deposits.
Definitions from Wiktionary.