Is this sentence grammatically correct?

The word 'destructive' is correctly used. It is an adjective meaning 'tending to destroy; causing destruction or much damage', and it modifies or describes the noun 'practice'.

You could have found this out using a useful kind of book called a 'dictionary', which all language learners should have. They are available online these days.

I thought the word 'as' in the sentence was used as a kind of preposition. And as far as I know, a noun is supposed to come after preposition. So, I assumed 'destruction' is grammatical than an adjective 'destructive' after 'as'.
I am not a native speaker and my knowledge is based on what I learned in English grammar class. So, I'd appreciate it if anybody can enlighten me about this in case I am mistaken. Thanks.

Would you be able to post that rule that you've learned, Nakjibaby?