I dare say it's a conjunction. Conjunctions join two like units, but 'His speech' and 'reported by the journal' are not structurally the same.Originally Posted by 760831
The first one is a possessive noun phrase, and the second one is a passive clause, wherein the main verb (is) has been omitted, and most importantly, where the subject of that clause has been replaced by 'as'.
Phrase: His speech
Clause: as (was) reported by the journal
Notice that 'as' functions as the subject of the clause. Moreover, that the entire clause modifies the noun 'His speech'. That is, it tells us more about 'His speech'.
Relative clause: His speech as reported by the journal
In short, 'as' is functioning as a relative pronoun: the subject of the passive clause:
EX: His speech as (was) reported by the journal is worth reading.