Comparisons - grammar
When you are comparing two things, be careful to match the preposition you use for the second item carefully with the term you use at the beginning.
If we compare with as, the matching preposition should also be as: "as pretty as a picture"; "as deaf as a post"; "as blind as a bat"; "as fast as the wind". As also follows the more informal structure where we use the first adjective without as: "his face was red as beetroot"; "hot as hell"; "Happy as Larry, he was."
So with an adjective or adverb is often not followed by any joining word. "He's so sharp [~ intelligent] he'll cut himself". If a joining word is necessary, it is the conjunction that.
Errors of this sort (using the wrong preposition in a comparison) are quite common for non-native writers. However, they are not unknown among native speakers - especially when they write longer, more complex comparisons in academic English. Be careful to think about what exactly you mean.