The main struggle that Montag goes through in "The Sieve and the Sand" is trying to understand what he reads.
Explanation:
In the second part of the novel "Fahrenheit 451" Montag is seen pursuing the reason which led his society to destructive behaviors. He knew the answers can only be found in the books, which are being intensively destroyed from the society.
He begin to memorize Bible verses. The struggle that Montag had in "The Sieve and the Sand" was that he was not able to understand what was written in the books. He asks Faber to help him understand the meaning. Montag was looking for the meaning that those books contain. He struggles to pursue his desire of intellect in the society which bans literature, or access to any books written by dead people. He struggles to get out of the society to seize from making the old mistakes.