If the player chooses to side with Amita after the City of Pain mission, leading to the mission Advanced Chemistry, they will be tasked with securing the brick factory. If they choose to side with Sabal, leading to the mission Basic Chemistry, they will be tasked with destroying the factory.
It is not solely used just as a brick factory, as Amita and Sabal reveal that the opium poppies from the Kyra Tea Factory are sent there for processing into Heroin. If Ajay sides with Amita, he will hallucinate once inside the factory. This may be drug-induced (likely a hallucinogen), caused by a drug being produced or processed in the factory, however the dialogue in-game never specifies what exactly is being produced (while it is known that it produces at least Heroin, it is not a hallucinogen, so there must be something else being made there).