We don't know for sure.
The most popular theory going around is that his flames are merely a hotter version of Drogons' and Rhaegal's.
Motherboard has a compelling theory that Viserion is breathing a compound called dicyanoacetylene, which would burn hotter (and bluer) than the flames he spewed as a living dragon. The hotter flames might also cause the Wall to vaporize, which is what happened — regular flames would have melted it, resulting in large quantities of meltwater. If all that's true, it means Viserion is now more dangerous than Drogon and Rhaegal, who are presumably breathing regular old fire.
Viserion's Newly Blue Flames, Explained - Cosmopolitan
It's doubtful that his fire is any kind of the fire of a Ice Dragon.
"Of all the queer and fabulous denizens of the Shivering Sea, however, the greatest are the ice dragons. These colossal beasts, many times larger than the dragons of Valyria, are said to be made of living ice, with eyes of pale blue crystal and vast translucent wings through which the moon and stars can be glimpsed as they wheel across the sky. Whereas common dragons (if any dragon can truly be said to be common) breathe flame, ice dragons supposedly breathe cold, a chill so terrible that it can freeze a man solid in half a heartbeat."
A World of Ice and Fire
Viserion doesn't quite meet any of these qualifications other than blue eyes, which leads us to believe that he is still a dragon, just dead.
Game of Thrones has never been limited by what is and isn't possible in the real world. Dragons, zombies, and shadow assassin babies all populate the world of Westeros, meaning that Viserion could breath whatever George R. R. Martin wanted Viserion to breath. However, it certainly didn't look like Viserion was breathing any kind of "ice beam" or anything type of cold breath that would imply that Viserion is an ice dragon. Viserion is still a fire-breathing Dragon, although a dead one. However, that doesn't mean his blue fire shouldn't be feared. If anything, his blue breath proves that Viserion may now be more powerful than Drogon or Rhaegal.
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So, the most probable explanation is that his fire is much hotter than Drogon and Rhaegal's.