Mazut is a heavy, low quality fuel oil, used in generating plants and similar applications. When blended the end product is Diesel. (source:www.wikipedia.org)
Sounds as though Mazut might be very similar to what we in the US have long named as "Bunker C" fuel oil. Thick, soupy stuff, which in cold weather required steam lines inside tenders, to keep it liquified. Boxcab E50