A method and apparatus for expanding particles of a lightweight aggregate material are disclosed. The apparatus comprises a housing having an upper chamber separated from a lower chamber by a partition having a plurality of openings. Hot air is introduced into the lower chamber and flows through the partition to fluidize particles introduced into the upper chamber. Means are provided for independently regulating the velocity of air through separate sections of the partition. Particles are introduced at the inlet end of the upper chamber. The particles are fluidized and transported toward the outlet end of the upper chamber by the air flow. The temperature of the air flow causes the particles to expand and become lighter. The velocity of the air flow through separate sections of the partition is adjusted to maintain uniform fluidization of the particle bed in the upper chamber.