That's a common misconception due to somewhat of a mistranslation. None of the logias are simple solids (sand is granular matter and has fluid properties in bulk; smoke is a colloid of solid suspended in gas, also having fluid properties). The more accurate translation would be to say that Kuzan is a "Cold" person, or a "Freezing" person. He doesn't turn into "ice" but rather "cold" which deposes the gases around him and gives him solid form temporarily (thus explaining his badass "steam" effect, which is just sublimation of those gases).
As for actual theories:
This idea of non-solid logias also serves as evidence against any predictions of Wood, Metal, Stone, Earth, etc. logias.
So far we have: granular matter (sand), colloid of solid in gas (smoke), gas undergoing combustion (fire), electric charge (electricity), mass (gravity), photons (light), colloid of solid in liquid (mud), cold (freezing), and melted solid (magma)
My prediction is that the only other logias will be: gas, liquid, foam (colloid of gas suspended in liquid), and gel (colloid of liquid in solid).