There are many factors in mineral buildup, but the primary ones are:
- Concentration of dissolved solids in the nutrient solution (fertilizer and what was already in the water)
- Capillarity of the medium
- Humidity
- Temperature
The higher the dissolved solids content of the water, the less evaporation it takes to cause precipitation to occur.
The better the capillarity (wicking action) of the medium, the more minerals are transported upward to replace the evaporated moisture.
The lower the humidity, and higher the temperature of the environment, the faster the evaporation will be.
Added thought: all of the above applies to all media, not just the inert stuff used in s/h.