A recent study by Marcela Cosarinsky and Flavio Roces examines turret-building by the chaco leafcutter ant:
…workers do not simply pile clay over sands or sands over clay after replacement of the available materials, and evince some complexity in construction behavior. The micromorphological analysis of the final wall demonstrated that the imported materials were distributed and combined tending toward a kind of microstructural balance that may be related to the maintenance of a porous but mechanically-stable structure.
source: Cosarinsky, M. I., & Roces, F. 2011. The Construction of Turrets for Nest Ventilation in the Grass-Cutting Ant Atta vollenweideri: Import and Assembly of Building Materials. Journal of Insect Behaviour. Online First DOI: 10.1007/s10905-011-9290-8.