  We present findings from uniaxial (oedometric) compression tests on two
cohesive industrially relevant granular materials (cocoa and limestone powder).
Experimental results are presented for the compressibility, tested with two
devices -- the FT4 Powder Rheometer and the custom made lambdameter. We focus
on the stress response and the slow relaxation behavior of the cohesive samples
tested. After compression ends, at constant volume, the ongoing stress
relaxation is found to follow a power law consistently for both cohesive
powders and for the different testing equipments. A simple (incremental
algebraic evolution) model is proposed for the stress relaxation in cohesive
powders, which includes a response timescale along with a second, dimensionless
relaxation parameter. The reported observations are useful for both the
improvement of discrete element simulations and constitutive macroscopic models
for cohesive granular materials.
