In building materials, such as mortars, cellulose ethers (CE) are used as water-retention agents. They prevent the loss of water due to absorption into the substrate during curing stage. They maintain wet conditions for proper hardening and final properties. However, the cause of retention remains unknown. First, we show through filtration tests and usual characterization methods (microscopy, light scattering) that CE solutions are composed of polydisperse aggregates of several tens of microns in diameter. We then show that a CE solution passing through a model porous medium (sieve) progressively block the sieve, even if the mesh size is much larger than the aggregates diameter, due to a statistical effect of blocking. This jamming effect also occurs during the flow through a 3D porous medium (stacks of glass beads), which led to the stopping of the fluid after a certain penetration depth of the medium. Finally, coming back to the retention standard tests with the real material (mortar) and comparing them to the filtration test we show that the latter is a good alternative test to characterize the CE retention capacity
