A previous X-ray absorption study of tile adhesive mortars identified a new phenomenon during drying, namely the movement of small cement particles from the interior of the specimen to its surface. An analysis based on Stokes equation indicated that this particle movement is consistent with the very high viscosity of these tile adhesive mortars. In addition to this particle movement, the concurrent formation/growth of large scale flaws (voids) within the specimens was observed. The goal of the present study was to observe this void instability and growth in real time by using three-dimensional X-ray microtomography. Mitigation strategies for avoiding the formation of these flaws were also investigated.