Mine backfill has become a common practice in many modern mining operations around the world. Pastefill is an innovative tailings management method that returns much of this waste material to the underground mine. At the same time, it can be used as a structural element participating in the ground support. This allows the mine operator to extract a greater amount of the underground ore. In addition to the mill tailings, pastefill contains enough water to obtain the desired consistency and various quantities of binders that give this composite the needed cohesion. The purpose of this paper is to bring some new evidence to better understand of the hardening processes that occur in this complex material. It has been demonstrated that the strength development of pastefill depends on the physical, chemical and mineralogical properties of the tailings, the water chemistry and the amount of water and finally, the binder type and proportion. The hardening is caused not only by direct hydration of binder reagents but also by precipitation of hydrated phases from the pore water of paste backfill. The sulphates have a significant influence on the mechanical performance of the pastefill in different ways depending on their concentration in the pore water: inhibition or participation of the hardening phenomenon or, after the material has hardened; sulphates can also cause strength losses due to the precipitation of expansive species. (C) 2003 Elsevier Ltd. All rights reserved.