The invention relates to a barrier cap material including a blend of: a plurality of aggregate particles, and a plurality of manufactured composite particles including a hydratable sealant material. In a particular embodiment, the segregation of a blend of aggregate particles and composite particles, when dropped through a water column can be controlled by varying blends of aggregate and composite particles. In another embodiment, the permeability of the barrier cap material, when hydrated on a surface, can be controlled from 1.times.10.sup.-3 cm/sec to 1.times.10.sup.-8 cm/sec by varying blends of aggregate and composite particles.