It has been demonstrated that in low w/c ratio concrete, replacement of the coarsest cement particles by inexpensive inert fillers should result in only a small (7 % or so) reduction in compressive strength development. In concrete with an initial w/c ratio between 0.25 and 0.30, replacement levels are likely limited to about 15 % to 20 %; for higher replacements levels, substantial (> 10 MPa or 10 %) losses in compressive strength were projected. The optimal replacement level will depend on the actual concrete mixture proportions being used in practice. This implies that a blended inert/cement mixture of specific proportions will only be optimal for a finite range of high-performance concrete mixture proportions.