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All simulations were conducted using version 2.0 of the NIST CEMHYD3D program [4,5] and the new prototype Virtual Cement and Concrete Testing Laboratory web-based interface [6]. Three-dimensional starting microstructures were created based on the measured particle size distribution (PSD) and phase composition of Cement and Concrete Reference Laboratory proficiency cement sample Number 135, issued in January of 2000 [7]. Six different microstructures were examined, three with a water-to-solids (w/s) mass ratio of 0.25 and three others with a w/s of 0.30. For the w/s=0.30 systems, one system had zero replacement and the other two had the coarsest 14.5 % and 22.3 % mass fraction of the cement particles replaced by inert fillers, respectively. For the w/s=0.25, in addition to the base system with no replacement, replacement levels of 20.5 % and 30.8 % by mass were investigated. The complete measured PSD for Cement 135 is provided in Fig. 1. Typically, we are replacing all the particles larger than 20 µm to 27 µm in diameter by inert (nonreactive) particles. All systems were then hydrated for 4000 dissolution/reaction cycles of the hydration model (representing about 200 d of real hydration time [7]). Comparisons were then made on the basis of the development of degree of hydration with time and compressive strength development predicted using Power's gel-space ratio concept [1,4] with a strength prefactor (the strength achieved for a gel-space ratio of 1.0) of 100 MPa for ASTM C109 mortar cubes [7]. By using a constant strength prefactor, we are implicitly assuming that the contribution of the inert filler to the compressive strength of the composite is the same as that of the unreacted cement particle cores. We will then be evaluating the ability of the smaller cement particles to hydrate more completely to compensate for the original hydration provided by the now missing coarser cement particles. Due to local space limitations within the microstructure (real and computer modelled), the smaller cement particles may not be able to totally compensate for the lack of the coarser ones.