After many years of discussion, in 2004, the ASTM C150 standard specification for portland cement was modified to allow the incorporation of up to a 5 % mass fraction of limestone in ordinary portland cements [1]. An extensive survey of the literature conducted by the Portland Cement Association [2] concluded that "in general, the use of up to 5 % limestone does not affect the performance of portland cement." Even higher contents of ground limestone could potentially be utilized in lower water-to-cement ratio (< 0.45) systems, where a substantial fraction of the cement clinker particles remains unhydrated, effectively acting as a rather expensive filler material [3-5].
Because concretes made with limestone-containing cements are often prepared at a water-to-solids ratio (w/s) similar to the water-to-cement ratio (w/c) of the concrete with no limestone, the effective w/c of the limestone-filled concrete can be substantially increased from that of the original mixture. This will naturally modify the hydration characteristics of the concrete. Further, the additional surface area provided by the limestone particles may provide sites for the nucleation and growth of hydration products, generally enhancing the achieved hydration. Finally, the ground limestone is slightly reactive with the portland cement, mainly forming a monocarboaluminate phase [6-9]. Being able to predict the influence of a specific limestone substitution on the hydration behavior of a specific cement paste (concrete) should expedite the usage of these filled cements and allow for a priori design of concrete mixtures that meet desired performance criteria. In this paper, the CEMHYD3D hydration model developed by NIST will be extended to consider the above influences of limestone fillers on cement hydration and validated against experimental measurements.