Incorporating Ultrafine Particles in Microstructural Models of Cement Hydration
Jeffrey W. Bullard
Models of cement paste microstructure that discretize particles on a regular mesh generally must use very fine meshes to resolve the smallest particles. Particularly if the particle size distribution is broad, this leads to undesirably large numbers of mesh points and a corresponding increase in computation time. In this paper, an alternative method is presented that accounts for the reactivity of fine particles by ``coarse graining'' them in the microstructure. All particles at or below the resolution limit x0 are represented as compact clusters, each cluster of size x0. A mean dissolution bias, zeta, is applied to clusters to simulate their enhanced dissolution rate. zeta can be calculated directly from knowledge of the particle size distribution. This approach is applied to the NIST hydration model, CEMHYD3D, for two Type I ordinary portland cements. Predictions of degree of hydration versus time are compared to experimental measurements. For typical portland cements, the values of zeta appropriate for CEMHYD3D are approximately unity. It is shown that this is due to that model s fine resolution limit and the way that it partitions the particle size distribution. Nevertheless, the concept of a dissolution bias for coarse-grained clusters should have general utility for other types of microstructure development models using finite difference or other continuum-based approaches.