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The basic model for the structure of C-S-H at the nanometer level is based on a hard core-soft shell representation . The individual C-S-H particles are represented as spheres, each consisting of an impenetrable hard core surrounded by a concentric soft shell which can freely overlap both the hard cores and the soft shells of other particles. While any size distribution of particles can be matched using the model , for simplicity, we will limit our representations to monosize particles. The particles will be placed in a three-dimensional cubic box using periodic boundary conditions such that a desired interparticle porosity is achieved. For C-S-H, some overlap of these ideal spherical particles is necessary to achieve the desired low porosities (lower than dense sphere packings), and is also indicated in observing TEM images of the gel .
This continuum representation can then be digitized at various resolutions for the production of images and computation of properties as described below. For example, a 3-D system 25 nm on a side can be digitized into a 3-D array of 200 x 200 x 200 pixels where each pixel is 0.125 nm on a side. For this model, the size of the individual spheres must be chosen; here, it will be based on an existing interpretation of SANS data from the literature  as detailed in the results section.
For our model C-S-H structure, a two level (macro/micro) "self-similar" structure will be generated to represent the C-S-H particles between scales of 1 and 100 nm. The C-S-H will be considered as a system of "large" (tens of nanometers) spherical agglomerates, each composed of many smaller (nanometers in size) spherical particles. Such a self-similar multi-scale model is not uncommon for gel-like materials, as it serves as a conceptual basis for structural models of aerogel materials . Evidence for such a structure for C-S-H is provided both by SANS data and TEM images [12,13,14].