A model for the microstructure of calcium silicate hydrate in cement paste

Abstract

A model is proposed for the structure of calcium silicate hydrate (C-S-H) as it is formed during the hydration of Portland cement. One purpose of the model is to move toward an ability to evaluate the microstructure quantitatively, so that it can be related to properties on the one hand and processing on the other hand. It is a hypothesis intended to promote discussion and motivate experiments. Furthermore, the model is an attempt to rationalize disparate measurements of specific surface area reported in the literature by describing an underlying structure, which, when observed by different instruments, gives different results. It is a simplified representation of the microstructure within the size range of about 1 to 100 nm. The basic building block is a unit of C-S-H that is roughly spherical and approximately 2 nm across with a specific surface area of about 1,000 m 2/g. These building blocks flocculate to form larger units. This paper describes the structure of the basic units and how they pack to form larger structures and microstructures. The model also explains a number of variant observations for such measured attributes as specific surface area, pore size, and density as determined by different techniques, as well as water content at different relative humidities.