The Surface Area of Hardened Cement Paste as Measured by Various Techniques

Hydrated cement paste has a high specific surface area due almost entirely to the calcium-silicate-hydrate reaction product. The surface area of cement paste is closely related to many crucial properties, including strength and permeability, and is therefore a useful predictive measurement. It is also a useful parameter for studying the nature of the calcium-silicate-hydrate gel itself. Unfortunately, the surface area of cement paste is difficult to measure accurately, and different techniques have given widely varying values. This review discusses these different techniques, summarizes the surface area results given by each, and attempts to rationalize the differences in these results by considering the different physical principles by which each technique generates a surface area value. The surface area of a porous material, as given by the total internal boundary between the solid phase and the pore system, is one of the most useful microstructural parameters for defining its properties. For example, given a known total porosity, the surface area gives a measure of the fine pores present, and indicates the potential for reactions between solid phases and intruded active species. In the case of disordered porous microstructures such as occur in cement paste and concrete, measurement of the total surface area can give an indication not only of the fineness of the microstructure, but also of the tortuosity of the pore phase, and hence of the permeability. Hardened cement paste, which is the ubiquitous product formed upon mixing cement and water, is the main ingredient of concrete - the world's most widely-used manufactured material. Cement paste possesses a reactive porous microstructure known to contain a high internal surface area, and a reliable measure of the surface area, as a function of the hydration time and conditions, should provide important diagnostic information on the evolution of the properties as hydration proceeds. However, the microstructure of cement paste is quite complex, containing several reaction products as well as unreacted clinker grains and porosity, and measuring a definitive value of surface area for cement paste is beset with several difficulties. One major challenge to the measurement of surface area in hydrating cement paste arises from the heterogeneous microstructure involved, and the very wide length-scale range applicable (from a few nanometers to tens of micrometers). Mindess and Young [1] have classified cement paste pores by size, and Table I lists the different size ranges and the properties influenced by each. It should be emphasized that there is considerable overlap in both the size range and role of the different pore types [2]. While this pore scheme has proved useful in cement paste and concrete microstructure characterization, much recent evidence suggests that the fine pore space in and around the C-S-H should not be considered simply as 'inert' voids (of various shapes) within a monolithic solid phase.