Texture and Moisture Characterization of Hardened Cement Pastes and Concretes from Water Vapour Sorption Measurements

Abstract

This paper presents results obtained for several hardened cement pastes and concretes in the field of texture (notably specific surface area and pore-size distribution) and moisture properties. In particular, very high-performance materials (mixes that have a low water-cement ratio and contain silica fume and superplasticizer) were studied.

Because of the inability of classical pore investigation methods (mercury intrusion, nitrogen adsorption) to provide the complete pore network characteristics of cement-based materials, in particular for very high-performance materials, a more pertinent type of experiment was also carried out: water vapour desorption and adsorption tests. From these experiments, isotherms were determined at hygrometrical equilibrium and at constant temperature (T = 23 °C).

Not only does the analysis of these curves give textural parameters in the mesopore range, which is not accessible by most other methods, but it also allows study of the water/solid interaction. Thus, from the adsorption curve, the specific surface area accessible to water molecules was calculated for the hardened materials by the B.E.T. method. The statistical thickness of water adsorbed on the solid surface was also determined. Application of the B.J.H. method on the desorption branch provides relevant porosity values, the pore-size distribution, and, in particular, the intrinsic characteristics of C-S-H hydrates.

This makes it possible to point out the similarities and differences in microstructure and moisture properties between the various materials, and to quantify the influence of mix parameters such as water-cement ratio and silica fume content.