Moisture Dependence of Pore Size and Specific Surface Area of Hardened Cement Paste Determined with Saxs and Inverse Gas Chromatography

Abstract

The main constituent of hardened cement paste is the calcium silicate hydrate (CSH). Pore size and specific surfae areas change depending on relative humidity. New applications with two different techniques support these findings. A new interpretation of small angle X-ray scattering (SAXS) data correlates almost perfectly the fractal information (deduced from the linear Porod regime) with the pore sizes determined with Mercury intrusion porosimetry (MIP). In particular the moisture dependent pore size changes from MIP [2] are now shown independently by the noninvasive SAXS at least below 60% rh. Another issue is the specific surface area, that varies depending on relative humidity as previously reported from SAXS data. This is now supported by another technique - the inverse gas chromatography (iGC). In the iGC technique pulses of organic probe molecules are injected in almost infinite dilution into a carrier gas of controlled humidity and the retention time of the probe molecules is measured. From the retention times various thermodynamic properties can be determined, like the dispersive part of the surface energy [3]. However, it is also possible to gain the adsorption isotherms although limited to very low relative pressures due to the almost infinite dilution. Applying the ESW (Excess Surface Work Sorption Model) [4], allows to recover the isotherm so far that it is possible to determine the monolayer (with ESW or BET) and accordingly the specific surface area.