Sorption of water on compacts of bottle‐hydrated cement. II. Thermodynamic considerations and theory of volume change

Abstract

In order that the hydrated‐cement—water sorption may be adequately analysed, two thermodynamic processes connected with the material, encountered in adsorption, are discussed.The B.E.T. surface area computed by the use of a modified form of the Gibbs adsorption equation, is found to be in close agreement with the value obtained by an experimental method of determining zero adsorbed water. The surface area calculated from the pore‐size distribution is also in agreement with these values. It is found that over the range of vapour pressures from 0 to 25% the Gibbs adsorption and Bangham equations are obeyed.Beyond 25% R.H. physical adsorption and the readmittance of the interlayer hydrate water of tricalcium silicate occur simultaneously. The secondary hysteresis observed in the sorption isotherm and other anomalies previously observed in the system are explained with these data.Mechanisms of volume changes for the whole extent of the isotherm are presented; thermodynamic equations are applied to different regions of the isotherm to express the volume change and spreading force developed in the system during sorption. These are compared with those used in other volume‐change mechanisms described for this system and the thermodynamic consistency of these is discussed. Calculations of Young's modulus from the above equations lead to the conclusion that the values obtained are for the individual elements of the body and that as such they will yield values larger than those obtained for the body as a whole.