A datum point for estimating the adsorbed water in hydrated portland cement

In order that it may be possible to differentiate between adsorbed water and water chemically bound in hydrated Portland cement, a study was conducted of the dimensional‐change sorption characteristics at low relative vapour pressures. By use of compacts of bottle‐hydrated cement fabricated at loads between 5000 and 65, 000 lb., equilibrium as well as dynamic values were obtained for dimensional change and moisture change relationships below 20% R.H. The apparatus enabled experiments to be performed in high vacuum. For the first rewetting and second drying it was found that for equilibrium points from 7.2 to 17‐6 and 17‐7 to 7‐9% R.H. the plots of ( Δ l/l ) vs (Δ W/W ) were both linear. After degassing the specimens beyond 7.9% R.H., the dynamic points continued along the straight line traced out by the equilibrium points in the plot mentioned and a sharp transition point was obtained. The vapour pressure in the region of this transition point, referred to as datum point, was less than 10 μ. Further degassing produced another linear region. It was concluded with supporting evidence from work on other porous materials that the datum point where the transition between the linear portions of the curve occurs, will yield a good estimate of the true value for adsorbed water and will thus approximate to the point for zero adsorbed water; it will also separate the processes of desorption and dehydration. This establishes therefore a method for determining the adsorbed water in hydrated cement. The results obtained from the compacts of bottle‐hydrated cement indicate that by this method the surface area, as calculated from the B.E.T. theory, will be considerably lower than that obtained from the high value for the evaporable water (determined by the method of Copeland & Hayes).