Shrinkage mechanisms of hardened cement paste

Abstract

Due to its large specific surface, hardened cement paste can be looked upon as a colloidal system. This means that shrinkage due to a change in relative humidity must be related to the interaction between surfaces of colloidal particles and adsorbed water films. Following the Munich model shrinkage is due to a change in surface free energy for relative humidities lower than 40 % (Bangham shrinkage), and due to the disjoining pressure for higher relative humidities. The first hypothesis of this model was checked by a measure of shrinkage on thin samples of hardened cement paste. In order to study the effect of the disjoining pressure in a colloidal system an experimental set up has been designed. A series of tests was carried out to measure the interaction force between two surfaces in the presence of an adsorbed water layer and an external pressure. The forces of interaction of such surfaces separated by a water film can be divided into attractive and repulsive components. In a coherent xerogel the disjoining pressure cannot separate the surfaces freely and indefinitely. The hygral length change, which is due to the disjoining pressure, can be obtained by subtracting the Bangham shrinkage from the total shrinkage. The shrinkage in a xerogel due to the disjoining pressure develops steadily from 100 % to 40 % RH due to a gradual restraining action of the skeleton of the xerogel on the interaction forces.