A new model for a better understanding of the cohesion of hardened hydraulic materials

Abstract

A model is presented in order to explain most of the macroscopic features of set plaster in a wet and in a dry state. Set plaster is made of entangled gypsum needles, and our model implies that there are thin liquid layers of water molecules between the gypsum needles in the zones where they are in contact. The effect of interparticle forces like van der Waals forces and double layer interactions is important only for very small objects in the colloidal domain. But, in the case of set plaster, because of a very good parallelism of the crystalline facing surfaces, such forces exert themselves on unusually large areas and are, for a large part, responsible for the cohesion and mechanical strengths of set plaster. During the drying of set plaster, capillary effects have also to be taken into account and they become important only when the quantity of moisture gets smaller than 2% of the weight of the material; from this moment the remaining liquid water is in the form of menisci with small curvatures, in the vicinity of the different needles contact zones. In the absence of external forces applied on the sample, the thickness of each water film corresponds to the distance for which the repulsive double layers interactions equilibrate the sum of attractive van der Waals and capillary forces. The thickness of the water molecules layers separating the gypsum facing surfaces is expected to vary between about 125 Å for water saturated set plaster and about 15 Å for dried set plaster. In this last case, the water molecules are expected to be very regularly packed like in a solid crystal, which results in very strong adhesive forces between the gypsum needles. It is thought that such a model taking into account interparticle and capillary forces can be used to explain the cohesion of many other minerals obtained by a crystallisation process. Particularly, the case of hardened cement-based materials is considered and the physical properties and the structure of these materials are described. A large part of the hydrated cement pastes is constituted of CSH gels which are very small particles of calcium silicate hydrates associated with variable quantities of water. In order to explain the cohesion of cement pastes with our model, a new description of the internal structure of the CSH gels is proposed: the CSH flat particles, which repel one another at short distances, have no solid contact: they are arranged in parallel layers, with thin films of liquid separating them, thus forming large dimensions foils. Such foils partially or completely fill the space between the other hydration products, for respectively dilute or dense cement pastes. During the drying, the liquid films between the CSH particles become thinner and thinner, and that results in continuous changes of the internal porosity of the cement hydrated paste. Finally, a few experimental results which corroborate the presented model in the case of cement pastes are presented.