The significance of pore liquid pressure and disjoining pressure on the desiccation shrinkage of cementitious materials

There are several conflicting opinions in the literature concerning the significance and role of bulk pore liquid pressure and disjoining pressure on the desiccation shrinkage of cementitious materials. While the applicability of the Kelvin–Laplace equation in fine pores (due to the unstable nanosized meniscus) has been questioned by some, disjoining pressure has been advocated to be the primary mechanism driving desiccation shrinkage in cementitious materials. In order to elucidate the proper contribution and understanding of these two mechanisms, a thermodynamics-based mechanistic approach has been utilized here to derive expressions for both the bulk liquid pressure and the disjoining pressure. The validity of the poromechanical approach to modeling shrinkage of a porous body has also been examined. It has been concluded that the determination of pore liquid pressure via the Kelvin–Laplace equation does not require the presence of a stable meniscus and is applicable to nanosized pores. As such, the pore liquid pressure is found to be the primary mechanism associated with the desiccation shrinkage of cementitious materials. While disjoining pressure does not induce any change in the bulk liquid stress during drying, it plays a significant role in counterbalancing the liquid pressure in the thin film separating the vapor phase from the pore wall, which justifies poromechanical shrinkage models that consider pressurization to occur only in the portion of the pores containing bulk pore liquid.