Expansion and shrinkage of early age cementitious materials under saturated conditions : the role of colloidal eigenstresses

Abstract

Mixing water with anhydrous cement powder and other additives results in a viscid cement slurry and triggers a set of complex exothermic reactions. As the cement slurry transitions from a suspension to a gel and ultimately to a stone-like porous solid, the material develops mechanical properties. This transition, however, is also accompanied by bulk volume changes, which -if restrained- lead to premature cracking of materials and structures. The main objective of this study is to relate bulk volume changes as measured at a macroscopic scale to their finer colloidal origin under controlled temperature and pressure conditions. To achieve this goal, an original set of macroscopic scale experiments is designed and a multiscale microporomechanics model is employed to rationalize the experimental results. While bulk volume changes have been classically attributed to capillary pressure, surface tension, and disjoining pressure that all relate to changes in relative humidity, we herein argue that they are a consequence of eigenstresses that develop in the solid phase of the hydrating matter due to attractive and repulsive colloidal forces at mesoscale. To prove our hypothesis, we experimentally investigate volume changes under saturated and drained conditions that eliminate any volume changes associated with humidity and (effective) pressure changes. Under these conditions, we observe first a volume expansion followed at later stages of hydration by volume shrinkage that cannot be explained by classical theories. By analyzing both expansion and shrinkage within the framework of incremental micro-poro-mechanics, we suggest that the expansion is caused by the relative volume change between the reactive solids and hydration products in the hydration reaction. After the solid percolates, this volume change is restrained by the percolated solid phase. This induces a compressive eigenstress in the solid phase that entails a swelling of the material under overall stress-free conditions. In return, as the material further densifies, the attractive forces between charged C-S-H grains prevail causing the whole system to shrink. These attractive (tensile) forces compete with the compressive solid eigenstress development, reversing the expansion into shrinkage. By carrying out tests at different temperatures, we provide strong experimental evidence that this tensile eigenstress development is an out-of-equilibrium phenomenon that occurs close to jamming. Furthermore, the tensile eigenstresses calculated from our shrinkage measurements agree qualitatively with those from meso-scale coarsegrained simulations of C-S-H precipitation originating from the electrostatic coupling between charged C-S-H particles mediated by the electrolyte pore solution.