Hysteresis from Multiscale Porosity: Modeling Water Sorption and Shrinkage in Cement Paste

Abstract

Cement paste has a complex distribution of pores and molecular-scale spaces. This distribution controls the hysteresis of water sorption isotherms and associated bulk dimensional changes (shrinkage). We focus on two locations of evaporable water within the fine structure of pastes, each having unique properties, and we present applied physics models that capture the hysteresis by dividing drying and rewetting into two related regimes based on relative humidity (RH). We show that a continuum model, incorporating a poreblocking mechanism for desorption and equilibrium thermodynamics for adsorption, explains well the sorption hysteresis for a paste that remains above approximately 20% RH. In addition, we show with molecular models and experiments that water in spaces of ≲1 nm width evaporates below approximately 20% RH but reenters throughout the entire RH range. This water is responsible for a drying shrinkage hysteresis similar to that of clays but opposite in direction to typical mesoporous glass. Combining the models of these two regimes allows the entire drying and rewetting hysteresis to be reproduced accurately and provides parameters to predict the corresponding dimensional changes. The resulting model can improve the engineering predictions of long-term drying shrinkage accounting also for the history dependence of strain induced by hysteresis. Alternative strategies for quantitative analyses of the * Corresponding author. hmj@mit.edu PHYSICAL REVIEW APPLIED 3, 064009 (2015) 2331-7019=15=3(6)=064009 (17) 064009-1

Drying-induced atomic structural rearrangements in sodium-based calcium-alumino-silicate-hydrate gel and the mitigating effects of ZrO2 nanoparticles

“…The strain values presented in Figure 5 are within an order of magnitude of those measured at the macroscopic level for pure C-S-H gel (0.03), 36,69 OPC paste (0.0005 to 0.003) 71,72 and previous studies on silicate-activated slag (0.006) 25 and hydroxide-activated slag pastes (0.017). 70 Interestingly, the excessively large strain values reported from XRD-measured changes in the basal spacing for C-S-H gel (0.2), 36,69 which we also partially observe in the reciprocal space data in the Supporting Information for silicate-activated slag (0.6Å/14Å = 0.04) are not captured by the PDF data, and therefore the PDF peak shifts better represent the magnitude of nanoscopic shrinkage that occurs within the sample. The data presented in Figures 2 and 4b show that at 43% RH there are limited changes occurring to the atomic structure of the C-(N)-A-S-H gel, and therefore the corresponding strain values ( Figure 5) are minimal (almost an order of magnitude smaller than those measured for 0% RH).…”

The Pore Characteristics of Geopolymer Foam Concrete and Their Impact on the Compressive Strength and Modulus

“…The pores in foam concrete, either Portland cement based or geopolymer based, consist of interlayer pore/space, gel pore, capillary pore, and air void, and the sizes of these pores vary from nanometer scale to millimeter scale. For example, in Portland cement foam concrete, the cement binder will have (1) interlayer pores/space in calcium silicate hydrates with width smaller than 1 nm (this type of pores may be not considered in some definition), (2) gel pores between calcium silicate hydrates with width between 1 and 10 nm, and (3) capillary pores between gel clusters with width larger than 10 nm (Pinson et al, 2015), in addition to the introduced voids with width from several micrometers to 1-2 mm (Nambiar and Ramamurthy, 2007a). The total volume of pores, namely porosity, correlates with density and plays an important role that affects the compressive strength of foam concrete (Matusinović et al, 2003).…”

Capillary Stress and Structural Relaxation in Moist Granular Materials

“…In general, we find that capillary forces facilitate macroscopic stress relaxation in granular or colloidal cohesive media, which present rough and glassy potential energy landscapes having an abundance of meta-stable states. For the case of cement paste drying shrinkage [134], this mechanism is able to quantitatively explain the experimentally observed volume shrinkage (see Fig.4). Reactive heterogeneous materials such as cement have residual stresses due to the out-of-equilibrium solidification process [135][136][137].…”