Finely ground mineral powders are known to accelerate cement hydration rates. This "filler effect" has been attributed to the effects of dilution (w/c increase) when the cement content is reduced or to the provision of additional surface area by fine powders. The latter contribution (i.e., surface area increase) is speculated to provide additional sites for the nucleation of the hydration products, which accelerates reactions. Through extensive experimentation and simulation this study describes the influence of surface area and mineral type (e.g., quartz or limestone) on cement reaction rates. Simulations using a boundary nucleation and growth (BNG) model and a multiphase reaction ensemble (MRE) indicate that the extent of the acceleration is linked to the: (1) magnitude of surface area increase and (2a) capacity of the filler's surface to offer favorable nucleation sites for hydration products. Other simulations using a kinetic cellular automaton model (HydratiCA) suggest that accelerations are linked to: (2b) the interfacial properties of the filler that alters (increases or decreases) its tendency to serve as a nucleant, and (3) the chemical composition of the filler and the tendency for its dissociated ions to participate in exchange reactions with the calcium silicate hydrate product. The simulations are correlated with accelerations observed using isothermal calorimetry when fillers partially replace cement. The research correlates and unifies the fundamental parameters that drive the filler effect and provides a mechanistic understanding of the influence of filler agents on cementitious reaction rates.

Like many others, silicate solids dissolve when placed in contact with water. In a given aqueous environment, the dissolution rate depends highly on the composition and the structure of the solid, and can span several orders of magnitude. Although the kinetics of dissolution depends on the complexities of both the dissolving solid and the solvent, a clear understanding of which critical structural descriptors of the solid control its dissolution rate is lacking. Through pioneering dissolution experiments and atomistic simulations, we correlate the dissolution rates -ranging over four orders of magnitude -of a selection of silicate glasses and crystals, to the number of chemical topological constraints acting between the atoms of the dissolving solid. The number of such constraints serves as an indicator of the effective activation energy, which arises from steric effects, and prevents the network from reorganizing locally to accommodate intermediate units forming over the course of the dissolution.

Partial replacement of ordinary portland cement by silica fume (SF) accelerates its rate of hydration reactions. This acceleration is attributed to the enhanced heterogeneous nucleation of the main hydration product, i.e., calcium−silicate−hydrate (C−S−H), on the extra surfaces provided by SF. However, such enhancement of C−S−H nucleation is suppressed in the presence of polycarboxylate ether (PCE) dispersant, which is added to regulate the fluidity and rheological properties of fresh paste. A generalized phase boundary nucleation and growth model with time-dependent growth of C−S−H is used to fit the hydration rates of plain and binary (10% to 30% SF) cement pastes prepared with and without PCE. The results show that while SF accelerates cement hydration, increments in hydration rates are significantly smaller in relation to the extra surface area provided by SF. This is because of the agglomeration of SF particles which renders up to 96% of their surface area unavailable for C−S−H nucleation. Furthermore, it is shown that the hydration of cement, in both plain and binary pastes, is suppressed in relation to the PCE dosage. This is because of (a) adsorption of PCE molecules onto cement and SF surfaces resulting in inhibition of sites for product nucleation and (b) interaction of PCE with C− S−H, which suppresses growth of C−S−H throughout the hydration process. It is shown that the effects of nucleation site inhibition by PCE are more pronounced in SF as compared to cement. The outcomes of this study improve our understanding of the mechanisms that drive the hydration of cement in the presence of SF and PCEs.

Tricalcium silicate does not undergo hydration at relative humidities (RH's) below 80%. But, the rate at which its hydration rate decreases as a function of the RH has not yet been elucidated. By invoking correspondence between RH and water activity (a H , unitless), both of which are related to the chemical potential of water, the reaction evolutions of triclinic tricalcium silicate (i.e., T1-Ca 3 SiO 5 or C 3 S) are tracked in water + isopropanol (IPA) mixtures, prepared across a wide range of water activities. Emphasis is placed on quantifying the: (a) rate of hydration as a function of a H , and (b) the critical (initial, a H0c or the achieved) water activity at which hydration effectively ceases, i.e., does not progress; here identified to be % 0.70. The hydration of tricalcium silicate is arrested even when the system remains near saturated with a liquid phase, such that small, if any, capillary stresses develop. This suggests that changes in chemical potential induced via a vapor-phase or liquid-phase route both induce similar suppressions of C 3 S hydration. A phase boundary nucleation and growth (pBNG) model is fit to measured hydration rates from the onset of the acceleration period until well beyond the rate maximum when the water activity is altered. The simulations suggest that for a fixed hydrate nucleation density, any water activity reductions consistently suppress the growth of hydration products. Thermodynamic considerations of how water activity changes may influence reactions/hydrate evolutions are discussed. The outcomes improve our understanding of the chemical factors that influence the rate of Ca 3 SiO 5 hydration.