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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.

By invoking analogies to lime mortars of times past, this study examines the carbonation of portlandite (Ca­(OH)2) by carbon dioxide (CO2) in the liquid and supercritical states as a potential route toward CO2-neutral cementation. Portlandite carbonation is noted to be rapid; e.g., >80% carbonation of Ca­(OH)2 is achieved in 2 h upon contact with liquid CO2 at ambient temperatures, and it is only slightly sensitive to the effects of temperature, pressure, and the state of CO2 over the range of 6 MPa ≤ p ≤ 10 MPa and 8 °C ≤ T ≤ 42 °C. Additional studies suggest that the carbonation of anhydrous ordinary portland cement is slower and far less reliable than that of portlandite. Although cementation is not directly assessed, detailed scanning electron microscopy (SEM) examinations of carbonated microstructures indicate that the carbonation products formed encircle and embed sand grains similar to that observed in lime mortars. The outcomes suggest innovative directions for “carbon-neutral cementation.”

The partial replacement of ordinary portland cement (OPC) by fine mineral fillers accelerates the rate of hydration reactions. This acceleration, known as the filler effect, has been attributed to enhanced heterogeneous nucleation of C-S-H on the extra surface provided by fillers. This study isolates the cause of the filler effect by examining how the composition and replacement levels of two filler agents influence the hydration of tricalcium silicate (T1-Ca 3 SiO 5 ; C 3 S), a polymorph of the major phase in ordinary portland cement (OPC). For a unit increase in surface area of the filler, C 3 S reaction rates increase far less than expected. This is because the agglomeration of fine filler particles can render up to 65% of their surface area unavailable for C-S-H nucleation. By analysis of mixtures with equal surface areas, it is hypothesized that limestone is a superior filler as compared to quartz due to the sorption of its aqueous CO 3 2À ions by the C-S-H-which in turn releases OH À species to increase the driving force for C-S-H growth. This hypothesis is supported by kinetic data of C 3 S hydration occurring in the presence of CO 3 2À and SO 4 2À ions provisioned by readily soluble salts. Contrary to prior investigations, these results suggest that differences in heterogeneous nucleation of the C-S-H on filler particle surfaces, caused due to differences in their interfacial properties, have little if any effect on C 3 S hydration kinetics. K E Y W O R D S C 3 S, filler effect, limestone, nucleation, quartz Correspondence

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