Influence of water activity on hydration of tricalcium aluminate-calcium sulfate systems

The hydration of tricalcium silicate (C₃S)—the major phase in cement—is effectively arrested when the activity of water (a_H) decreases below the critical value of 0.70. While it is implicitly understood that the reduction in a_H suppresses the hydration of tricalcium aluminate (C₃A: the most reactive phase in cement), the dependence of kinetics of C₃A hydration on a_H and the critical a_H at which hydration of C₃A is arrested are not known. This study employs isothermal microcalorimetry and complementary material characterization techniques to elucidate the influence of a_H on the hydration of C₃A in [C₃A + calcium sulfate (C$) + water] pastes. Reductions in water activity are achieved by partially replacing the water in the pastes with isopropanol. The results show that with decreasing a_H, the kinetics of all reactions associated with C₃A (eg, with C$, resulting in ettringite formation; and with ettringite, resulting in monosulfoaluminate formation) are proportionately suppressed. When a_H ≤0.45, the hydration of C₃A and the precipitation of all resultant hydrates are arrested; even in liquid saturated systems. In addition to—and separate from—the experiments, a thermodynamic analysis also indicates that the hydration of C₃A does not commence or advance when a_H ≤0.45. On the basis of this critical a_H, the solubility product of C₃A (K_C3A) was estimated as 10^−20.65. The outcomes of this work articulate the dependency of C₃A hydration and its kinetics on water activity, and establish—for the first time—significant thermodynamic parameters (ie, critical a_H and K_C3A) that are prerequisites for numerical modeling of C₃A hydration.     

Formation and stability of gismondine-type zeolite in cementitious systems

Microcrystalline zeolites of the gismondine family are often reported in alkali-activated and blended cement systems. However, little is known about gismondine's compatibility with other cementitious phases to determine stability in long-term phase assemblage. Experimental studies were conducted to investigate the compositional field of gismondine stability in the lime-alumina-silica-hydrate systems, with a particular focus on understanding the compatibility of gismondine with other cement phases such as C-S-H, ettringite, monosulfate, strätlingite, katoite, gypsum, calcite, portlandite, alkali, silica, and aluminosilicate phases. Results show that gismondine-Ca forms readily at ~85°C in high aluminosilicate compositions; and persists in the presence of calcite, gypsum, ettringite, katoite solid solution, low Ca tobermorite-like C-S-H, silica and aluminosilicate phases, at 20-85°C. However, gismondine-Ca reacts with: (a) monosulfate, producing ettringite-thaumasite solid solution; (b) portlandite, forming tobermorite-like C-A-S-H gel and siliceous katoite at >55°C; (c) aqueous NaOH, generating gismondine-(Na,Ca), a garronite-like zeolite P solid solution; and (d) strätlingite leading to the conversion of strätlingite to gismondine indicating the metastability of strätlingite with respect to gismondine at 55°C. The outcomes are discussed to provide insights into the long-term phase assemblage of relevant cement systems such as lime-calcined clay, alkali-activated materials, and potentially ancient Roman concrete.     

Psychogenic myoclonus

Action potentials were recorded intracellularly from single diaphragmatic fibers, in vitro, of newborn (3-10 d, n = 18) and older (> or = 21 d, n = 10) rats using flexible microelectrodes. At 20 and 50 Hz phrenic nerve stimulation (1 s duration), action potential transmission failure was significantly higher in the newborn than in the older fibers. During the failure periods, small and highly variable depolarizations were observed which were most likely EPPs. These results show that failure of action potential transmission across the neuromuscular junction is more prevalent in the newborn, and we speculate that this failure is due to inadequate release of neurotransmitter in newborn muscle fibers.