Self desiccation

Jiang Zhengwu, Sun Zhenping and Wang Peimi. Study on self-desiccation effect of high performance... 

Self-desiccation is the process in which very fine porosity drains water from the larger capillary pores where water is not as strongly bonded. 

Another similar application of isothermal calorimetry is the assessment of the thermal power at different curing regimes see Figure 2.11. Sealed conditions, which are the usual case in isothermal calorimetry, are compared to pastes with extra water added on top of the sample. This extra water causes an increase in cumulative heat and thus an increased hydration degree. The effect becomes more pronounced at lower w/c, where the samples tend to undergo self-desiccation. The same effect can be seen also with other types of cement, such as calcium sulfoaluminate cements. As this type of cement needs a higher w/c for complete hydration compared to Portland cement (around 0.5-0.6 instead of 0.4 see Winnefeld and Lothenbach 2010), the effect of the extra water is already very evident at a w/c of 0.7 see Figure 2.12. 

Wadso, L., and A. Anderberg (2002). A method for simultaneous measurements of heat of hydration and relative humidity . Self-Desiccation and Its Importance in Concrete Technology, Proceedings of the Third International Research Seminar in Lund, 14-15 June 2002 103-112. 

Comparing the data obtained by different methods, it should be kept in mind that the moisture state of the hydrating material (saturated or self-desiccated) affects the development of hydration. Figure 3.22 illustrates possible differences between the developments of hydration of sealed and saturated (open) samples. 

The sample dimensions are critical when measuring chemical shrinkage for longer periods and on dense pastes. The size should be sufficiently large to provide accurate measurements and sufficiently small to reduce sources of error as possible temperature effects and self-desiccation due to depercolation of the capillary porosity. 

Measurement of chemical shrinkage of cementitious materials is used for quality control, kinetic studies and prediction of self-desiccation and proportioning of internal curing water. 

Note that only water-hlled pores can be detected. If cement pastes are kept sealed, empty voids develop due to chemical shrinkage and self-desiccation (at around 8 vol.% for conventional cements [Bentz 2008]) which are not detected by NMR experiments. 

Self-desiccation Internal drying due to the consumption of water by chemical reactions. 

Fig. 1. A general, simplified and self-explanatory layout of the wheat breeding programme at the Volcani Center, Israel. The selection for drought resistance is affected at about the F4 and later generations, by using infra-red thermometry, chemical desiccation and yield under stress. A summer (off-season) nursery is incorporated as means for selection for heat tolerance.

The point is also made [134] that the very high surface areas and the richly interconnected three-dimensional networks of these micron-sized spaces, coupled with periods of desiccation, could together have produced microenvironments rich in cat-alytically produced complex chemicals and possibly membrane-endosed vesides of bacterial size. These processes would provide the proximate concatenation of lipid vesicular precursors with the complex chemicals that would ultimately produce the autocatalytic and self-replicating chiral systems. A 2.5 km2 granite reef is estimated to contain possibly 1018 catalytic microreactors, open by diffusion to the dynamic reservoir of organic molecules. .. but protected from the dispersive effects of flow and convection [134] as well as protected from the high flux of ultraviolet radiation impinging on the early Earth. [123,137]... 

Zinc Benzenediazonium Chloride. C6H5N NG-ZnCl2,C6HsN2Cl3Zn mw 276.87 N 10.12% solid. A self-reactive compd which has been known to expld after 15 hrs storage in a vac desiccator... 

Radium Barium Platinocyanide.—-Since the majority of platino-cyanides fluoresce under the influence of radiations from radioactive substances, the radium salt should be self-luminous. The radium barium salt was prepared1 by dissolving radioactive barium chloride in water and boiling the solution with excess of silver platinocyanide. After filtering off insoluble silver salts the solution was allowed to crystallise in a desiccator. 

Glassware and normal syringes can be dried in an oven then cooled in a desiccator over P2O5 or self-indicating silica gel, but it is not advisable to heat microlitre syringes and these should be dried under vacuum in a desiccator. 

The submitters have obtained yields of about 70% in runs eight times as large as that described. This amino aldehyde undergoes self-condensation on standing, especially in a desiccator, and so the product should be used immediately. 

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