Hydrated products formation

The strength development of sulphate-aluminate cement mortars is strongly influenced by the SOj/AljOj ratio. This ratio affects the phase composition of paste, as well as the rate of hydration products formation Klein and Mehta [136] exammed the phase composition of hydration products, formed in the mixture C4A3S+CaSO +CaO+H20. The results are shown in Fig. 9.29. 

Synthetic pine oil is produced by the acid-catalyzed hydration of a-pinene (Fig. 1). Mineral acids, usually phosphoric acid, are used in concentrations of 20—40 wt % and at temperatures varying from 30—100°C. Depending on the conditions used, alcohols, chiefly a-terpineol (9), are produced along with /)-menthadienes and cineoles, mainly limonene, terpinolene, and 1,4- and 1,8-cineole (46—48). Various grades of pine oil can be produced by fractionation of the cmde products. Formation of terpin hydrate (10) from a-terpineol gives P-terpineol (11) and y-terpineol (12) as a consequence of the reversible... 

The formation of cotar none from cotar nine methine methiodide by the action of potash (IX—X) led Roser to represent cotarnine and its salts by the following formulae, the loss of a molecule of water in the formation of cotarnine salts being explained by the production of a partially reduced pyridine ring, which is fully hydrogenated in the reduction of cotarnine to hydrocotarnine. In the reverse process, oxidation of liydrocotarnine to cotarnine, Roser assumed the scission of the ring at the point indicated, with the formation of a hydration product, and oxidation of the latter to cotarnine thus —... 

NaClO, or else in the two-phase system but with a quaternary ammonium (viz. AUquat) ion as a phase-transfer catalyst, overoxidation to the corresponding carboxylic acid is obtained (entry 4). Therefore, by proper choice of the experimental conditions, a synthetically useful distinction in products formation can be made for the oxidation of primary alcohols, even though we are far from a satisfactory understanding of the reason behind this different behaviour. In fact TEMPO, as a well-known inhibitor of free-radical processes is allegedly responsible for the lack of overoxidation of an aldehyde to carboxylic acid (entry 3) this notwithstanding, TEMPO is also present under those conditions where the overoxidation does occur (eutry 4). Moreover, a commou teuet is that the formation of the hydrated form of an aldehyde (in water solution) prevents further oxidation to the carboxylic acid however, both entries 3 and 4 refer to water-organic solutions, and their... 

There are few reported oxidations of this type with TPAP in organic solvents, one of the advantages of the reagent being that the alcohol-to-aldehyde oxidation rarely proceeds further. One natural product which did involve such a step is antascomicin B using TPAP/NMO/PMS/CH Cl [85], In aqueous base however [RuO ] is a much more powerful oxidant than TPAP in organic media, perhaps because oxidation of aldehydes to carboxylic acids may proceed via an aldehyde hydrate, the formation of which is inhibited by the molecular sieves used in catalytic TPAP systems. 

The mechanism of the inhibitive action of LiOH proposed by Stark et al. [7] is attributed to the formation of lithium silicate that dissolves at the surface of the aggregate without causing swelling [7], In the presence of KOH and NaOH the gel product incorporates Li ions and the amount of Li in this gel increases with its concentration. The threshold level of Na Li is 1 0.67 to 1 1 molar ratio at which expansion due to alkali-silica reaction is reduced to safe levels. Some workers [22] have found that when LiOH is added to mortar much more lithium is taken up by the cement hydration products than Na or K. This would indicate that small amounts of lithium are not very effective. It can therefore be concluded that a critical amount of lithium is needed to overcome the combined concentrations of KOH and NaOH to eliminate the expansive effect and that the product formed with Li is non-expansive. 

The overall process of cement hydration and setting results from a combination of solution processes, interfacial phenomena and solid-state reactions which lead to the formation of complex products. Some of the hydration products formed from the different mineral components of cement are shown in Table 7.31 [125]. Admixture-cement interactions are essentially interactions between admixtures and the initially formed cement hydrates the influence of admixtures on cement hydration is best considered by reference to the evolution of the reaction with time. Five stages can be identified [125, 126] ... 

These processes may be affected by chemical admixtures, particularly the formation and properties of the protective layer. Also, admixtures remaining in the pore solution may further influence nucleation and growth of the hydration products, causing volume expansion, outward mechanical pressure on the protective gel layer and its subsequent disruption. 

It is also possible to determine the nature of the excited molecule reaction leading to product formation by kinetic methods. For example, variation in the rate of formation of dimethyluracil hydrate with water concentration in acetonitrile-water mixtures is convex to the water concentration axis (Fig. 15).65 The rate of formation of uracil hydrate under similar conditions is linear with water concentration. The first of these is not the shape of curve to be expected if the function of the water molecules were simply to quench an excited state according to the common mechanism ... 

Dilaktorsky, N. L. Galibina, E. A. 1955. The processes of mineral formation during the heating of oil shale ash and hardening of hydrate products of burning. In Oil Shale Ash Materials in Building. Estonian Academy, Tallinn, 31—46 (in Russian). 

Upon addition of water, the hydration reactions initiate, and the hydraulic cement begins to gain strength. This process is very complex, but the strengthening effect is due basically to the formation of three types of hydration products colloidal products such as C2S xH20, which have a size of less than 0.1 p.m submicrocrystalline products such as Ca(OH)2, Al +, Fe +, and S04 phases with sizes from 0.1 to 1 tim and microcrystalline products, primarily of Ca(OH)2, with particle sizes greater than 1 p,m. The most common type of hydraulic cement, Portland cement, usually contains mostly colloidal products. 

The generally accepted pathway for the hydration of alkynes are the generation and subsequent tautomerization of an intermediate enol. The use of fairly concentrated acids, usually H2S04, is necessary to achieve suitable reaction rates. Addition of catalytic amounts of metal salts, however, greatly accelerates product formation. In most cases mercury(II) salts are used. Mercury-impregnated Nafion-H [with 25% of the protons exchanged for Hg(II)] is a very convenient reagent for hydration 35... 

However, other researchers suggest that considerably less methane can be generated by in situ hydrate production. Hyndman and Davis (1992) indicated that an unaccountably high concentration of gas was required for hydrate formation. Minshull et al. (1994), Pauli et al. (1994), and Klauda and Sandler (2005) suggest that for in-place formation, under the best conditions the maximum amount of hydrate that can fill the sediments is 3%. 

Chloral has three electronegative chlorine units attached to the a-carbon (CI3C-) to the aldehydes. The carbonyl carbon bears a partial positive charge so such electronegative elements destabilizes the carbonyl and favors the equilibrium towards significant formation of the hydrate product. For acetaldehyde, on the other hand, the equilibrium constant is 1. 

When water comes into contact with the cement (stage I), wetting of the highly hygroscopic cement particles and the solubilization of a variety of ionic species, e.g. Na+, K+, Ca++, S04-, OH-, by complete or selective solubilization (surface hydrolysis) of the various phases present in cement occurs. Surface hydrolysis quickly leads to the formation of a thin layer of both amorphous and gel products. Beyond the initial solubilization phenomena, the formation of any of the solid hydration products (Table 7.31) will be governed by nucleation processes which may occur homogeneously from the solution phase or heterogeneously at a solid-solution interphase [64, 125],... 

The relative reactivity of the different mineral phases of cement with water is usually given as C A>C S>C S>C AF. Aluminate phases and their hydration products therefore play an important role in the early hydration process. Because of the high reactivity of calcium aluminate, the aluminate hydration reaction is carried out in the presence of sulfate ions. The latter provide control of the reaction rate through the formation of mixed aluminum sulfate products (ettringite and monosulfoaluminate) Calcium sulfate which is added to the cement clinker hence controls the properties of the aluminate hydration products. Sulfates thus play a crucial role in cement hydration and the influence of chemical admixtures on any process where sulfates are involved may be expected to be significant [127],... 

In subsequent experiments, using other crystal systems, such as ferrous sulfate and sodium hydrogen phosphate, it was similarly observed that the first crystallization product to form was the one most closely resembling the structure of the solvent (Nyvlt, 1995). For the case of citric acid, this is the monohydrate, which more closely resembles the aqueous structure. As the temperature of the solution is increased, the structure of the solvent, as well as the solubility of the crystal, changes, resulting in a more thermodynamically stable anhydrous product. This conversion between the kinetic and thermodynamic product occurs at a critical transition temperature, below which the structure of the solution favors the formation of the hydrated product. As the transition temperature is surpassed, the anhydrous product becomes favored. 

In the context of this discussion, it is useful to recall that solvent uptake may be attained by mechanical treatment of unsolvated crystals. Even gentle grinding of a powder product to prepare a sample for powder diffraction may lead to the formation of a hydrated product. 

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