Calcium hydroxide crystals

Outside this, a zone of perhaps 10 to 30 m thickness, which, in reasonably well-hydrated systems, is largely occupied by relatively massive calcium hydroxide crystals, with occasional interruptions of more porous regions. 

During electrolysis, per mole of potassium permanganate one mole of potassium hydroxide is produced, which has to be recovered. This can be achieved, for example, by evaporating the mother liquor to 750 g KOH per L, whereupon the dissolved potassium manganate(Vl) and calcium hydroxide crystallize out and are removed. The potassium hydroxide can be returned to the dissolution step. Other dissolved impurities from the ores, such as silicates or aluminates, have to be removed from the alkali cycle. 

Crystal structure. Calcium hydroxide crystals have a hexagonal symmetry. The particles in commercial hydrated limes consist of micro-crystalline agglomerates. 

As it has been shown by Barret [7], the nucleation of C-S-H occurs very quickly. It is provided also by the rapid drop of silica concentration in solution. Also the addition of CH nuclei, though it causes some acceleration of the process, does not eliminate the induction period [16], There are some reports indicating the poisoning of the CH nucleation process by diluted silica solution and a slowed growth of calcium hydroxide crystals as a result of silicate anions adsorption on the 001 planes [17,18]. However, this has not been proved in the case of C3S hydration a low CH crystallization rate is important because it prevent fast calcium ions removing from the solution. 

There was also presented the hypothesis linking the strength reduction of the paste by the portlandite crystals [99]. This was concluded from the observations of crack propa tion in the pastes, surrounded usually the areas rich in CH crystals [100]. However, as it has been shown in recent experiments, the strength of CH is similar to C-S-H strength [101, 102]. Thus, the morphology of calcium hydroxide crystals in the interstitial transition zone, as well as its significant porosity in ordi-... 

The XRD studies of the interfacial transition zone (material produced by abrasion of paste layers) [16], as well as the SEM observations with EDS analysis [16] revealed the presence of transition zone surrounding the aggregate grains, determined by Maso as an aureole [ 10]. This relates to the former water film around the aggregate. This area shows higher w/c ratio and subsequently cement components can readily dissolve, as well as the hydration products crystallize from the solution. Calcium hydroxide crystallizes in this interfacial transition zone and the crystals are oriented in such a way that their (001) axis is perpendicular to the surface of aggregate, as it was reported by Barnes et al. [17]. The C-S-H is then formed and the two products occur together as a duplex film about 1 pm thick (Fig. 6.7). 

Neither the duplex film, nor the oriented portlandite crystals in the interfacial zone were found by all the authors [19, 20]. However, there is a common opinion that this area is enriched with calcium hydroxide crystals and exhibits higher porosity. Consequently, it will have a great impact on the corrosion resistance of concrete. The transition zone of high porosity is the weakest micro-area where the corrosion of concrete will begin [21]. For this reason, the interfacial transition zone became a subject of numerous studies [16,17,22-28]. The constitution of this zone can be easily observed on the model proposed by Rooij et al. [28] (Fig. 6.9). 

The authors are not consistent as the constraction of duplex film is concerned however, there is a commonly accepted opinion that this transition zone is emiched with calcium hydroxide crystals, well developed and oriented in relation to the aggregate surface [10, 16, 41,42],... 

Cauliflowerlike calcium hydroxide crystals (presumably epezite), from hydration of free lime (air slaking), and round, gray periclase crystals in a nest, suggesting coarse dolomite in the feed. (S A6744)... 

Photograph 9-22 Calcium hydroxide crystals (presumably epezite) growing on free-lime substrate in clinker. Portlandite crystals in upper center. (S A6745)... 

Photograph 9-23 Calcium hydroxide crystals (presumably epezite) derived from free lime in clinker. Indicative of air slaking. (S A6746)... 

Bricklaying mortar is made by mixing slaked lime, Ca(OH)2, with sand and water. The mortar hardens as the mixture dries and calcium hydroxide crystallizes. Over time, however, the mortar sets to a harder solid as calcium hydroxide reacts with carbon dioxide in the air to form calcium carbonate crystals that intertwine with the sand particles. ... 

Moreover, applied to concrete, the silicate platelets provided by the clay, being pozzolanic in nature, react with the calcium hydroxide crystals in the concrete matrix to produce C-S-H units, providing all of the associated benefits, i.e., increased strength, reduced permeability Additionally, these new C-S-H crystals form around the polymer chains, resulting in what is essentially a fiber reinforced concrete, though the reinforcing is at a scale and consistency never before achieved. The result of this latter effect is an increase in ductility of the concrete during failure (5). 

Yang,T., B. Keller, E. Magyar , K. Hametner and D. Gunther (2003). Direct observation of the carbonation process on the surface of calcium hydroxide crystals in hardened cement paste using an atomic force microscope . Journal of Materials Science i8 9)-. 1909-1916. 

To prepare the standard pH buffer solutions recommended by the National Bureau of Standards (U.S.), the indicated weights of the pure materials in Table 8.15 should be dissolved in water of specific conductivity not greater than 5 micromhos. The tartrate, phthalate, and phosphates can be dried for 2 h at 100°C before use. Potassium tetroxalate and calcium hydroxide need not be dried. Fresh-looking crystals of borax should be used. Before use, excess solid potassium hydrogen tartrate and calcium hydroxide must be removed. Buffer solutions pH 6 or above should be stored in plastic containers and should be protected from carbon doxide with soda-lime traps. The solutions should be replaced within 2 to 3 weeks, or sooner if formation of mold is noticed. A crystal of thymol may be added as a preservative. 

Lithium Hydroxide. Lithium hydroxide monohydrate [1310-66-3], Li0H-H2 0, is prepared industrially from the reaction of lithium carbonate and calcium hydroxide in aqueous slurries. The calcium carbonate is subsequently separated to yield a lithium hydroxide solution from which lithium hydroxide monohydrate can be crystallized. Lithium hydroxide is the least soluble alkaH hydroxide, and solubiHty varies Htfle with temperature. 

Precipitated Calcium Carbonate. Precipitated calcium carbonate can be produced by several methods but only the carbonation process is commercially used in the United States. Limestone is calcined in a kiln to obtain carbon dioxide and quicklime. The quicklime is mixed with water to produce a milk-of-lime. Dry hydrated lime can also be used as a feedstock. Carbon dioxide gas is bubbled through the milk-of-lime in a reactor known as a carbonator. Gassing continues until the calcium hydroxide has been converted to the carbonate. The end point can be monitored chemically or by pH measurements. Reaction conditions determine the type of crystal, the size of particles, and the size distribution produced. 

Adipic acid and HMDA are obtained from nylon-6,6 by die hydrolysis of die polymer in concentrated sulfuric acid (Fig. 10.7). The AA is purified by recrystallization and the HMDA is recovered by distillation after neutralizing die acid. This process is inefficient for treating large amounts of waste because of die required recrystallization of AA after repeated batch hydrolyses of nylon-6,6 waste. In a continuous process,5 nylon-6,6 waste is hydrolyzed with an aqueous mineral acid of 30-70% concentration and the resulting hydrolysate is fed to a crystallization zone. The AA crystallizes and the crystals are continuously removed from the hydrolysate. Calcium hydroxide is added to neutralize the modier liquor and liberate the HMDA for subsequent distillation. 

Continuous recovery requires AA crystals having an average diameter of ca. 40-50 nm.5 Such crystals are obtained by continuously introducing die hot hydrolysate containing 10-20% AA into an agitated crystallization vessel while maintaining an average temperature of 20-30°C. The slurry obtained from the crystallization vessel is filtered to collect die AA crystals, and the filtrate which contains the HMDA acid salt is continuously neutralized with calcium hydroxide. The calcium salt formed is removed by filtration, and the HMDA in the filtrate is isolated by distillation. 

In this system, the least soluble component is sodium bicarbonate, so this crystallizes out. On calcination it yields sodium carbonate and the carbon dioxide is recycled. The ammonia is recovered by adding calcium hydroxide, producing calcium chloride waste and liberating the... 

Thann A process for making crystalline calcium hypochlorite by passing chlorine into an aqueous slurry of calcium hydroxide. There are several such processes in this one, some of the filtrate is recycled in order to produce larger crystals. Invented by J. Ourisson in France in 1936. 

Crystallization of magnesium hydroxide by a continuous mixed suspension mixed product removal crystallizer was conducted to make clear the characteristics of reactive crystallization kinetics of magnesium hydroxide, which was produced by the precipitation from magnesium chloride with calcium hydroxide. The following operating factors were investigated affecting the crystallization kinetics the initial concentration of feeds, residence time of reactants, feed ratio of reactants, and concentrations of hydroxide and chloride ions. 

The present work deals with the reactive crystallization of magnesium hydroxide, a well-known sparing soluble material, from magnesium chloride with calcium hydroxide. Magnesium hydroxide is produced industrially by the precipitation from brine with... 

Experimental conditions. Table 1 lists the experimental conditions. Co shows the apparent initial concentration of reactants in the crystallizer. Series I, and 1 were conducted with stoichiometric feed ratio, with changing the feed ratio of reactants and with sodium chloride addition under the constant Co of magnesium chloride and calcium hydroxide, respectively. 

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