Calcium reaction with silica

Reaction with silica and alumina. Hydrated lime reacts with pozzolans (materials containing reactive silica and alumina) in the presence of water to produce hydrated calcium silicates and aluminates. The reactions may take months to proceed to completion at ambient temperatures, as in mortars (section 26.6) and lime treated soil (section 26.3), but proceed within hours at elevated temperatures and water vapour pressures (e.g., in steam at 180 °C and a pressure of 10 bar — see sections 26.10,26.11 and 26.12). This pozzolanic reaction is the basis of the strength generated by hydraulic quicklimes (section 26.9). 

The liquid phase formed due to local eutectic, accelerate the calcium carbonate decomposition and the reactions of calcium oxide with silica, as well as with alumina and iron oxides. 

The hypothesis supporting the decisive role of Ca(OH)2 was advanced by Hansen [134], as early as in 1944. According to Hansen the Ca + ions can replace alkalis in a reaction product and hence they promote their further reaction with silica, with the new batch of expansive gel formation. This problem was then developed by Chattegi [106], as it has been mentioned earlier. Recently, Thomas returned to the Hansen s hypothesis (see Fig. 6.36) [135]. Studying experimentally gel in the concrete samples from an old dam, he found the proves of potassium ions by calcium ions replacement. This process occurs when the gel is migrating to the areas occupied by rich in calcium hydroxide cement matrix. 

The reaction of calcium hydroxide with silica is significantly accelerated by autoclaving, because the rate of reaction increases exponentially with temperature (Arrhenius equation). Simultaneously, at higher temperature the solubility of silica is higher (Fig. 4.54), which is of significantly importance for the rate of reaction with calcium ions. In these conditions C-S-H is formed relatively quickly, and its Ca0/Si02 molar ratio is below 1. 

Finally, it is known that the increase of ferrite phases content in clinker is advantageous. They are formed at low temperatures and produce the melt, which promotes the calcium oxide reaction with silica. The ferrite phases react quickly with water, giving with lime the hydrated componnds, analogous to the aluminate hydrates. 

A little less sulphuric acid is used than indicated by Equation 12.1. This is to ensure that the product contains no free sulphuric or phosphoric acids which would render it hygroscopic and lead to caking. In practice not all the fluorine is removed from the rock and some hydrofluoric acid can be produced by reaction with silica impurities (12.2). The efficiency of reaction (12.1) depends on the grade of phosphorite rock which is used, and is limited by the crystallisation behavionr of the by-product, calcium sulphate. The latter is complex and is influenced, not only by the silica and fluorine, but by the level of the other impurities which are present. The phosphate rock (phosphorite) should contain not more than about 1% MgO and 5% AI2O3 + Fe203, and the content of other impurity (potentially toxic) elements such as Cd, Se and Ni must be very low. 

Modem production of elemental phosphoras uses a technique similar to the metallurgical processes described in Chapter 20. Apatite is mixed with silica and coke and then heated strongly in the absence of oxygen. Under these conditions, coke reduces phosphate to elemental phosphoms, the silica forms liquid calcium silicate, and the fluoride ions in apatite dissolve In the liquid calcium silicate. The reactions are not fully understood, but the stoichiometry for the calcium phosphate part of apatite is as follows ... 

Five common desiccant materials are used to adsorb water vapor montmorillonite clay ([(Na,Cao.5)o.33(Al,Mg)2Si40io(OH)2 H20], silica gel, molecular sieves (synthetic zeolite), calcium sulfate (CaS04), and calcium oxide (CaO). These desiccants remove water by a variety of physical and chemical methods adsorption, a process whereby a layer or layers of water molecules adhere to the surface of the desiccant capillary condensation, a procedure whereby the small pores of the desiccant become filled with water and chemical action, a procedure whereby the desiccant undergoes a chemical reaction with water. 

Yellow phosphorus (known also as white phosphorus) is produced by reducing phosphate rock (calcium phosphate or calcium fluorophosphate) with carbon in the presence of silica as flux heat of reaction is furnished by an electric-arc furnace. 

Silica is reduced to silicon at 1300—1400°C by hydrogen, carbon, and a variety of metallic elements. Gaseous silicon monoxide is also formed. At pressures of >40 MPa (400 atm), in the presence of aluminum and aluminum halides, silica can be converted to silane in high yields by reaction with hydrogen (15). Silicon itself is not hydrogenated under these conditions. The formation of silicon by reduction of silica with carbon is important in the technical preparation of the element and its alloys and in the preparation of silicon carbide in the electric furnace. Reduction with lithium and sodium occurs at 200—250°C, with the formation of metal oxide and silicate. At 800—900°C, silica is reduced by calcium, magnesium, and aluminum. Other metals reported to reduce silica to the element include manganese, iron, niobium, uranium, lanthanum, cerium, and neodymium (16). 

Thenaldehyde (thiophene-2-carbaldehyde) is readily available via the Vilsmeier-Haack reaction of DMF with thiophene catalyzed by phosphorus oxychloride. The Sommelet reaction with 2-chloromethylthiophene also gives reasonable yields (63AHC(l)l). Likewise, thiophene is readily acylated with acyl anhydrides or acid chlorides (equation 14), using mild Friedel-Crafts catalysts, such as tin(IV) chloride, zinc chloride, boron trifluoride, titanium tetrachloride, mercury(II) chloride, iodine and even silica-alumina gels or low-calcium-content montmorillonite clays (52HC(3)l). 

The way in which salts such as calcium chloride and calcium formate operate is not fully understood, but it is clear that the mechanism involves an acceleration of the C S and C S hydration. It has been proposed [23] that the initial products of cement hydration form a sort of membrane which acts as a restraint to the diffusion process which in turn leads to the dormancy period . It seems likely that the chloride ion, by virtue of its small size and high mobility, is able more easily to penetrate the pores of the restraining layer allowing the diffusion process to proceed more rapidly. The resultant tobermorite gel has a higher lime-silica ratio and a more open, accessible structure, based on a crumpled foil morphology rather than the usual spicular. The considerable reaction with,... 

Chromatography on silica gel yielded 13.2 g racemate of lactone - trans-(+/-)-5-(4-fluorophenyl)-2-(l-methylethyl)-N,4-di-diphenyl-l-[2-(tetrahydro-4-hydroxy-6-oxo-2FI-pyran-2-yl)ethyl]-lFI-pyrrole-3-carboxamide. This racemate was divided by chiral synthesis which was made analogously the method in US Pat. No. 4,581,893. Then each isomer was saponificated with NaOFI and purificated by FIPLC. The calcium salt corresponding acid was prepared by reaction with 1 eq. of CaCl2-2Fi20 in water. 

Cationization of starch by dry reaction with 2,3-epoxypropyltrimethylammonium chloride is a commercially significant process. The key to a dry reaction is an intimate, homogenous mixture of the reagent and the catalyst. One process38,39 describes an activator consisting of spray dried, precipitated silica with a surface area of 190m2/g (BET) that contains an alkaline agent such as calcium oxide or calcium hydroxide and/or silicates. Different ratios of silica to alkali and 1-3% catalyst (based... 

Potassium Phosphates.—Basic or neutral phosphates (Rhenania phosphates) may be made directly from rock phosphate by mixing it with potassium chloride, some form of carbon and soda, and heating to over 1000° C. in an electric furnace.2 One of these products has the composition Ca2KNaP208, with some silica. Such products may contain 23 to 31 per cent, of soluble phosphoric acid. Potassium superphosphate has been made by mixing potassium sulphate and calcium carbonate with concentrated phosphoric acid in a lead-lined vessel.3 The CaS04.2Ha0 is separated and the filter-pressed solution is evaporated to dryness by steam heat. The residue may be treated with more phosphoric acid and again evaporated. The reaction is expressed by the equation... 

Since calcium oxide is more than sparsely soluble and its reaction with phosphoric acid or a soluble phosphate is highly exothermic, researchers have used less soluble salts of calcium to react with the phosphates and form a phosphate ceramic [4-12]. In the acidic medium of the phosphate solutions, the salts of calcium dissolve slowly and release Ca (aq) into the solution, which subsequently reacts with phosphate anions and forms calcium phosphates. The best calcium minerals for forming CBPCs are combination of oxides of calcium and insoluble oxides such as silica or alumina, e.g., calcium silicate (CaSi03) and calcium aluminate (CaAl204), or even a phosphate of calcium such as tetracalcium phosphate (Ca4(P04)2 0). These minerals are reacted with acid phosphate salts to form phosphate cements. 

Calcium silicates. In [25] it was demonstrated that the reactions with the participation of hydrated silica are more profitable from thermodynamic viewpoint ... 

It was stated that weakly crystallized tobermorite was synthesized in a mixture of calcium hydroxide and silica gel. The reaction is most efficient at the reagent ratio CaO Si02=5/6 and water-to-solid ratio 4 1. At short activation time, the formation of poor crystallized calcium silicates of B type with tobermorite-like structure was observed, Mechanochemically obtained tobermorite is stable till 800°C. Above this temperature, it... 

According to data [10,11], practically monophase anorthite can be synthesized after mechanical treatment of a mixture of calcium oxide with aluminium hydroxide and hydrated silica followed by thermal treatment at 1000°C for 4 h. Fig. 7.3 shows X-ray pattern of this mixture. After activation for 5 min (Fig. 7.3, curve 1), the reaction mixture exhibits a small amount of individual crystal calcium oxide (0.240 nm) and aluminium hydroxide (0.482, 0.436, 0.245 nm), while the major part of CaO is bound into an X-ray amorphous product. The annealing of the activated mixture at 1000°C gives anorthite (Fig. 7.3, curve 3) while the thermal treatment of non-activated mixture brings to the formation of intermediate product, wollastonite (Fig. 7.3, curve 2). The lines of this product are absent in X-ray patterns of the activated mixture. 

AIPO4 is isoelectronic with silica and, as such, readily forms glasses and Si02-like crystalline materials. As well, framework stmctmes similar to zeolites may be prepared by the use of amines as templates. Like zeolites, these are active in catalytic reactions such as methanol conversion to hydrocarbons (seeZeolites) As a ceramic material, AIPO4 is an infusible material that is insoluble in water but is soluble in alkali hydroxides. It is often used with calcium sulfate and sodium silicate for dental cements. AIPO4 is also used as a white pigment that also acts as a corrosion inhibitor. 

In the foregoing examples the spectral data indicated a Lewis acid-base reaction on the surface where the alkali and alkaline earth cations acted as the electron acceptors while the adsorbates were the electron donors. It is quite natural that the reverse situation might be possible that is, the adsorbent be basic while the adsorbate show acidic properties so that in the chemisorption electron transfer will occur in the reverse direction. Several examples of such adsorption have already been discussed in this chapter. Kortiim (22) found another example in the adsorption of symmetrical trinitrobenzene on magnesia and on alumina. Whereas trinitrobenzene adsorbed on calcium fluoride or silica was colorless, on magnesia it was red with an absorption maximum at 4650 A (Fig. 26) and the spectrum of the adsorbed species was very... 

Two reactions can convert alkali silicate glass into a solid mass with which contaminated sludges can be bound (1) by adding acid to form silica gel, whereby the evaporation of water does not occur (alternative to vaporization ) and (2) reaction with multi-valent metal ions (e.g., calcium chloride) while forming aqueous metal silica gel, where heavy metals are precipitated and are mechanically bonded into the gel structure. The CHEM-FIX-Process is primarily used in the United States for inorganic contaminants the Belgian SOLIROC-Process contains additives to make it usable for the solidification of organic wastes. 

The weathering of alkaline rocks is essentially a natural form of carbon dioxide capture and storage, but is a very slow process. Carbon dioxide can form stable carbonates through reaction with minerals that contain magnesium or calcium. A typical example would be the transformation of serpentine, a common silicate mineral, to magnesium carbonate, silica and water, i.e.. 

The geochemical balance of a 103 acre watershed underlain by silicate bedrock was investigated. Base flow composition of the stream water was essentially constanty but flood flows showed a decrease in concentration of silica, bicarbonate, and sodium and an increase in sulfate, magnesium, calcium, and potassium. Laboratory experiments indicate that fresh rock or soil reacts rapidly with distilled water and achieves a composition similar to the stream water, suggesting control of water composition by reaction with the silicate minerals. The aluminosilicate minerals react with CO charged water to form kaolinite, releasing cations and silica to solution. The products of weathering are removed as particulate matter (0,28 metric tons per year) and dissolved material (1,5 metric tons per year). 

Flux refers to the reaction of solid substances to form a melt. Calcium oxide is frequently used to react with silica, alumina and iron oxides to form a molten slag. 

Calcium fluoride (fluorspar) CaF2 is a very stable compound. The easiest way to decompose this is to mix the finely powdered sample with silica (or broken glass), and heat the mixture with concentrated H2SO4 in a platinum crucible. Silicon tetrafluoride gas is formed and can be detected (cf. Section 4.17, reaction 1). The residue contains calcium sulphate. This can be extracted with 5% sodium ethylenediamine tetraacetate in the presence of 2m ammonia, and calcium ions can be tested for in the solution (cf. Section 3.33). 

Other impurities can enter the slurry by being leached from the refractories themselves. Calcium and Magnesium are impurities that can have very deleterious effects on the stability of colloidal silica. Some refractories like Zircon are acidic in their reaction with colloidal silica and tend to reduce the pH over time. The stability of alkali stabilized colloidal silicas depend both on minimizing the presence of impurities (especially polyvalent metal ions) as... 

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