Lime-silica reaction

Silica and Alumina. The manufacture of Pordand cement is predicated on the reaction of lime with siUca and alumina to form tricalcium sihcate [12168-85-3] and aluminate. However, under certain ambient conditions of compaction with sustained optimum moisture content, lime reacts very slowly to form complex mono- and dicalcium siUcates, ie, cementitious compounds (9,10). If such a moist, compact mixture of lime and siUca is subjected to steam and pressure in an autoclave, the lime—silica reaction is greatiy accelerated, and when sand and aggregate is added, materials of concrete-like hardness are produced. Limestone does not react with siUca and alumina under any circumstances, unless it is first calcined to lime, as in the case of hydrauhc lime or cement manufacture. 

Although hydration under hydrothermal conditions may be rapid, metastable iatermediate phases tend to form, and final equiUbria may not be reached for months at 100—200°C, or weeks at even higher temperatures. Hence, the temperatures of formation given ia Table 6 iadicate the conditions under saturated steam pressure that may be expected to yield appreciable quantities of the compound, although it may not be the most stable phase at the given temperature. The compounds are Hsted ia order of decreasiag basicity, or lime/siHca ratio. Reaction mixtures having ratios C S = 1 yield xonotHte at 150—400°C. Intermediate phases of C—S—H (I), C—S—H (II), and crystalline tobermorite ate formed ia succession. Tobermorite (1.13 nm) appears to persist indefinitely under hydrothermal conditions at 110—140°C it is a principal part of the biader ia many autoclaved cement—silica and lime—silica products. 

These ate the main reactions ia Pordand cements because the two calcium siHcates constitute about 75% of the cement. The average lime—silica ratio (C S) may vary from about 1.5 to about 2.0 or even higher, the average value is about 1.7. The water content varies with the ambient humidity, the three moles of water being estimated from measurements ia the dry state and stmctural considerations. As the lime—silica ratio of the C—S—H iacreases, the amount of water iacreases on an equimolar basis, ie, the lime goes iato the stmcture as calcium hydroxide, resulting ialess free calcium hydroxide. 

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,... 

AR = alumina ratio (alumina modulus). ASR = alkali silica reaction. LSF = lime saturation factor. SR = silica ratio (silica modulus). C, = analytical (total) concentration of x, irrespective of species, [x] = concentration of species x. x = activity of species x. RH = relative humidity. =... 

Portland cement is a finely ground, powdered mixture of compounds produced by the high-temperature reaction of lime, silica, alumina, and iron oxide. The lime (CaO) may come from limestone or chalk deposits, and the silica (Si02) and alumina (AI2O3) are often obtained in clays or slags. The blast furnaces of steel mills are a common source of slag, which is a byproduct of the smelting of iron ore. 

Radiation sources including X-rays, y-rays, and ultraviolet produce preferential light absorption at color centers in glass by formation of free electrons and holes. These are trapped at a defect such a vacancy, an interstitial atom, a multivalent impurity, or a nonbridging oxygen T The types of color center induced in siliea, soda-lime-silica, borate, and phosphate glasses are elucidated by optical and electron spin resonance studies of irradiated samples. Table 1 summarizes composition variables and reaction types that induce damage. 

Another important criterion in the manufacture of high-purity DB MgO is the CaO/Si02 ratio. It is desirable to have a lime/silica ratio >2 1 as this reaction leads to the formation of dicalcium silicate bonds see Figure 9.2. 

The alkali silicate glasses are easily corroded by aqueous acids and the main reaction is an exchange between H3O+ of the solution with the alkali M+ of the glass the corrosion rate increaseswith the ionic radii ofM+, that is, K > Na" " > Li" ". The chemical durability is greatly increased by addition of divalent CaO or trivalent AI2O3 oxides, leading to commercial soda-lime-silica compositions. 

Wang, F.F., and F.V. Tooley. (1958). Detection of reaction products between water and soda-lime-silica glass. Amer. Ceramic Soc., 41(11), 467-469. 

In the temperature range between -323 K and 423 K, it is found that the time to fracture and temperature follow an Arrhenius relation. The activation energy for soda-lime-silica glass was found to be equal to 18.1 kcal/mole. This kind of behavior shows that the static fatigue is a temperature-dependent activated process, similar to any chemical reaction or diffusion. 

Boron carbide from boron oxide and carbon Calcium silicate from lime and silica Calcium carbide by reaction of lime and carbon Leblanc soda ash... 

Technology Description Waste solidification techniques based on lime products usually depiend on the reaction of lime with a fine-grained silica... 

High levels of silica in the raw water supply can lead to serious risks of deposition in boilers, especially if cycles of concentration (COC) also are high. The incoming silica can be reduced by adsorption on magnesium hydroxide [Mg(OH)2] precipitate during lime-softening processes, or by the addition of magnesium hydroxide in a reaction tank, followed by filtration. 

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