Lime, free

In clinkers and cements, expansive hydration of free lime produces popcorn or cauliflowerlike crystals of calcium hydroxide, called epezite, surrounding the original particle of free lime (see Brown and Swayze, 1938). Formation of epezite via air slaking normally causes clinker disintegration. 

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Mvailable lime, the total free lime (ie, CaO) content in a quicklime or hydrate, is the active constituent of a lime. It provides a means of evaluating the concentration of lime. 

Tricalcium phosphate, Ca2(P0 2> is formed under high temperatures and is unstable toward reaction with moisture below 100°C. The high temperature mineral whidockite [64418-26-4] although often described as P-tricalcium phosphate, is not pure. Whidockite contains small amounts of iron and magnesium. Commercial tricalcium phosphate prepared by the reaction of phosphoric acid and a hydrated lime slurry consists of amorphous or poody crystalline basic calcium phosphates close to the hydroxyapatite composition and has a Ca/P ratio of approximately 3 2. Because this mole ratio can vary widely (1.3—2.0), free lime, calcium hydroxide, and dicalcium phosphate may be present in variable proportion. The highly insoluble basic calcium phosphates precipitate as fine particles, mosdy less than a few micrometers in diameter. The surface area of precipitated hydroxyapatite is approximately... 

Other Phases in Portland and Special Cements. In cements free lime, CaO, and periclase, MgO, hydrate to the hydroxides. The in situ reactions of larger particles of these phases can be rather slow and may not occur until the cement has hardened. These reactions then can cause deleterious expansions and even dismption of the concrete and the quantities of free CaO and MgO have to be limited. The soundness of the cement can be tested by the autoclave expansion test of Portiand cement ASTM C151 (24). 

Industrial by-products are becoming more widely used as raw materials for cement, eg, slags contain carbonate-free lime, as well as substantial levels of silica and alumina. Fly ash from utility boilers can often be a suitable feed component, because it is already finely dispersed and provides silica and alumina. Even vegetable wastes, such as rice hull ash, provide a source of silica. Probably 50% of all industrial by-products are potential raw materials for Pordand cement manufacture. 

At atmospheric pressure, calcium carbonate almost completely calcines to free lime, and it is this that captures the sulfur dioxide. As the free lime is not completely sulfated, the resulting sorbent ash is veiy alkaline, consisting primarily of CaS04 and CaO, with small amounts of CaCO,3. 

In addition to the four compounds discussed above, the final Portland cement may contain gypsum, alkali sulfates, magnesia, free lime and other components. These do not significantly affect the properties of the set cement, but they can influence rates of hydration, resistance to chemical attack and slurry properties. 

Both sodium silicate and silico fluoride solutions are applied to clean, dry, sound concrete floors as dilute aqueous solutions (10-15 per cent solids) in two to three applications, taking care to ensure that all material penetrates and is absorbed into the concrete surface. The silicate or silico fluoride reacts with the small amount of free lime in the cement to form glassy inert materials in the surface, and the successful application of both materials depends upon filling the micropores in the surface of good-quality concrete, leaving its surface appearance and non-skid characteristics virtually unchanged. 

It is important to stress that neither sodium silicate nor silico fluoride will improve the performance of a poor, low-strength, dusty concrete floor and if the surface is too porous, there is no way that all the material applied can react with the relatively small quantity of free lime in the concrete surface. All that will happen is that the pores will be filled with non-reacted powder, producing a most unpleasant alkaline dust, which can be very irritating to the skin and eyes when the floor is put into service. 

Two absorbents are required, one for water vapour, the other for carbon dioxide. The absorbents for water vapour which are generally employed are (a) anhydrous calcium chloride (14-20 mesh), (b) anhydrous calcium sulphate ( Drierite or Anhydrocel ), and (c) anhydrous magnesium perchlorate ( Anhydrone ). Both (b) and (c) are preferable to (a) (c) absorbs about 50 per cent of its weight of water, but is expensive. Anhydrous calcium chloride usually contains a little free lime, which will absorb carbon dioxide also it is essential to saturate the U-tube containing calcium chloride with dry carbon dioxide for several hours and then to displace the carbon dioxide by a current of pure dry air before use. 

The potential for expansion because of free lime or magnesia in the steel slag is of particular concern, which could result in pavement cracking if ignored. It is recommended that no detectable soft lime particles or lime-oxide agglomerations be present.36... 

Free-flow electrophoresis, 9 752 Free lime, 75 27 hydration, 5 478... 

Conduction calorimetric curves of Portland cement hydrated isothermally containing various quantities of triethanolamine are shown in Fig. 5.3 [8]. On initial contact with water each sample evolves heat (not shown in figure) that can be attributed to heat of wetting, hydration of free lime and reaction of C3 A with gypsum to form... 

Type and amount of aluminates, calcium sulfate and free lime present in the mixture governs the rate at which ettringite forms. The presence of lime is reported to be essential for both the initial and subsequent stages of ettringite formation because it maintains a solution phase saturated with calcium ions [81]. Crystal habit, particle size and range of particle size in CSAs determine its rate of hydration and duration of expansion (Fig. 6.11). 

Rapid cooling of the clinker is preferred for many reasons, notably to prevent the reversion of alite to belite and lime in the 1100 1250 °C regime and also the crystallization of periclase (MgO) at temperatures just below 1450 °C. The magnesium content of the cement should not exceed about 5% MgO equivalent because most of the Mg will be in the form of periclase, which has the NaCl structure, and this hydrates slowly to Mg(OH)2 (brucite), which has the Cdl2 layer structure (Section 4.6). Incorporation of further water between the OH- layers in the Mg(OH)2 causes an expansion that can break up the cement. Accordingly, only limestone of low Mg content can be used in cement making dolomite, for example, cannot be used. Excessive amounts of alkali metal ions, sulfates (whether from components of the cement or from percolating solutions), and indeed of free lime itself should also be avoided for similar reasons. 

Tha question to be solved, therefore, ie—in what manner does the free lime prevent the decomposition that would otherwise ensue Many chemists explain the difficulty, by supposing that tho lime—CaO— enters into combination with the sulphide of calcium. In the first place, however, it must be borne In mind that this supposed compound has never yet been isolated, so that its precise composition cannot, therefore, be ascertained. 

The conclusion that combination of sulphide of calcium with lime does ensue, appoars to have beon arrived at, not from an analysis of tho pure compound, for, as mentioned above, it has never yet been obtained, but simply because, in the presence of free lime, the sulphide of calcium does not dissolve when treated with water. This is indeed tho only argument stated by Dumas to account for tho existence of his 2 Ca S Ca.O and certainly tho fact that an additional proportion of lime in the preparation of black ash does render the sulphide of calcium more insoluble, seems a power-... 

In one case, the fumigation of a church freshly painted with iron-free lime paint led to dark stains caused by the polymerization of hydrogen cyanide D. Grosser, E. Rofimann,... 

Some free lime (CaO) is usually present. Calcium arsenate is used extensively as an insecticide and as a fungicide. 

Calcination. Phosphate rock normally is used as a dry rock or in slurry form. However, in some cases, particularly where the raw phosphate is high in carbonaceous matter or it is desirable to have a clean acid, the rock is calcined. Also, in a few cases, the phosphate rock is calcined, the product slaked, and free lime separated as a beneficiation step. Calcination is energy intensive and produces a less reactive rock and, in some cases, a less filterable gypsum. Therefore, the use of calcination is diminishing, and is being replaced by a wet oxidation step to produce green acid.16 In separating calcium carbonate, flotation, where it is successful, is favored over calcination because of its lower cost. 

The definition of LSF is theoretically based (Section 2.3.3), and in the form given above applies to clinkers if corrected by subtracting O.7SO3 from CaO, it may be applied to cements. It largely governs the ratio of alite to belite and also shows whether the clinker is likely to contain an unacceptable proportion of free lime, a value of 1.0 or above indicating that the latter will be present at equilibrium at the clinkering temperature and thus liable to persist in the product. In practice, values up to 1.02 may be acceptable typical values for modern clinkers are 0.92-0.98. Other parameters similar in... 

In another approach, widely used in the USA and elsewhere, the quantitative phase composition is estimated using a procedure due to Bogue (B24). It is necessary also to know the content of free lime, which may be determined by a chemical extraction method (Section 4.3.3). The calculation is as follows ... 

Deduct from the CaO content the amounts attributable to C AF, CjA and free lime, and solve two simultaneous equations to obtain the contents of CjS and CjS. 

This leads to the following equations, in which CaO is assumed to have been corrected for free lime ... 

As with LSF, the approach is applicable to cements if CaO is further corrected by deducting O.7SO3. Because minor oxide components are ignored, the total for the four main phases plus free lime will not add up to 100%. It is implicit in the approach that all the MgO is assumed to occur as periclase. The phase composition calculated by Bogue s method is related to the LSF in that a ratio of 38 to CjA 0.546, calculated without correcting the CaO content for free lime, corresponds to an LSF > 1.0 (D2). 

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