Lime; Cement, and Plaster of Paris

Good lime is nearly pure calcium oxide; it is one of the most important substances used in chemical industry, and is prepared in enormous quantities by calcining calcium carbonate at a bright red heat. If the carbonate used (limestone, chalk, or the shells of mollusks) contains much silica, iron, alumina, or other impurity, the lime does not slake freely with water, and is said to be "poor" or "lean." If but small quantities of these impurities are present, a fair lime is produced, when properly burned. But such impure carbonates are very difficult to burn, since slight overheating causes semi-fusion of the lumps, and the lime combines with Witt'3~'~iery slowly and incompletely, and is said to be "burned to death." A pure lime, which combines readily with water to form a fine white powder, free from grit, and which makes ,a smooth stiff paste with an excess of water, is called a "fat" lime.
Calcium carbonate begins to decompose below a red heat into calcium oxide and carbon dioxide, but the decomposition is not complete until a bright red heat (800° or !")QO° C.) is reached. The temperature should not rise much above 1000° to 1200° C.; as there is danger of overheating the lime. For successful burning, it is essential that the gases escape freely from the kiln, the draught usually being sufficient to remove them as they form. This escape may be acceleritted by blowing stemn or air into the kiln during the burning, or even by wetting the carbonate as it is introduced. If the gases are retained, they cause pressure in the kiln and thus hinder the decomposition; and on cooling, the carbonic acid recombines with the lime.
There are two general classes' of limekilns, continuous and periodic. (See Calcination, p. 18.) The former are preferred where fuel is expensive, and where a large regular output is desired. They are tall, narrow furnaces (shaft kilns), built of brick or of iron plates, and vary much in size, but are usually from 40 to 45 feet the lime taken out at the bottom, without interrupting the process. Figure 56 shows a furnace £01' burning with "long flame." The fuel is burned on the grate (A), and only the flames and combustion gases pass through (B) to come in contact with the charge in the shaft. The ashes fall into the chamber (D), and are thus kept separate from the lime, which is withdrawn through (C) at regular intervals, causing a slow descent of the charge. Thus a clean lime is obtained.
In the continuous kilns which use the "short flame," the carbonate and fuel are charged alternately at the top, and the lime, contaminated with ashes, is taken out at the bottom. Less fuel is needed than for "long flame burning."
Fuel gas for lime burning has been successfully introduced in m:1llYplaces. This gives a very clean lime, burned at a constant temperature.
In this country, long-flame, periodic kilns are generally used. These require much fuel and time, but are probably preferred because of the simplicity and cheapness of building. They are made of brick or of large blocks of limestone. Two or three feet from the ground an arch (A) (Fig. 57), of large blocks of limestone, is turned, numerous small openings being left, through which the flames may pass into the interior of the kiln. The fire is built under the arch, and on top of the latter the limestone is piled, the lumps varying in size from that of a cocoanut, just above the arch, to that of a goose egg at the top of the kiln. When the during six or eight hours, to prevent the limestone arch from crumbling; then the temperature is kept at a full red heat for two days or more, when the fire is allowed to burn out, and the kiln cools. During the time of cooling, discharging, and recharging, the kiln stands idle, and thus much time is lost. :J\Ioreover, a large amount of fuel is necessary to heat the walls of the kiln after each recharging, so that the method is not an economical one.
Excepting the limekilns in the ammonia-soda works, no attempt is made in this country, to save the carbonic acid gas which escapes from the top of the kiln. But in Europe the gas is often collected and used for technical purposes.
:Freshly burned lime is usually called :, caustic" lime or "quicklime," because of its corrosive action on organic matter. 'When pure, it is white and amorphous, but iron gives it a yellow or brown color. The crystalline limestones and pure marble yield the best lime. Owing to the loss of water, organic matter, and carbon dioxide during the burning, there is great reduction in the weight of the charge, but only a slight decrease in its volume. As a rule, 100 pounds of good limestone yield about 57-50 pounds of lime, but the shrinkage in bulk is not over 10-15 per cent of the original volume of the limestone. Nor is there much change in the hardness, though lime is much more porous than the limestone, and absorbs considerable water before slaking.
Pure lime is infusible at the temperature of the oxy-hydrogen flame (hence its use in the "calcium light "), but if silica, iron, alumina or other substance is present, the lime combines with it to form a fusible substance (glass or slag). Considered chemically, lime is calcium oxide, and is a powerful base. It combines with acids to form calcium salts; it has great affinity for water, and when wet the lumps expand and fall to a powder of calcium hydroxide (slaked lime), with the C\'olution of much heat, especially in the case of "fat lime." ~When exposed to the air lime absorbs carbon dioxide and moisture, and soon falls to a powder called" air slaked lime," consisting of a mixture of calcium carbonate and hydroxide. Lime for mortar and many other purposes is always slaked immediately before use.
If the limestone contains more than from 8 to 10 per cent of silicate of aluminum (clay), and is burned at a moderate temperature, " hydraulic lime" is obtained. This does not slake freely, and if kept in contact with water after slaking it soon hardens again. This hardening is due to a secondary reaction between the water and the anhydrous silicates and aluminates of calcium, which have been formed in the burning, and which combine with the water to form hydrated compounds, having considerable hardness. Hydraulic limes are chiefly employed in mortar and cement mixtures. Since much heat is liberated in the slaking of lime, the storage and shipment is attended with some danger. If water comes into contact with the lime in presence of combustible material, fire is very apt to ensue.
The uses of lime in the arts are too numerous for extended mention, but the following are a few of the most important: in mortar and cement mixing; in bleaching powder; in the Leblanc soda process; for purifying illuminating gas; in the preparation and purification of many chemicals, such as acetic, citric, oxalic, and tartaric acids, potassium chlorate, caustic soda and potash, etc.; for purifying sugar solutions; in bleaching and dyeing cotton; in tanning; in glass making; in metallurgical operations; for disinfecting, etc.
Mortar is an aqueous pasty mixture of slaked lime, sand, and other materials, which dries without excessive shrinkage and becomes hard on exposure to the air, owing to absorption of carbon dioxide and formation of carbonate of lime. It will not harden while it remains wet, and this is one of the chief differences between mortar and cement. The hardening of the two substances is due, in part at least, to different causes.
If a paste of freshly slaked lime is allowed to dry by exposure to the ail', it shrinks considerably, and if in thick masses, numerous qracks are formed. '1'he admixture of three or four volumes of sharp sand prevents this shrinkage by separating the lime paste into very thin layers, which fill the spaces between the grains of sand. The sand also gives the mortar a porous structure, which facilitates the penetration of the carbon dioxide during the hardening period. The interlacing crystals of calcium carbonate enclose the sand grains and join them together, thus increasing the hardness and strength of the mortar. This addition of sand also cheapens the mortar by increasing the mass obtained from a given amount of lime. "Fat lime" requires a much larger proportion, which is replaced in part by the impurities in "poor" lime.
For a good mortar it is very necessary that the lime be thoroughly slaked. The proper quantity of water should be added all at once, or the product is apt to be granular and lumpy. The mass is covered with a layer of sand, 01' with boards or cam"as, to retain the heat and moisture, and should not be stirred while slaking, but should be allowed to swell and fall to powder without disturbance. Water is then added, and the paste allowed to stand for several days, or e\'en weeks, well protected from the air, before being stirred up with more water for use in mortal'.
The first change noticeable in a mortar is the" set," which is a solidification of the mass, due to the loss of its water through evaporation or absorption by the bricks, etc. But it is not until after the mass becomes dry that the real haruening begins. This is very slow, since it progresses from without towards the interior of the mass; and the surface layer of calcium carbonate first formed is but slowly penetrated by carbon dioxide from the air. The interior of thick walls will often show an alkaline reaction after the lapse of a century or two, but after twenty-five years the change is very slight under ordinary conditions. After several hunched years there appears to be a certain amount of combination between the silica of the sand and the calcium carbonate to form a hydrated silicate of calcium. This secondary reaction docs not increase the hardness of the mortar. Hardening is a true chemical change, and should not be too rapid for the best results. In order to hasten the hardening of mortar and plastering in new houses, builders sometimes build coke or charcoal fires in open grates or baskets. But this is liable to cause uneven drying and excessive shrinkage, resulting in cracks or scaled places. In certain mortars hair or other fibrous material is added to increase the toughness, especially while wet.
Since mortar does not harden until dry, it should never be used in damp places, such as foundations and cellars, nor in very thick walls. Sometimes it is mixed with some cement, increasing its strength and usefulness. When thoroughly Iml"llened, good mortar is about as hard as limestone, and adheres firmly to the bricks or stones of the wall.