Common Lime

COMMON LIME.

Lime is produced by heating a pure or nearly pure limestone in a kiln to such a temperature as will drive off the carbon dioxide and leave calcium oxide. When fresh lime is brought into contact with water it will rapidly absorb nearly a quarter of its weight of that substance. This absorption is accompanied by a great rise of temperature, by the evolution of a hot and slightly caustic vapor, by the bursting of the lime into pieces; and finally the lime is reduced to a powder, the volume of which is from two and a half to three and a half times the volume of the original lime—the increase of bulk being proportional to the purity of the limestone. In this condition the lime is said to be slaked, and is ready for use in making mortar.

On exposure to the air a paste of lime absorbs carbon dioxide, the oxide of calcium slowly changes back to carbonate, and the mortar sets or hardens.

In making mortar, sand is mixed with lime paste for three reasons: (1) to prevent shrinkage of the paste through the drying out of the water; (2) to cheapen the resultant product; and (3) to subdivide the lime paste into thin films so that the carbon dioxide in the air may have better access to the calcium oxide. Lime mortar hardens very slowly, owing to the small amount of carbon dioxide that the calcium oxide can absorb from the air; and what is more important from a builder's point of view, the mortar in the interior of the wall never fully hardens, as only the exposed portions have an oppor tunity to absorb carbon dioxide. (Lime does not harden at all under water.) It is probable that a certain amount of chemical action takes place between the lime and the sand, as the strength of sand lime brick (see 1 86) is chiefly due to such action; but at atmospheric pressure and temperature, this action is inappreciable and is of no practical importance in construction.

Fat vs. Lean Lime.

If the limestone is nearly pure, the resulting lime will be nearly white, and will slake to an unctuous paste that is impalpable to both sight and touch; and hence such lime is called a fat or rich lime. If the limestone contains considerable impurities, as silica, alumina, iron oxide, etc., the resulting lime will

not be white, but will vary from a yellowish white to a gray or a brown, according to the amount and kind of the impurities present. Such a lime is known as a lean or meager lime, and exhibits a more moderate rise of temperature, evolves less vapor, slakes more slowly, seldom reduces to an impalpable powder, yields a thin paste, and expands less than a fat lime.

High-0alcium vs. Magnesian Lime.

If the limestone is nearly a pure calcium carbonate, it will yield nearly pure calcium oxide, and the product will be known commercially as a high-calcium lime; but if the limestone should contain any considerable quantity of magnesium carbonate, the resulting lime will be a mixture of the oxides of calcium and magnesium, and will be known commercially as magnesian or dolomitic lime. Magnesian limes usually slake more slowly, evolve less heat, expand less, set more slowly, and make a stronger mortar than the high-calcium limes. The differ ences between these two classes of limes vary with the amount and nature of the constituents and with the temperature at which each is burned. The two classes of lime shade gradually one into the other; but commercially any lime containing less than 10 per cent of magnesium oxide is known as pure or high-calcium lime, and a lime containing more than 10 per cent of magnesium oxide is known as magnesian or dolomitic lime. The high-calcium limes are known as "hot" or "quick" limes; and the magnesian limes, as "cool" or "slow" limes.

Table 10 shows the relative strength of the two kinds of lime. Notice that the magnesian lime sets more slowly, but finally gains greater strength than the high-calcium lime.

The results in Table 10 are for. briquettes (§ 167) 1 inch square, with all sides exposed to the air; and consequently in actual practice lime mortar does not gain its strength as rapidly as shown in Table 10. Further, the briquettes were composed of 1 volume of slaked lime to 2 volumes of sand, and were probably comparatively porous; and therefore, for this reason also, the briquettes gained strength faster than lime mortar is likely to do in practice.