The subject is one almost too intricate for a treatise of such an elementary character as this, in which much chemical knowledge cannot be presupposed. Nevertheless it is touched upon, for without some idea of the principles involved, all dealing with these important materials must be conducted entirely by rule of thumb, or guess work.
A very slight acquaintance, however, with the changes that take place during the different operations will enable the student more easily to remember, and more intelligently to avail himself of, the several characteristics of different limes and cements.
From the experiments made with limes of which the composition is accurately known, it is evident that the differences in their slaking and setting properties are due to the nature and proportion of the foreign constituents they contain. These are chiefly clay and magnesia. Pure or fat limes contain none of these foreign constituents.
Pure carbonate of lime1 contains nothing but lime, carbonic acid, and water. When it is calcined the carbonic acid and water are driven off by the heat, and pure quicklime remains.
Such a quicklime, when slaked, shows very violent action, great heat is evolved, the mass is greatly swollen, and thoroughly disintegrated.
The residue left after slaking is soluble in water, and has within itself no constituent which will enable it to solidify, except to a very slight extent It is therefore constantly soft when in a moist situation, and will dissolve under water.
Such portions of its surface, however, as are exposed will imbibe carbonic acid from the air, and will be reconverted into a crust of carbonate of lime, as before described (p. 147).