Lime Mortars

The lime, being slaked, is ready to be made into mortar. A lime paste is formed; and to this, sand is added. The sand prevents the shrinkage of the lime in drying out, just as it does in preventing the shrinkage of clay brick. The sand particles assist (by their coarseness) the action of the carbon dioxide in the air on the calcium oxide in the mortar. A very important advantage of the sand mixture is that it reduces the mortar cost.

Very pure limestones produce fat limes, almost white and slaking to an impalpable flour. Where silica, alumina, and other impurities are present, lean limes result. These are yellowish or grayish in color. They slake slowly.

If the limestone is nearly a pure cal cium carbonate, the lime is known as a high-calcium ]hie. If the limestone contains over 10 per cent of magnesia, the Hine is known as magnesian or dolomitic lime. The high-calcium limes are called hot or quick limes; and the magnesian limes are known as cool or slow. These terms refer to their action in contact with water. In slaking the hot limes, it is better that all the water should be added at once; with the cool limes, the water should be added only gradually. In the latter case, too rapid addition of water would cool the lime mass and stop the slaking, as much less heat is evolved in the breaking-down of magnesian limes. They set more slowly, but finally gain more strength, than the high-calcium limes.

The proper slaking of lime requires knowledge and close attention. Many mortars are low in bonding efficiency, owing to want of care in their making. Builders are regarding with increased favor the hydrated limes that are available. These are slaked to a dry powder in the factory, are perfectly slaked to maximum volume, and arc ready for immediate use.

A good lime should be free from impurities, such as cinders, etc., and should slake readily in water to a smooth paste and without any residuum.

The quality of the sand used in mortar-making has a marked effect on the strength of the product. In the specifications, the essentials demanded are expressed in the sentence: "The sand shall be sharp, clean, and coarse." Ocean and lake sands are to be preferred to glacial sands as a rule, the latter containing frequently 'large quantities of soft and easily decomposed constituents. considered desirable, because the ihgular grains of sand invite more ready adhesion to the lime or cement, and offer an arching resistance to compression. Sharp sands, however, present more voids in the mass; and efficiency of mix demands a greater proportion of lime or cement. As few sands have grains of marked sharpness, and the minimum degree of sharpness is not specified, the fulfilment of the specification in this regard is for the most part disregarded. The purchaser gives most attention to the sand's cleanness and coarseness. Sand, if clean, will not show to form lumps by compression in the hand while damp. If the sand does not dirty the hands to any marked degree during its handling, it may be considered clean. Foreign matter, such as wood chips, leaves, paper, etc., cannot be permitted in mortar sands. The presence of clay particles in the sand may he said to he injurious to rich mortars and cements, and beneficial to lean mortars, when in proper proportion, of course, to the degree of leanness.

Coarseness of sand insures easy mixing and the use of the least quantity of cementing material. As a general statement, the coarser sands produce the strongest mortars. The quality of mortar, the thoroughness of the mix, and the percentage of voids would all have to be taken into account in any special consideration of mortar strengths.