Portland Cement

Storage of Cement. Since cement readily absorbs moisture from the atmosphere, becom ing lumpy or even a solid mass—in which con dition it is worthless and must be discarded—it must be stored in a dry place. If an attempt is made to break up lumpy cement and use it, it will be found to have lost most of its adhesive power and hardening qualities, and so to have lost the greater part of its strength and value as a building material.

In the store-room, the cement should not be piled directly on the floor. Blocks should be laid on the floor, and planks placed over them, thus providing a platform slightly raised above the store-room floor. On this platform, the cement should be piled, and a covering of canvas or roofing paper should be thrown over the pile.

If properly stored, where there is no absorp tion of moisture sufficient to harden it, Portland cement can be carried from one season to an other without deterioration. Some makes, in fact, have shown improvement. Long storage of cement tends to retard its setting, though not preventing its ultimate acquisition of full hard ness and strength.

Setting and Hardening.

When cement is ready to be used, and is mixed with a sufficient amount of water to bring it to a paste, it soon loses its plastic nature, and finally reaches a point at which it can no longer be disturbed without producing a rupture. This change of condition is known as setting, and should be dis tinguished from the hardening of the mixture. Setting is usually complete within a few hours, whereas hardening may continue for months or even years. A further distinction is made be tween initial setting and final setting. The time required for setting will depend largely on atmospheric and weather conditions, as setting proceeds more slowly in cold weather than in warm. The initial set generally takes place within thirty minutes, and final setting within four to five hours.

General Requirements.

The general require ments of a good Portland cement may be very briefly stated thus: The final set should take place within a few hours after mixing. The cement should have a certain tensile and com pressive strength, and a considerable part of its ultimate strength should be developed within a few days. Age should bring no deterioration or loss of strength in the concrete made from the cement, but rather an increase of strength. And even under conditions of moisture and temper ature far more severe than any that are ever likely to be met, the cement should demonstrate a quality of permanency; that is, the tendency to shrink or swell or be distorted or disintegrate should be so small as to be a negligible quantity.

The possession or lack of these requisite qualifications on the part of any particular brand or shipment, can be determined only by a thorough testing.

Keene's Cement Keene's cement is a special form of gypsum plaster which has certain unique and valuable properties. It is much harder than ordinary gypsum plaster, will take a good polish, and can be washed with damp cloths without injury. For these reasons it is often used for wainscot ing, column coverings, casts, mouldings, etc., intended for places of unusual exposure, com mon gypsum plaster being hard enough for all ordinary conditions. Hydrated lime is often present in the American product, but imported Keene's cement is generally pure.

This form of cement is made by saturating lump gypsum which has been subjected to intense heat, in a solution of one part alum to thirteen parts water. The rock so treated is ground, then treated again with the same alum solution, and finally ground again.