Quicklime is produced by burning limestone in a kiln, the carbonic acid is driven off, and the result is quicklime.

Slaking is effected by thoroughly wetting a quicklime and covering it up. It then swells, becomes hot, gives out puffs of steam and falls to powder, which is called slaked lime.

The slaking process is very violent with rich limes, less so with poor limes, and very slight in the case of hydraulic limes.

Setting

When a lime or cement is made with water into a pat, and exposed to the air, it will harden less or more according to its quality, until in most cases it becomes quite hard throughout its bulk. With hydraulic limes and cements the hardening will take place even better if the pat is placed under water.

Pure, Rich, or Pat Lime is that produced from pure limestones, such as marble or chalk, containing nothing but carbonate of lime. Such a lime slakes furiously, but a pat made from it will never thoroughly set or harden, even in the air, and if placed under water it will simply dissolve away. Rich limes cannot, therefore, make good mortar or concrete, but are the best for whitewashing and sanitary purposes.

Poor Lime is from limestone containing useless impurities, and it shares all the defects of rich limes.

Hydraulic Limes are produced from limestones which contain from 5 to 30 per cent of clay in a peculiar form. They slake with more or less difficulty, but will set, becoming quite hard in air or under water, and are therefore adapted for making good mortar and concrete.

Test

To ascertain whether a limestone is hydraulic, it should be made red hot, to drive off the carbonic acid. The resulting quicklime should be slaked, made up with water into a pat, and then placed under still-water, to see if it will set there. If it does not set, but dissolves or becomes disintegrated, it will show that the lime is not hydraulic.