108. Hydraulic limes are those containing, after burning, enough lime to develop, more or less, the slaking action, together with sufficient of such foreign constituents as combine chemically with lime and water, to confer an appreciable power of setting under water, and without access of air.

The process of setting is entirely different from that of drying, which is produced simply by the evaporation of the water. Setting is a chemical action which takes place between the water, lime and other constituents, causing the paste to harden even when under water.

Hydraulic lime or cement should not be used after it has commenced to set, as the setting will not take place a second time and the strength of the mortar will be lost.

In the great majority of natural hydraulic limes commonly used for making mortar, the constituent which confers hydraulicity is clay, although silica also has the same effect.

Hydraulic limes containing clay may be arranged in three classes, according to their amount of hydraulic energy :

1. "Feebly hydraulic - containing 10 to 20 per cent, of impurities. This slakes in a few minutes, with crackling, heat and emission of vapor. If made into a paste and immersed in water in small cakes, it will harden so as to resist crushing between the thumb and finger in from twelve to fifteen days.

2. "Ordinary hydraulic - containing 17 to 24 per cent, of impurities. Slakes after an hour or two, with slight heat and fumes, without crackling. Sets under water in six or eight days.

* William Wallace, Ph.D., F. R. S E., in London Chemical News, No. 281.

3. "Eminently hydraulic - containing at least 20 per cent of impurities. Slakes very slowly and with great difficulty, with slight heat. Sets under water in twelve to twenty hours and becomes hard in two to four days." *

Artificial hydraulic lime can be manufactured by mixing together, in proper proportions, thoroughly slaked common lime and unburnt clay, then burning and grinding in much the same manner as in the manufacture of Portland cement; but as the process of manufacture is nearly as expensive as for making Portland cement, it is more profitable to make cement, on account of its superior hydraulic energy.

No hydraulic lime is manufactured, artificially, in the United States, and but very few hydraulic limes are in use.

A gray lime is obtained at Morrison and a few other locations in Colorado which hardens under water and makes very strong mortar. It is also sometimes used for making concrete.

A very simple experiment will determine if a lime is hydraulic or not: Make a small cake of the lime paste, and after it has commenced to stiffen in the air, place it in a dish of water so that it will be entirely immersed. If it possesses hydraulic properties it will gradually harden, but if it is not hydraulic it will soften and dissolve.

Hydraulic lime mortar is made in the same way as common lime mortar, care being taken to use sufficient paste to coat all grains of sand and to fill up the voids between them.

109. Pozzuolanas is a name given to certain clayey earths containing 80 to 90 per cent, of clay, with a little lime and small quantities of magnesia, potash, soda, oxide of iron, or manganese.

When finely powdered in their raw state and added to lime mortar they confer hydraulic properties to a considerable degree.

Natural Pozzuolana is a naturally-burnt earth of volcanic origin found at Pozzuoli, near Vesuvius, and in the caverns of St. Paul, near Rome. It is found in the form of powder, and when sifted is used all along the Mediterranean coast for making hydraulic mortars.

Beton (similar to concrete) as prepared in that region is generally made of Pozzuolana, lime and aggregates in the following proportions:

Pozzuolana...................................

12

parts.

Sand.................................................

6

"

Good quicklime..................................

9

"

Small stones...................................

13

"

Ground slag...........................

3

"

* Ira O. Baker in "Masonry Construction."

Pozzuolana is not used in this country, but as the name is frequently found in books on masonry construction, it is well for the young architect to know what it is.

Brick dust, mixed with common lime, produces a feebly hydraulic mortar, and adds materially to its strength.