Hydraulic Cement

HYDRAULIC CEMENT The most important cements are the prod ucts of the calcination of an argillaceous limestone, i.e., are a com bination of lime, silica, and alumina (see § 102). Such cements may be divided, according to the method of manufacture, into three classes, viz.: portland cement, natural cement, and pozzolan. The first two differ from the third in that the ingredients of which the first two are composed must be roasted and then pulverized before they acquire the property of hardening under water, while the ingredients of the third need only to be pulverized and mixed with water to a paste.

Whenever cement is referred to as a material of construction, one or the other of the above kinds is nearly always intended; but there are two other forms of hydraulic cement that are of enough interest to warrant a brief mention here.

Cemen

t. A cement has recently been made by using limestone and iron ore instead of limestone and clay. It is known as iron-ore cement. Such cement if ground to the fineness of commercial portland is more slow-setting, but ultimately attains . greater strength than portland cement or any of the limestone-clay cements mentioned in the precgding section. If iron-ore cement is ground very much finer than ordinary portland cement, it becomes as quick-setting as the latter, and, besides, contains 70 to 80 per cent of active material as against the 30 to 40 per cent in ordinary port land cement (§ 139). In this respect iron-ore cement is much su perior to portland, since if portland cement were ground extremely fine it would become too quick-setting for practical use. It is

not yet proved that with present grinding machinery it is eco nomical to grind iron-ore cement fine enough to make it as quick setting as portland cement. If the finer-ground iron-ore cement can he made commercially, it will make possible the use of either a stronger mortar or a leaner mortar than at present. Iron-ore cement is superior to portland also in that it is not injured by immersion in saa water.

Iron-ore cement has been manufactured on a commercial scale for a few years past in Hamburg, Germany; and the claim is that it will soon be made in the United States.

Magnesia Dement.

A mixture of magnesium oxide and magnesium chloride makes the strongest hydraulic cement known. This discovery was made in about 1853 by Sorel, a French chemist; and the cement is known as Sorel or magnesia cement. The mag nesium oxide, or magnesia, is prepared either by calcining magnesite, a comparatively rare material, or from sea-salt. The cement is made by wetting the pulverized magnesium oxide with bittern water, the refuse of sea-side salt works, which contains magnesium chloride.

Magnesia cement was used about 1870 to a considerable extent in making emery wheels and in a small way in making artificial stone, Sorel stone (0 529) ; but at present it seems not to be in use owing to its great cost, quick setting, and lack of durability.