Pozzolan

POZZOLAN. Pozzolan is a term applied to a combination of silica and alumina which, when mixed with common lime and made into mortar, has the property of hardening under water. There are several classes of materials possessing this property.

Pozzolan proper is a material of volcanic origin, and is the first substance known to possess the peculiar property of hydraulicity. The discovery was made at Pozzuoli, Italy, near the base of Mount Vesuvius,—hence the name. Vitruvius and Pliny both mention that pozzolan was extensively used by the Romans before their day; and Vitruvius gives a formula for its use in monolithic masonry, which with slight variations has been followed in Italy ever since. It is as follows: " 12 parts pozzolan, well pulverized; 6 parts quartz ose sand, well washed; and 9 parts rich lime, well slaked." Trass is a volcanic earth closely resembling pozzolan, and is employed in substantially the same way. Arenes is a species of ocherous clayey sand that makes a fair air-lime mortar by adding water, and by adding also fat lime it makes a fair hydraulic cement.

Slag Oement.

Slag cement is by far the most important of the pozzolan cements. It is sometimes, but inappropriately, called pozzolan cement. It is the product obtained by grinding together. powdered slaked lime and granulated blast-furnace slag, without previous calcination. This cement is likely to contain so much sulphur in the form of sulphides as to make it unfit for use in the air, since on exposure the sulphides change to sulphates and, in so doing, expand; and hence such cements are liable to destroy the structure. But slag cement can safely be used under water and generally in posi tions where constantly exposed to moisture, as in foundations, sewers, etc., and in the interior of heavy masses of masonry or concrete. It is claimed that slag cement will not stain the stone laid with it.

Slag cement was formerly made at nine works in seven States, but in recent years most of the mills have been idle, partly because of the increased manufacture of portland cement from limestone and clay, and partly because of the use of the blast-furnace slag in the manufacture of a true portland cement.

A careful distinction should be made between slag cement as defined above and a portland cement made by calcining a mixture of slag and lime. Notice that the slag cement is made simply by mixing and grinding blast-furnace slag and hydrated lime, while the portland cement is made by mixing, calcining, and grinding the slag and the lime. Cement made in the last way differs in no material respect from portland cements made of limestone and clay.

Slag cement may be known in its powdered form by a light lilac color, by the absence of grit (due to fine grinding and the slaked lime), and by its low specific gravity (see 136); and in the form of hardened mortar it may be known by the intense bluish-green color in the fresh fracture after long submersion in water, due to the presence of sulphides, which color fades if exposed to dry air.

Cement is generally sold by the barrel, although not necessarily in a barrel. Imported cement is always sold in barrels, but domestic cement is usually sold in bags, sometimes in barrels, less frequently in bulk.

Portland cement usually weighs 400 pounds per barrel gross, and 376 pounds net. A bag of portland usually weighs 94 pounds, of which four are counted a barrel.

Natural cement made in or near Rosendale, N. Y., formerly weighed 318 pounds per barrel gross, and 300 net. Cement made in Akron, N. Y., Milwaukee, Wis., Utica, Ill., Louisville, Ky., formerly weighed 285 pounds per barrel gross, and 265 net. Cloth bags usually contain one third, and paper bags one fourth of a barrel.

Slag cement formerly weighed from 325 to 350 pounds net per barrel.

In 1904 most of the national engineering societies adopted specifications for the reception of cement, which require that a bag of cement shall weigh 94 pounds net, and that four such bags shall constitute a barrel of portland and three a barrel of natural.

Portland Cement.

In 1905 the average selling price of portland at the mills was 94 cents per barrel in cloth bags, and the average price in the Lehigh Valley district was 81 cents, not including the cost of the bag which may be returned. The price in paper bay: is usually about 5 cents per barrel more than in cloth; and the price in wood is about 15 cents per barrel more than in cloth.

Of course the 'cost to the consumer includes freight, but during the above year portland cement could be had in car-load lots at almost any place in the upper Mississippi Valley for $1.50 to $1.75 per barrel in cloth.

Natural Cement.

In 1905 the average price of natural cement at the mill was 54 cents in cloth, not including the cost of the bags, the lowest average for a State being 40 cents and the highest 69 cents.* In localities where there was sharp competition between different natural-cement manufacturers, or between natural and portland cement, natural cement has been sold at the remarkably low price of 60 cents per barrel in paper bags, including freight for 100 to 200 miles.

Slag Cement.

In 1905 slag cement sold at the mill at from 71 to 76 cents per barrel.