686 CEMENTS, LIMES, AND PLASTERS. 



in order to distinguish them from the slag bricks made by mixing granu- 

 lated slag with slaked lime, which have been discussed in previous sec- 

 tions of this chapter. 



Slag blocks, if properly made, are stronger than slag bricks. They 

 are, however, impervious to air and moisture; and on that account 

 are not good building materials, for dwellings constructed of them are 

 apt to be damp and unhealthful. Their chief uses are for foundations 

 or for paving blocks, for the latter of which they are particularly well 

 adapted. 



Many smelters and furnaces have made small amounts of slag blocks 

 for local use. For the past thirty years or so a considerable quantity 

 have been made in the Lehigh iron district of Pennsylvania, their earliest 

 recorded use being in the slag-block pavements laid in Philadelphia 

 about 1876. 



The properties required in a slag block to be used for paving work 

 are density, resistance to abrasion, toughness, and roughness of surface. 

 These properties are found to vary with the chemical composition of 

 the slag, the rapidity with which the slag is allowed to cool, and the 

 character of the moulds used. By properly varying the last two factors, 

 slags of almost any composition can be utilized in this industry. 



The three requisite properties first mentioned i.e., density, resist- 

 ance to abrasion, and toughness vary directly with the rate of cooling, 

 the slowly cooled blocks being the best. Blocks cast in sand molds 

 and heavily covered with loose sand, cool very slowly, and give very 

 much better results than those cast in iron molds. Slowness in cooling, 

 however, requires much greater storage space than if rapid cooling is 

 practiced; and casting in sand molds demands a higher grade of work- 

 manship than casting in iron molds. 



The roughness of surface or non-slipperiness of blocks intended 

 for paving use is highly important, especially as slipperiness has been 

 the chief defect charged against slag blocks, which defect is prevalent 

 in blocks cast in iron moulds. In English practice it has been overcome 

 by casting the block in a double size mold, having a projection inside 

 which results in a notch on the slag block. The block is, after coating, 

 split apart at this notch, and the rough fracture-surface of each half 

 is laid uppermost in paving. This method of avoiding slipperiness adds 

 considerable to the labor cost of the blocks, and is therefore not well 

 adapted to American practice. Slag blocks cast on a sand bed are 

 free from the defect noted (slipperiness); or at least it can be avoided 

 if sufficiently coarse sand be used. 



Slag blocks manufactured by the Tees Scoria Brick Company, of 



