676 CEMENTS, LIMES, AND PLASTERS. 



and for the local market, but apparently no attempt has been made to 

 extend the manufacture. 



Methods of manufacture. The slags used are basic blast-furnace 

 slags, but a somewhat greater range in composition is allowable for 

 slag bricks than when the slags are to be used in cement-mam 

 facture. The analyses quoted in the present chapter may be regarded 

 fairly representative of the class of slags used in slag-brick manufe 

 ture. It will be seen that the silica ranges from 22.5 per cent to 35 

 cent; the alumina and iron oxide together, from 16.1 per cent to 21 per 

 cent; the lime, from 40 per cent to 51.5 per cent. As in slag cements, 

 sulphur is an objectionable constituent. Much of it, fortunately, is 

 removed during the process of granulating the slag. 



The general steps in slag-brick manufacture may be stated as follows; 

 Slags of proper composition are granulated by being run into a stream 

 of cold water immediately upon issuing from the furnace. This causes 

 the slag to break up into little porous particles, thereby greatly reducing 

 the expense of subsequent grinding: Granulation also confers hydraulic 

 properties on the slag, and removes part or all of its contained sulphur. 

 The granulated slag is dried and pulverized. Powdered slaked lime 

 is added in sufficient uantity to bring the total calcium oxide content 

 of the mixture up to about 55 per cent. This mixing, as well as the 

 previous burning and slaking of the lime, must be carefully and thor- 

 oughly done in order to prevent subsequent disintegration of the bricks. 

 Usually, during or after the mixing, a small amount of water is added. 

 The mixture is then molded into shape, either by hand or in a brick- 

 machine. After shaping, the bricks are dried in the open air, this usu- 

 ally taking six to ten days in dry weather. In the best practice, the 

 bricks are retained for several months, after drying, in order that they 

 may be well hardened before marketing. 



Though over 90 per cent of the total production of slag brick is at 

 plants following the above methods, three other methods may be briefly 

 noted. At a few plants the granulated slag is mixed, without drying, 

 with the unslaked lime; the slag furnishing sufficient water to slake 

 the lime. Slaking in this way is very imperfectly done, however, and 

 the practice should never be followed if high-grade bricks are expected. 

 At a few other plants, notably at the Bilbao plant described below, 

 slag is mixed with slag cement instead of with lime. At certain English 

 plants, also noted below, the slag bricks are hardened in steam cylinders 

 like the cylinders used in lime-sand brick manufacture. 



Slag bricks vary in color from a grayish white to dark gray. They 

 weigh less than clay bricks of equal size, are said to require less mortar 



