682 CEMENTS, LIMES, AND PLASTERS. 



ing of high-pressure steaming, it is to be understood that this refers 

 to any pressure above the atmosphere and low pressure to at or under 

 this. With other slags low pressure is quite as effectual as, and in some 

 instances is better than, high pressure. To determine which is the most 

 suitable process is a question for experiment with the particular slag. 

 Where low-pressure steaming is adopted the chambers may either be 

 made of thin sheet steel or tunnels may be constructed of brickwork. 

 In the author's experiments a steel high-pressure chamber was used 

 steaming up to 150 Ibs. pressure per square inch and for low pressure 

 a chamber .constructed of brickwork. In general, for low-pressure steam- 

 ing for about forty hours, and for high-pressure steaming twenty hours, 

 will be found most suitable and convenient. The author has not formed 

 a definite opinion as to what element in the slag causes the different 

 effect in the action of high- and of low-pressure steam, but is inclined 

 to think that it is principally due to the proportion of sulphur in the 

 slag. During the steaming some sulphur is driven out of the bricks, 

 and the final hardening does not seem to be completed until this vola- 

 tile or unfixed sulphur is driven out or combined. It is probable that 

 the sulphide of calcium present is slowly being split up, the hydrogen 

 of the water combining with the sulphur forming sulphuretted hydro- 

 gen, and the oxygen with the calcium forming lime. By subjecting 

 the slag to steam, thus keeping it moist and hot at the same time, this 

 action is accelerated. 



" Generally slags high in sulphur can be hardened best under pro- 

 longed low-pressure steam, and in one or two instances no hardening 

 effect was produced by high-pressure steam, whereas low-pressure steam 

 produced the desired effect. From this it would seem that the chemical 

 action is only accelerated up to a certain temperature, and that at a 

 higher temperature a different effect is produced; or it may be that 

 at a higher temperature the action is too violent, causing an expansion 

 and separation of the particles without actually producing cracks or 

 disintegration of the bricks, but sufficient to prevent the final com- 

 bination. Seemingly the presence of this unfixed sulphur retards the 

 action of the lime on the silicates and aluminates, and only when it is 

 finally driven off can the full combination be effected. 



" In the case of a slag which falls to powder on exposure to the atmos- 

 phere a grinding-mill is unnecessary; and with some slags of this 

 character it is only necessary to moisten and then press it into bricks 

 and harden as before. Again, with others it would be necessary to 

 grind a portion to dust in a ball mill before the setting could be obtained. 

 In the former case the slag powder would consist of a fine dust mixed 



