The next product to be described is the manufacture of what is called slag-cement. The word cement has sometimes been objected to in connection with this material, because it is generally manufactured in a wet state, and must be used within a few hours of its being made. Upon this point Wood expresses no opinion, simply mentioning the fact that, in point of strength, he finds little difference whether the materials are ground together in a dry or in a wet state. The cost of production, however, is as nearly as possible 4 to 1 in favour of the wet state. It is made by grinding under edge-runners, for about 1 hour (the finer the better), 70 Per cent. of slag-sand. 15 of common lime, and 15 of iron oxides, calcined ironstone, or spent pyrites. Following is an analysis of this cement, lately made by Patterson and Stead:-
Phosphoric acid .
Total water .
Less oxygen of the lime combined with sulphur.
The large quantity of water held in suspension in the slag-sand, is quite sufficient to make the mass in the mill into a semi-fluid state, but this water is mostly taken up in setting, as water of crystallization. It is therefore necessary that the cement should be used before setting takes place. This cement is usually employed for making concrete, by mixing 1 part of the cement to 5 of slag-shingle. The shingle is made by the slag-shingle machine before described.
The shingle, before being used, is well wetted; and when the concrete is put into place, it is beaten lightly down in a soft state, until the water and cement begin to rise on the top; 2 days afterwards it has become sufficiently set to allow of the building-boards being taken down, and at the end of a week it will be fairly hard, and will go on hardening for months. It is perfectly hydraulic, and will harden under water. It will be Been by this that it requires a longer time to set than Portland cement, and is perhaps not quite so hard; but there is a remarkable toughness, which has surprised all those who have used it, and this toughness makes it valuable for heavy machinery foundations, etc.; and, when made in proximity to the furnaces, the cost of the cement will not exceed 6s. per ton, whilst concrete made of this cement and slag-shingle will cost only 5s. 6d. per cub. yd.
These prices are absolute figures of cost, that of the concrete being arrived at after having executed many hundred cub. yd. upon the Tees Iron Works, at the new railway station at Middlesbrough, and elsewhere. The Slag Works buildings, the walls of which are between 70 and 80 ft. high, are built entirely with it, the basement walls being 2 1/2 ft. thick.
Whilst the underground walls of the Slag Works were being executed, they were twice immersed, through exceedingly high tides, with the result that this part of the building is the hardest of all; and to give an idea of the strength, Wood mentions that when it was necessary to cut 2 openings at different points through the basement walls, 3 1/2 ft. wide and 6 ft. high, this employed 2 good workmen, with steel bars and sledge hammers, at least 4 days for each doorway. He knows of no material at a similar cost which can compete with it, and he is satisfied that it has only to be widely known to be more extensively used. Personally, where time can be given, he employs nothing else for all heavy foundations for rolling machinery, for which purposes, as a conglomerate or monolithic man, it is peculiarly adapted. Slags from the furnaces making Bessemer iron are better adapted for this cement even than those from the Cleveland ores.
Mention has been made of the necessity of keeping the products from slag-sand in a damp state for a length of time after manufacture, in order to give them time to harden, or, in other words, to allow the material to absorb or take up as much water as will chemically combine with the lime, silica, and alumina; but whether this water becomes water of crystallization, or water of hydration, or a combination of both, is not at all certain. Wood is, however, strongly impressed with the idea that water in a fixed state, more particularly in a compound state, plays by far a more important part in the setting of cements than is generally supposed; that the presence of water in a chemically combined state forms as much a constituent part of cement as does the lime, silica, and alumina, seems certain from the results of the analysis shown further on. For instance, if Portland cement be heated to redness, so as to evaporate the fixed water, the cement loses at once its strength, and becomes rotten. Again, with gypsum, where the water of crystallization amounts to more than 1/5 of its bulk; if this is driven off at a red heat, we have little better than a powder left.
And it seems clear that the quicker this crystallization takes place, the quicker is the setting; and, on the contrary, as in the slag cements and the brick, the slower the water is in becoming fixed, the slower is the hardening; thus showing the necessity of keeping them damp during the process.
At Wood's request, Patterson and Stead made many analyses with the object of testing this point. Samples of Portland and Roman cements were mixed with water in the usual way, some specimens being supplied by the cement manufacturers themselves, as vast pieces, from their works, and had consequently been under water for terious periods. These were all reduced to powder, and carefully dried by keeping them for several hours at a temperature of 212° F. (100° C.), so as to evaporate every particle of free mechanically-mixed water. A very careful determination of the chemically-combined water was then made, with the following interesting results:-
Combined Water. 4 days in water. 6 days in water. Portland. Roman. Portland. Roman. 5-75% 5-25% 6-8% 6-78%
7 days in water. Portland. Slag cement. Slag brick. 7-75% 10-50% 5-70%