From this it seems certain that the hardening follows closely in proportion the quantity of water which becomes chemically combined, and that the slag-cement undergoes a similar change to that which takes place in Portland or Roman cements. That other chemical changes occur there seems also to be no doubt, and with Portland, or Roman, or slag cement - time being left out of the question-the same chemical changes apparently ensue.

Ransome, the well-known inventor of artificial stone, has recently taken out a patent for mixing the slag-sand in its wet state with chalk, and then burning the whole together in a cement kiln into clinker, after which he grinds it down in the same way as Portland cement. The results given are remarkable, exceeding Portland cement in strength by nearly 30 per cent.

Reid made some experiments on the application of slag to the manufacture of Portland cement, and could not get very satisfactory results; but that was before Wood's granulated slag was to be had. He found the greatest difficulty with the sulphur. He got in his slag about 1 1/4 Per cent. of calcium sulphide, which would amount to about 2.84 per cent. of lime sulphide in the cement when finished, an amount which made the cement very unreliable. He found also that, in working up the slag with the lime, it was very difficult to incorporate the 2 thoroughly; but it was very possible that Wood's granulated slag, which was very finely divided, might be more easily mixed with lime. The amount of sulphur usually contained in Portland cement, according to Grant, is nearly 2 per cent. If you were to make cement from slag already containing 12 Per cent. of sulphur, you would get 4 Per cent. of lime sulphate in it, which would render it unreliable. This process was patented by Bodmer in 1866; the specification stated that the invention consisted in the manufacture of a cement by mixing together slag cinder or scoria, whether naturally or artificially produced from furnaces, with a certain proportion of lime or calcareous matter, and with or without alumina.

Egleston. has referred to blast-furnace slags being used for various things, and amongst others for the manufacture of cements. He said that the possibility of having them in the form of granulated slag, reduced the price of the pulverized material to such a figure that, in certain parts of Germany, an artificial cement equal in every respect to the best Portland was manufactured at a large profit.

For Mortar

Mortar for building purposes is another material supplied at the Cleveland Slag Works. It is simply made by grinding the slag-sand with about 6 Per cent. of slaked lime in an ordinary mortar-mill, and (if ground fine) it makes a far better mortar than is generally employed by builders. Two years ago there was a very large demand for this material in Middlesbrough. There is only one objection made to it, viz., that it sets too quickly. Mortar supplied on the Saturday, left unused, would be worthless on the Monday. As with the other slag products, its remarkable strength and cheapness combined make it much liked by those who, in close proximity to the works, can obtain it freshly made.

For Bricks

At the Cleveland Slag Works the most important production, and the one which consumes by far the greatest quantity of slag, is that of concrete-bricks, known in the market as slag-bricks. These are made from the sand produced by the slag-sand machine before described. The sand is dropped from the railway wagons into hoppers, or depots, at the works, whence it is filled into large barrows, taken up a hoist to the top of the building, and tipped into a hopper, which supplies a measuring apparatus. Here it is mixed with a certain quantity of selenitic lime (General Scott's patent), with an addition of iron oxides; it then passes into the brick-press hereinafter to be described. The bricks are taken off the presses by girls, placed upon spring-barrows carrying 50 bricks each, and removed to air-hardening sheds; here they remain a week or 10 days, after which they are stacked in the air to further harden, and at the expiration of 5 or 6 weeks they are ready for the market. Here is the curious anomaly of bricks being made without burning, and of a wet season being favourable to the hardening process. The bricks thus produced are very tough; they do not split when a nail is driven into them, and are easily cut; they do not break in transit, and the frost has no effect upon them.

According to a certificate received from Kirkaldy's testing-works, some of these bricks, taken from stock 3 years eld, carried a pressure of 21 tons before crushing, whilst others only 4 months old crushed with 9 tons pressure; showing not only great toughness, but also that they greatly improve by age.

There are now 2 machines fully employed, making about 130,000 bricks weekly, consuming 250 tons of slag-sand and 30 tons of selenitic lime and oxides.

The preparation of this selenitic lime forms a necessary branch of the business. It is made in the following manner:-

80 Per cent. of unslaked common lime.

10 Per cent. of raw gypsum.

10 Per cent. of iron oxides calcined. These are all ground together, under edge-runners, into a fine dry powder. The composition is then passed through a fine sieve, 24 meshes to the inch, and is ready for the brick-press. To each 1000 of bricks, 6 cwt. of this lime is used; no water is added, sufficient being held in suspension in the slag-sand to thoroughly moisten the lime; in fact, it is no uncommon thing to find flowing from the brick-press a stream of water which has been squeezed out of the sand. The loss of bricks in manufacture is very small; in fact, after the bricks are once upon the barrows, the waste is not more than 1 1/2 per cent. At the present rate of production, there is a consumption of slag for this one article alone of about 14,000 tons per annum. The weight of these bricks is about 30 Per cent. lighter than ordinary red ones - 9 in. by 4 1/2 in. by 2 1/2 in. - weighing only 2 1/2 tons per 1000. Another interesting feature in connection with these bricks is the economy in manufacture, which - including all materials, labour, wear and tear of machinery, superintendence, power, and everything except interest on capital-does not exceed 10s. 6d. per 1000. The following is an analysis of these bricks, made by Patterson and Stead, and will be found worthy of notice, showing the hardening properties contained, the composition comparing favourably with the cements previously mentioned:-