CHAPTER XIII.

NATURAL AND ARTIFICIAL CEMENTS.

BY PROFESSOR N. W. LORD, OHIO STATE UNIVERSITY.

Cements, as considered in this chapter, embrace those materials obtained by burning or calcining certain rocks or mixtures of clay and limestone, slag, etc., and which, when mixed with sand and water to a mortar, "set," or harden, without exposure to air. They all have lime as the basis of their composition, but the mortars they furnish differ from ordinary lime mortars in having less plasticity, as a rule, and in this property of hardening and not disintegrating under water. Cements are more and more replacing ordinary lime in the construction of all masonry where strength and durability are desired.

Even where lime mortar is used, as for ordinary brick-work, it is becoming customary to mix with it a certain proportion of cement. It is claimed by Gillmore, and other writers, that the cement so added is useless, but many excellent masons and contractors so use it, stating that it gives greater body to the mortar and prevents any settling of the interior of the work. The lighter-burned natural cements are used for this purpose, and, as many of these contain an excess of clay and magnesia, they possibly act upon the free lime in the mortar, hardening it in a way that the artificial cements would not do.

Cement rocks, after burning, do not "slack," or fall to powder when wet with water, but require to be ground in mills before using. They are thus distinguished from certain "hydraulic limes" which, after slacking in the usual way, furnish mortars setting under water more or less powerful. Magnesian limestones belong to this class. They slack slowly, developing little heat, and furnish mortars which are decidedly hydraulic. The magnesia, probably, acts by gradually crystallizing as a hydrate. As an experiment, a mortar was made of pure calcined magnesia and sand. In a few hours it set hard enough to be handled, and was put in water. It retained its shape, and while not getting very strong, it gradually hardened like a true cement. From this it would seem that in a magnesia lime mortar, this setting of the

magnesia would account for the resistance to water, as it would prevent disintegration while the enclosed lime would gradually become carbonated and give strength.

Rocks which make cement by burning are very generally dis tributed, but, as a rule, the deposits are marked by great variability in quality. Of two layers in a quarry, one may be cement rock and the next a worthless shale; and even the same layer may, in a short distance, entirely change its character. From this cause, in part at least, the various attempts to work the smaller and more common deposits have more or less generally failed, and the natural cement of the market is obtained from only a few localities where large deposits of tolerably uniform rock occur. Of these, the two which furnish by far the larger part of the cement of the United States are Ulster county, New York, producing the Rosendale cements, and Louisville, Kentucky, from which are shipped the numerous brands of "Louisville cement."

The rock is, in both places, a shaly limestone. In New York, it lies between the Lower Helderberg and Niagara groups; and in Kentucky, on top of the Corniferous limestone, corresponding exactly to the beds covering the limestone at Columbus, Ohio. The total production of natural cement in the United States in 1883 was estimated at 4,100,000 barrels of 300 pounds (Mineral Resources of the United States, Albert Williams, Jr.,) of which Ulster county, New York, furnished nearly one-half, and Louisville by far the larger part of the remainder.

Many of the impure limestones and shales of Ohio would furnish. excellent cement when properly burned. Such are some of the magnesian limestones near Sandusky, a number of the Carboniferous limestones, the shales over and including top of the Corniferous. The Black (Huron) shale, where it lies over the limestone is, for the first fifteen or twenty feet, quite calcareous in some localities, and in such cases furnishes an excellent cement rock.

The efforts to make cement on a large scale from these various materials have not been very successful, probably from failure to understand the irregularity of the deposits and want of capital to duly press the enterprise. Works were started at Sandusky, but they did not continue. One of the most promising ventures in this direction was at Defiance, in the northwestern part of the state where Mr. E. H. Gleason manufactured the "Auglaize Cement" from the lower and most calcareous layers of the Huron shale. The cement was of excellent quality, and it is hard to account for the non-continuance of the works.

In the southeastern part of the state, cement has been, and still is, made at a number of points, the materials being the impure limestones of the Coal Measures. The cement used in the railroad bridge at Bellaire

was made by T. C. Parker & Sons, at Barnesville, L'elmont county, from the cement limestone lying between the Pittsburgh and Meigs Creek coals (coals Nos. 8 and 9.) At Barnesville, the stone is five feet five inches thick, and was mined by drifting. These works are no longer in operation. The works just below Bellaire are now making cement from the same stone, which is drifted for in the hills. The stone is burned by mixing it with coal slack and running through a kiln. It is then ground in buhrs and barreled. Near New Lisbon, in Columbiana county, also, cement is made from a similar limestone which belongs to the Lower Freeport horizon. These are the only attempts that have been made to work the Ohio material on a commercial scale.

It thus appears that the rocks which yield cement on burning are invariably more or less impure, that is, silicious limestones, and in mostcases strongly magnesian, though in some of the best foreign cements: this element is nearly absent. To illustrate this composition more fully, the following analyses are added, some of them made by the writer, and others gathered from various publications.

2. Rosendale, another make.

3. Louisville cement (analyzed by writer).

4. Buffalo Natural Portland (analyzed by writer).

Where analyses of cement or cement rock do not give the amount of the alkalies, they must be regarded as incomplete. Gillmore says in his book on "Limes, Hydraulic Cements and Mortars" that all cement rocks contain alkalies, which are important in determining their cementforming qualities. It is also known that in the manufacture of Portland cement from a mixture of clay and limestone a certain per cent. of alkali if not present in these materials, must be added. The amount is small, but it is essential.

The causes of the setting of a cement have been much discussed and a number of theories devised. Probably no one cause explains all that happens, and it may be that different cements bring into action dif ferent chemical actions; but for practical purposes, the following points may be considered as established, and as they have direct bearing upon the recognition and treatment of cement rocks, it will be necessary to discuss them in some detail.

In the first place, the principal constituent of all these rocks is lime. There can be no cement made from any shale, clay, or slate, alone, which does not contain this element in excess of others; and as the lime always occurs as carbonate, it is evident that the greater part of all such substances will be dissolved by warm, dilute muriatic acid. There will be, in all cases, a residue left undissolved by the acid, which is very fine and not sandy, and when separated and washed, is found to be like very fine clay. It contains the silica and almost all of the alumina of the stone but only a trace of the lime, that having been taken out by the acid. This clay-like character of the residue is characteristic of the cement limestones. Those which have a sandy, gritty residue, will have

no hydraulic properties when burned, even though the amount of such residue be considerable. This constitutes an easy approximate test for cement stone. When pulverized, it must dissolve in warm muriatic acid, giving a muddy liquid, the more residue the better, if it is not sandy and gritty.

After such a rock is burned at a bright red heat for some hours, it changes entirely in character. When powdered, it dissolves entirely or nearly so in acid, giving an almost clear liquid; and in the solution will be found the silica and the alumina of the stone. Chemistry shows that in this case the silica and alumina were combined with lime, while in the original clay residue, they were combined with each other in the clay. Therefore, a cement contains silicate and aluminate of lime. These compounds absorb water, uniting with it to form hard, crystalline minerals. Evidently this, then, is part of the action of the "setting." The extent to which this formation of aluminate and silicate takes place in the burning probably depends upon the stone used. It certainly does not have to be complete. If a clay containing some free lime be strongly burned, so as to cause a partial combination between it and the alumina and silica, and then, after finely pulverizing, be mixed with ordinary slaked lime, this mixture will "set," and the resulting mass contains crystalline hydro-silicates, exactly as in the case of a cement. Again, if some cement rocks be heated to the point where complete combination occurs, the mass may melt and give a hard, glassy "slag," having no cement properties whatever.

What the small amount of alkali has to do with the setting, it is difficult to state. It may aid in promoting the combination during burning, and it may also make the silicate formed more easily attacked. by water, and so facilitate the hydration, or finally, it may act to transfer silica to lime, forming silicate of potash, which is continually decomposed by the lime.

A cement rock, then, consists of a limestone, either magnesian or not, having intimately mixed with it fifteen to thirty-five per cent. of clay, and a little potash or soda. The hydraulic energy given to such a rock by burning will vary with the temperature and length of time it is heated. Where it is burned too hard, it may, as before stated, lose entirely its property of setting, forming even a slag-like mass like the furnace cinder in iron-making. This danger of over-burning is probably greater in the more magnesian stones, as it is well known that lime and magnesia form a more fusible combination with silica than either alone. This may be the reason that, while the artificial cements, having about the same composition as the natural ones, except that they contain only lime, are best and strongest when highly burned, the others are