PORTLAND CEMENT In 1908, Portland cement was manufactured in at least twenty-five of the forty-six States of the Union. The raw materials used by the different plants vary considerably, so that no absolutely universal rule can be laid down for the selection or proportioning of the ingredients from which Portland cement is made. It is equally impossible to give any exact formula for its chemical composition, since the latter, too, is found to vary through a considerable range. Nor, in fact, is it even possible to give a defini tion of Portland cement which will invariably afford an accurate guide as to its physical ingre dients or chemical composition. These things are not secret mysteries; but they are variable factors, no two brands being exactly the same in these respects. Thus, Portland cement eludes all attempts to reduce it to the uniformity of a positive chemical formula or universal standard of composition.
It is not to be supposed for a moment, how ever, that a true Portland cement is any uncer tain quantity or unworthy of implicit confidence as a structural factor. Its composition may vary, and does vary; but that is immaterial. From the structural point of view, the important thing is that the cement, and the concrete made from it, shall meet certain requirements as to strength, durability, constancy of volume, fire resistance, etc.; and in these respects, not only can Portland cement be manufactured with such perfection as to conform to all the positive standards that experience has shown to be de sirable; but in construction work—especially where failure would involve great destruction to property and possibly to human life—it should be the invariable practice to make assur ance doubly sure by careful testing, to see that the particular brand of cement whose use is pro posed does actually come up to standard require ments in every respect.
even such unpromising material as eggshells and street sweepings, a product could be ob tained which would pass all the standard tests, it would be fully entitled to be called a "Port land cement." Raw Materials. As a matter of fact, how ever, the ingredients from which true Portland cements can be manufactured, although very widely distributed in nature, appear to include only a comparatively limited list. In the main, they comprise calcareous or limey materials, argillaceous and siliceous materials, and blast furnace slag, with minor accessory materials added for the purpose of regulating the activity of the product.
The forms of calcareous matter generally used are three in number—limestone, chalk, and marl—each of which must contain the requisite amount of lime; for about sixty per cent, or nearly two-thirds by weight, of all Portland cement consists of this ingredient. Limestones are found almost everywhere. Chalk and marl are forms of limestone, usually of a purer cal cium content than the ordinary "limestone" so called. Marl is a finely powdered substance usually found at the bottom of lakes or in swamps, having been deposited there by the action of water; and consists almost entirely of pure carbonate of lime. It is dredged out of its bed or mixed with water, and pumped into tanks for future use, being employed in what is known as the "wet process" of cement making.
Clay, shale, and slate are used for the silica, alumina, and iron contents of the cement; and blast-furnace slag is also used for this purpose.
In physical form, Portland cement is an exceedingly fine powder, ordinarily of a dull bluish or greenish gray tint, though brands of pure "white" Portland are now also made.