Portland cement consists of the product of burning and grinding an artificial mixture of carbonate of lime and clay or slag, the mixture being very carefully proportioned so that the ingredients shall have very nearly the fixed ratio which experience has demonstrated to give the best results.

"If a deposit of stone containing exactly the right amount of clay, and of exactly uniform composition, could be found, Portland cement could be made from it, simply by burning and grinding. For good results, however, the composition of the raw material must be exact, and the proportion of carbonate of lime in it must not vary even by one per cent. No natural deposit of rock of exactly this correct and unvarying composition is known or likely ever to be found; therefore Portland cement is always made from an artificial mixture, usually, if free from organic matter, containing about 75 per cent carbonate of lime and 25 per cent clay." - S. B. Newberry, in Taylor and Thompson's "Concrete Plain and Reinforced."

As before stated, Portland cement is stronger than natural cement; it sets more slowly, which is frequently a matter of great advantage, and yet its rate of setting is seldom so slow that it is a disadvantage. Although the cost is usually greater than that of natural cement, yet improved methods of manufacture have reduced its cost so that it is now usually employed for all high-grade work where high ultimate strength is an important consideration.

In a general way, it may be said that the characteristics of Portland cement on which its value as a material to be used in construction work chiefly depends may be briefly indicated as follows:

When the cement is mixed with water and allowed to set, it should harden in a few hours, and should develop a considerable proportion of its ultimate strength in a few days. It should also possess the quality of permanency so that no material change in form or volume will take place on account of its inherent qualities or as the result of exterior agencies. There is always found to be more or less of shrinkage in the volume of cement and concrete during the process of setting and hardening; but with any cement of really good quality, this shrinkage is not so great as to prove objectionable. Another very important characteristic is that the cement shall not lose its strength with age. Although some long-time tests of cement have apparently indicated a slight decrease in the strength of cement after the first year or so, this decrease is nevertheless so slight that it need not affect the design of concrete, even assuming the accuracy of the general statement.

To insure absolute dependence on the strength and durability of any cement which it is proposed to use in important structural work, it is essential that the qualities of the cement be determined by thorough tests.