When cement is gaged with sufficient water to bring it to a paste, and is then left undisturbed, it soon begins to lose its plasticity and finally reaches such a condition that its form can no longer be changed without producing rupture. This change of condition is known as the "setting" of cement and is considered to be, in a measure, distinct from " hardening." Setting usually takes place within a few hours, or perhaps minutes, while the hardening is continuous for months or years.
The precise chemical changes that take place in the setting and hardening of cements are not thoroughly understood. The chief cementitious ingredient in Portland cement is considered to be a tricalcium silicate, 3 CaO, Si02; in contact with water it forms hydrated monocalcic silicate and calcium hydrate. This process is believed to contribute more to the final hardening of the mortar than to the setting, though the hydration of the finer particles of this important compound also contributes to the first setting. It is considered that the calcium aluminates play an important role in the first setting of cement, as they set rapidly in contact with water, and it has been suggested that they form the chief active constituents of natural cement.1
These chemical changes cause the formation of crystals which by their interlocking and adhesion give strength to the new compounds. For a scientific and detailed treatment of this subject, the reader is referred to the articles of M. H. Le Chatelier in Annates des Mines, 11, pp. 413-465, Trans. Am. Inst. Mining Engineers, August, 1893; to the conclusions of S. B. and W. B. Newberry, Cement and Engineering News, 1898; and to " The Constitution of Portland Cement from a Physico-Chemical Standpoint," a paper by Mr. Clifford Richardson read before the Association of Portland Cement Manufacturers at Atlantic City, June 15, 1904, Engineering Record, August 13 and 20, 1904, Engineering News, August 11, 1904.
1 S. B. Newberry, " Mineral Resources of the United States," 1892.
The setting of cement being a gradual and continuous process without well-defined points of change, it is necessary, in order to compare the rates of change in condition of different samples, to adopt an arbitrary standard. The method usually adopted is to determine the resistance of the mortar to the penetration of a wire or needle. The wires used by General Totten and recommended by General Gilmore for this purpose are now in general use in this country. One of the wires is 1/12 inch in diameter and is loaded to weigh 1/4 pound; the other is 1/24 of an inch in diameter and loaded to weigh one pound. The paste is said to have reached "initial set" and "end of set" when these two wires, respectively, fail to make an impression on the surface.
98. M. Vicat also suggested a needle test as follows: The cement paste is placed in a conical ring, 4 cm. in height and 7 cm. in diameter at the base. The consistency should be such that a rod 1 cm. in diameter and weighing 300 grams does not entirely pierce the mass. This consistency having been obtained by trial, a needle of circular cross-section having an area of 1 sq. mm. and loaded to weigh 300 grams, is gently lowered on the paste. The moment when this needle no longer penetrates the mass is called the beginning of the set, and the time in which it fails to make an impression upon it is called the end of setting. It may be mentioned in passing, that, according to a few comparative tests made by the author, when a cement paste has "set" by Gilmore's "heavy" wire, -fa inch weighing one pound, it requires about 1,100 grams weight on the Vicat 1 sq. mm. needle to make an impression on the paste. Vicat's method was indorsed by the Munich Conference and was suggested in the recent progress report of the Committee of the American Society of Civil Engineers.
99. M. LeChatelier has suggested a modification of this method by substituting for the rod 1 cm. in diameter a disc of the same diameter carried by a slender rod, the disc being loaded to weigh 50 grams, the normal consistency being such that the disc will stop midway in the ring, or "vase." The beginning and end of setting he would define by the penetration of the needle (1 sq. mm. in section) to mid-depth in the ring, the weights being 50 grams and 3,000 grams, respectively.
100. An approximate method of determining time of setting is also in use as follows: After mixing the cement paste to the proper consistency, place enough of it on a glass plate to form a thin cake, or "pat," about three inches in diameter and one-half inch thick at the center, thinning toward the edges. When the pat is sufficiently hard to bear a gentle pressure of the finger nail, the cement is considered to have begun to set, and when it is not indented by a considerable pressure of the thumb nail, it may be said to have set.
101. Mr. Henry Faija objected to all methods which are based upon the rates of acquiring hardness, on the ground that there are periods in the early stages of hardening that may be more rationally defined. He considers that the time at which the water leaves the surface of the pat, depriving it of its glossy appearance, is really the beginning of setting, and that this time may or may not correspond to the result obtained by the use of the needle.
Some of the qualities which determine the actual rate of setting of a cement are, its composition, degree of burning, age and fineness. Aside from these qualities of the cement itself, the addition of certain salts subsequent to the manufacture also influences the rate. The observed rate of setting will be influenced by the details of the test, such as the quantity, temperature and composition of the water used in gaging, the amount of gaging, the temperature of the cement, and the temperature and character of the medium in which the pat is placed after molding.