When the briquets are ready to be immersed, i.e., usually, twenty-four hours after made, they are placed in a tank, containing water that is kept fresh by frequent renewals. The water in the tank should also be maintained at a nearly constant temperature. It is sometimes the case that briquets are subjected to considerable variations of temperature while in storage. It also frequently occurs that the water is allowed to become stale. A few of the many experiments made at St. Marys Falls Canal to show the effect, on the tensile strength of natural cement briquets, of variations in the temperature of the water of immersion, are given in Table 36. The details of these experiments, as well as other tests on the same point, may be found in the Annual Report, Chief of Engineers, U. S. A., for 1894, page 2314.

The very marked effect which the temperature of the water may have on the rate of hardening of natural cements is clearly shown. When broken at the age of one day or seven days, the effect on the strength may not be evident, or the briquets stored in cold water may develop a greater strength, but the more rapid hardening of the briquets stored in warm water is usually very evident at twenty-eight days, and increases up to two or three months. Some samples of cement are affected less than others, and a few experiments indicated that the differences in strength due to the temperature of water of immersion decrease after three months and become almost nil at one year.

193. The conclusion drawn from these tests may be briefly stated as follows: Between certain limits the early strength of natural cement mortars is usually developed faster in cool water, but after the first seven days, and sometimes after a shorter time, the strength is developed more rapidly in warm water, and the strength at any time between seven days and three months is approximately proportional to the temperature. After three months, the effect of the temperature seems to diminish, and may entirely disappear in time.

Table 36. Variations In Temperature Of Water In Which Briquets Are Immersed

M. Paul Alexandre 1 made quite a number of experiments on this point with Portland cement. In these experiments the gaging was done in about the same temperature as that at which the water of immersion was maintained, so that a double cause of variation was present. However, it was found that in all cases the higher strength was attained at seven days by the briquets made and stored in the higher temperature (15° to 18° C, 60° to 65° Fahr.) while at twenty-eight days the briquets of the lower temperature (0° to 5° C, 32° to 40° Fahr.) were ahead in the case of neat cement, and nearly as high as the warm briquets in the case of mortar. At three months the differences seemed to disappear.

1 "Recherches Experimentales sur les Mortiers Hydrauliques".