190. The Time In Air Before Immersion

As soon as the briquets are molded they should be covered with a damp cloth until they are ready to be removed from the molds, when they should be transferred to a "damp closet," lined with zinc or other non-corroding metal. It was formerly the practice to immerse the briquets as soon as they were considered to be sufficiently set; but for the sake of uniformity, they are now left in moist air for twenty-four hours before immersion, whether the cement is quick or slow setting. Briquets which are to be broken at twenty-four hours, however, are usually immersed as soon as set hard.

Table 34. Variations In Length Of Time Briquets Are Left In Moist Air Before Immersion — Natural Cement

Note : — All briquets made by same molder. Each result is mean of ten specimens.

Table 34 gives the results obtained by allowing natural cement briquets to remain in moist air different lengths of time before immersion. In general, the strength is greater for seven and twenty-eight day tests the longer the briquets are allowed to remain in the moist air. It appears that, while the time in moist air should be made as nearly uniform as possible, a variation of a few hours will not cause an important difference in strength.

Table 35. Gain Or Loss In Strength Of Natural Cement Briquets By Immersion

Note : — All briquets made by same molder. Each result is mean of five specimens.

Table 35 shows the early action of the water on the briquets. These tests were made in sets of ten; five briquets of a set were immersed after twenty hours, forty-eight hours, etc., while the other five of the same set were broken at the time the first five were immersed. With this sample of natural cement, it appears that the briquets lose part of their strength by immersion, and that some time is required to regain this lost strength. Thus, with neat cement mortar the briquets broken at twenty hours without immersion were as strong as those broken at seven days which had been immersed the last six and one-fourth days. With briquets of one-to-one mortar, it appears that if immersed at the end of four days, the gain in strength during the last three days (in water) is about equal to the loss of strength due to immersion. If immersed earlier than this, the gain is greater than the loss, but if immersed later, the loss is greater than the gain.

191. For storing briquets the required twenty-four hours before immersion a moist closet is very convenient, tends to promote uniformity of treatment, and may be very easily made. The use of a damp cloth for covering briquets is inconvenient, as the cloth may dry out. If it is used, the end of the cloth should rest in a pail of water, so it will keep wet by capillarity; it should also be kept from touching the briquets by a wire screen or by wooden slats.

A moist closet may be made of slate, glass or soapstone, or of wood lined with metal. In the bottom of the box is a pan of water, or a sponge kept constantly wet. The shelves may well be of glass, and should be so arranged that any shelf may be removed without disturbing the others.