This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The time for the initial set is determined approximately as follows:
Mix a small amount of neat cement (that is, clear cement without sand) with water, to the consistency of stiff mortar; and observe the time it takes to set hard enough to prevent an indentation being made by a wire about 6 inches long and of 1/12-inch diameter loaded with a 1/4-pound ball on top, when set thereon. Such time should not be less than 30 minutes.
It is unwise to use cement which sets at once or in such short time that it is inconvenient to place it before such set occurs. Cement fresh from the works often takes its first set quickly; but after it has attained an age of two or three months before being used, is all right. Therefore, never specify "fresh cement," as none should be used within three months from the date of its manufacture.
It may be noted that the setting quality of cement consists in the 'permanent union of water with the cement. The setting of cement mortar is often referred to as the "drying out;" as a matter of fact, the water necessary for mixing the mortar never leaves it unless driven off by extreme heat. Take a tin can with tight cover; weigh out five pounds of dry cement; mix with it enough water to wet it; pack it in the can and thoroughly compress it. If water rises to the top - indicating that too much has been added - pour it off; put on the cover tight, weigh the whole, to determine the amount of water added; and set away for two weeks. Then open, and set in a warm, dry place for a few days so that it may be thoroughly dry, and weigh. It will be found that the weight is that of the original cement plus the water added, which still remains in the mass. Subject this to extreme heat, when it will be found that the water is thrown off, the cement is fine again, and its weight corresponds with that originally used.