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.
IVote. It may be noted that the setting quality of cement con sists 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.
Soundness. This is one of the most important of physical tests, for it indicates whether there probably exist elements which will eventually disintegrate the mass. The action of pats should be care fully observed; and if warping, cracking, or blotches appear, the material should not be used. Form on small pieces of clean glass four pats of cement mixed with only so much water as will turn the color of the cement. The pats are to be about 1-inch thick in the center, and 3 inches in diameter, tapering to the edge, which will be as thin as possible. After a day under a clamp cloth, put two of the pats in water for a week, and leave two in air, and examine for symptoms referred to above.
Should it be possible to test the material chemically, the follow ing points should be observed: The lime should be slightly over 3 times the silica. Thus, if there is 20 per cent silica, the lime should run from 60 to 62 per cent; this is a good balance. The silica should not fall below 10 per cent, nor rise above 22 per cent, which is about the range of good cements on the market.
1Iore than 2 per cent of sulphur should never be allowed. The reason for this is that the "setting" of cement is a process in which the various materials combined unite chemically to form one solid mass.
Sulphur in its various forms is an ingredient which does not work in harmony with the other materials; and while it does not at once prevent the satisfactory combination and setting of the other materials, yet, under conditions that are very likely to prevail, it sets up a chem ical action tending to disintegrate the mass in time.
Magnesia can generally be considered an ingredient which has no value either as sand or cement, being what is known as "inert" material. While 4 per cent may not injure masonry, wearing surfaces should not have over 3 per cent.
The manufacturers of Portland cement have tried to eliminate both of these ingredients—but so far without success—at a cost which would make it possible to sell the cement at a profit.
The specific gravity should not fall below 3.10, as a low specific gravity indicates poor burning of the clinker. These later tests, however, are delicate laboratory operations, and only experienced chemists are capable of getting satisfactory results.
Cements. There is a very general prejudice against the use of the ordinary Portland cement for setting limestone, sand stone, and marble, the theory being that the moisture from the mixture, in getting away, travels through thestone, carryingwith it certain stain ing matter, which is either deposited on the surface or unites chemically with ingredients in the air to stain the face. Thus there have come to be used certain materials of much less structural value than the Portland cement. These are called non-staining cements, and are sold at a high price. There are also on the market many materials intended to be put on the back of stone, which the manufacturers claim will prevent the passage of moisture and staining matter. While it can easily be demonstrated that water containing cement running over the face of soft stones does leave distinct traces, it is a different thing to show that such stone or marble can be stained by materials in the joints or on the back. It is well, before any amount of money is spent, to experiment in producing the stain on the stone used with the ordinary cement.
A small box can be filled with soft mortar, and a piece of stone or marble laid therein so that the top surface is inch above the mortar. 'Then, by tacking a piece of waterproof paper to the box sides, and cutting a hole somewhat smaller than the upper face of the stone or marble sample used, and by weights bringing the edge of the paper to the stone so that practically no moisture can pass except through the sample and off from the exposed surface, the results of the so-called staining qualities of ordinary cement can be observed.