CEMENT There is probably no other building material on which so much has been written within the last ten years; and there will be no attempt here to discuss the question except from an extremely prac tical standpoint. All things considered, Portland cement at $2.00 a barrel net is the most economical material to be used in general masonry. Lime is much cheaper, but of very doubtful value as generally used. The English, Scotch, and French use lime with great success; but it is always slaked in pits for at least three months prior to use, and is treated and worked so as to make it valuable for mortar, while in this country it is used after such crude treatment that it cannot be considered a material suitable for good work. Therefore lime in any form will not be further considered for masonry work.
There are now three classes of Portland cement on sale in this country. By far the largest class is that ground from an artificial clinker. Second are what is known as "slag" cements, ground largely from furnace slag. While in many of the second class the sulphur has been so far extracted as to make them identical in chemical analysis with those of the first class, and of great value in general masonry, it is wiser to avoid their use where surfaces are exposed to wear, as in sidewalks, steps, etc. Unless the work is of minor impor tance they should not be used unless such chemical test (independent of tests made by the manufacturers) can be obtained as will show that the sulphur therein is below 2 per cent.
The third class is one imported from Belgium under various brands carrying English names, such as "Eagle," "Star," etc., the packages, however, showing the place of manufacture. These cements are generally coarse and of low grade—possibly the factory's second grade, on which the manufacturer does not care to put his regular brand.
English cement is practically out of the market.
A few brands of German remain, like the "Dvkerhoff," which commands a high price and for which there is a limited demand from users who believe there are no other brands from which quite as good results can be obtained. For our purposes, however, the second and
third classes need not be considered; the range in No. 1 is wide, and little if anything is gained in price in using Nos. 2 and 3.
On important work, tests should be made to determine the fine ness of grinding, specific gravity, time of initial and final set, sound ness, the proportion of silica to lime, and the amount of sulphur and magnesia. Tests are also made of tensile strength; but as this is a quality never required of cement, and this test only indirectly bears on other necessary qualities; and as the presence of matter which is apt to cause final disintegration, tends often—in the early stages of use—to increase the strength, such tests should be in no way relied on.
In this paper, only three tests which it is possible for the student to make with appliances easily obtained, will be considered.
Fineness. What is known as a 100-mesh sieve—that is, a sieve with 100 spaces to an inch—should be obtained. Any first-class hardware store can furnish the wire either from stock or on order and a home-made sieve can be constructed. A small quantity of cement should be weighed and most carefully sifted; 90 per cent of the material should pass through the sieve. By referring to state ments relative to cement, under "Sand," the student will understand that extreme fineness is required in order that the cement may fit in and completely fill any voids which may exist between the particles of sand.
Initial Set. The time for the initial set is determined approxi mately 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 h--inch diameter loaded with a ¢-pound ball on top, when set thereon. Such time should not be less than 30 minutes.