Testing Cements

No. 74, 5,476 " No. 100, 10,000 f` Activity denotes the speed with which a cement begins to set. Cements differ widely in their rate and manner of setting. Some occupy but a few minutes in the operation, and others require several. Some begin setting immediately and take considerable time to com plete the set, while others stand for a considerable time with no ap parent action and then set very quickly. The point at which the set is supposed to begin is when the stiffening of the mass first becomes perceptible, and the end of the set is when cohesion extends through the mass sufficiently to offer such resistance to any change of form as to cause rupture before any deformation can take place.

Test of Activity.

To test the activity mix the cement with 25 to 30 per cent of its weight of clean water, having a temperature of between 65° F. and 70° F., to a stiff plastic mortar, and make one or two cakes or pats 2 or 3 inches in diameter and about i inch in thickness. As soon as the cakes are prepared, immerse in water at 65° F., and note the time required for them to set hard enough to bear respectively a wire loaded to weigh 4 pound, and a h-inch wire loaded to weigh 1 pound. When the cement bears the light weight, it is said to have begun to set; when it bears the heavy weight, it is said to have entirely set. The apparatus employed for this test is shown in Fig. 1, and is called "Vicat's Needle apparatus".• Quick and Slow Setting. The aluminous natural cements are commonly "quick-setting," though not always so, as those con taining a considerable percentage of sulphuric acid may set quite slowly. The magnesian and Portland varieties may be either "quick" or "slow". Specimens of either. variety may be had that will set at any rate, from the slowest to the most rapid, and no distinction can be drawn between the various classes in this regard.

Water containing sulphate of lime in solution retards the setting, This fact has been made use of in the adulteration of cement, pow dered gypsum being mixed with it to make it slow-setting, greatly to the injury of the material.

The temperature of the water used affects the time required for setting; the higher the temperature, within certain limits, the more rapid the set. Many cements which require several hours to set whvn mixed with water at a temperature of 40° F. will set in a few minutes if the temperature of the water be increased to SO° F. Below a cer tain inferior limit, ordinarily from 30° to 40° F., the cement will not

set, while at a certain upper limit, in many cements between 100° and 140° F., a change is suddenly made from a very rapid to a very slow rate, which then continually decreases as the temperature increases, until practically the cement will not set.

The quick-setting cements usually set so that experimental sam ples can be handled within 5 to 30 minutes after mixing. The slow setting cements require from 1 to 8 hours. Freshly ground cements set quicker than older ones.

Soundness denotes the property of not expanding or contracting or cracking or checking in setting. These effects may be due to free lime, free magnesia, or to unknown causes. Testing soundness is, therefore, determining whether the cement contains any active im purity. An inert adulteration or impurity affects only its economic value; but an active impurity affects also its strength and durability.

For the purpose of determining the amount of contraction or expansion the "Bauschinger" apparatus, Fig. 2, is used. A mould is used in which the test bars of cement are formed.

Tests of Soundness. The soundness of a cement may be determined by cold tests, so-called, the cement being at ordinary temperature; or by accelerated or hot tests.

To make the cold tests, prepare small cakes or pats of neat cement, 3 or 4 inches in diameter and about one-half inch thick at the center, tapering to a thin edge. Place the samples upon a piece. of glass and cover with a damp cloth fora period of 24 hours and then immerse glass and all in water for a period of 28 days if possible, keeping watch from day to day to see if the samples show any cracks or signs of distortion.

The first indication of inferior quality is the loosening of the pat from the glass, which usually takes place in one or two days. Good cement will remain firmly attached to the glass for two weeks at least, The ordinary tests, extending over a proper interval, often fail to detect unsoundness, and circumstances may render the ordinary tests impossible from lack of time. Under such circumstances resort must be had to accelerated tests, which may be made in several ways.

Test. Prepare the sample as before, and after allowing it to set, immerse in water maintained at a temperature of from 100° to 115° F. If the specimen remains firmly attached to the glass and shows no cracks, it is probably sound.