Specific Gravity

Mr. Mann found1 that the specific gravity of cement supplied by the best English manufacturers slightly exceeded 3.0.

The particles rejected by a sieve of 2900 meshes to the square inch had a specific gravity from 3.08 to 3.13, and the fine particles passed by that sieve a specific gravity of from 2.97 to 3'06, but in some cases the coarse and fine particles of the same cement had the same specific gravity.

In an inferior cement he found the specific gravity 2.80, and that of the finely sifted portion only 2'55.

Grant's experiments2 show the specific gravity of differently burnt cements to be as follows : -

Light burnt .....

3.130

Hard „ .

3.134

Medium,, .....

3.131

Mr. Grant's specification for specific gravity is "not less than 3"1."

The apparatus described in the next paragraph is recommended by Mr. Grant for ascertaining the specific gravity of cement.

Keate's Specific Gravity Bottle

This bottle consists of two bulbs, the lower somewhat exceeding the upper in capacity. The exact capacity of the lower bulb is of no importance. On the neck between the bulbs is a file mark b, on the neck of the upper bulb is a similar mark a.

The capacity of the upper bulb between the marks a and b must be accurately determined, and may conveniently be either 500 or 1000 grains in water measure at 60° Fahr.

In ascertaining the specific gravity of a solid in small fragments - small shot, for example - the following is the mode of procedure : fill the bottle with distilled water up to the mark b, accurately counterpoise the bottle so filled in a balance, drop the substance (of which the specific gravity is to be taken) carefully and gradually into the bottle until the water rises from b to a. Ascertain exactly the weight of the material so added. If the capacity of the upper bulb be 1000 grains of water, the weight of the material required to raise the water from b to a is its specific gravity; if the capacity of the bulb be 500 grains of water, the weight of the substance added must be multiplied by 2, which will give the specific gravity.

The principle of the apparatus is very simple; the capacity of the upper bulb is an exact measure of distilled water, and when the water is raised from b to a by dropping a solid into the bottle, the bulk of that solid equivalent to the given volume of distilled water is ascertained and the relation between the weights of the two is given by the weights of the substances added, which is either the specific gravity direct, if the capacity of the bulb is 1000 grains, or it can be ascertained by multiplying the weight of the solid by the number which represents the part of 1000 represented by the capacity of the bulb, etc.

If the solid be soluble iu water, any convenient liquid can be used in the place of water in making the experiment, the only thing necessary being carefully to counterpoise the bottle filled with the liquid up to 6 in this manner. Petroleum, oil, turpentine, or any liquid suitable to the nature of the material to be tested, may be used, all other things remaining the same.

The only precautions to be observed are that the air, which is apt to cling somewhat to the solid matter when dropped into the liquid, is carefully removed, and that if a very volatile liquid be used in the place of water the bottle should be stopped or corked to prevent evaporation.3

Method Of Weighing

In order that the cement may be accurately weighed, great care must be taken in filling the measure.

Method Of Weighing 30078

Fig. 82a.

1 M.I.C.E., vol. lxii. p. 224. 2 M.I.C.E., vol. lxii. p. 130.

3 Taken verbatim from Appendix iv. p. 129, M.I.C.E., vol. lxii. (Grant).

This may be done by allowing the dry cement to run down a board or shoot, inclined at an angle of 45°, into the measure, any superfluity being carefully struck off with a light straight-edge.

A vessel with holes in it is sometimes used for filling instead of the shoot. An accurate method is to fill the measure through a sieve of about 1/16 inch mesh held a short distance above it, or the cement may be poured through a hopper placed about two feet above the measure. A drawing of the hopper is sometimes supplied in connection with the specification.

Colour

This point should be examined, though it is not of very great importance. Bad cement may be of a good colour.

Good Portland cement, as received from the manufacturers, should be of a grey or greenish-grey colour.

A brown, or earthy colour, indicates an excess of clay, and shows that the cement is inferior - likely to shrink and disintegrate.

A coarse bluish-grey powder is probably overlimed and likely to blow.

The colour may best be observed by rubbing the cement on the hand or on a piece of white paper.