Portland Cement is produced by burning, with a heat of suf ficient intensity and duration to induce incipient vitrification, certain argillaceous limestones, or calcareous clays, or an artificial mixture of carbonate of lime and clay, and then reducing the burnt material to powder by grinding. Fully 95 per cent of the Portland cement pro duced is artificial. The name is derived from the resemblance which hardened mortar made of it bears to a stone found in the isle of Port land, off the south coast of England.
The quality of Portland cement depends upon the quality of the raw materials, their proportion in the mixture, the degree to which the mixture is burnt, the fineness to which it is ground, and the con stant and scientific supervision of all the details of manufacture.
Characteristics. The color should be a dull bluish or green ish gray, caused by the dark ferruginous lime and the intensely green manganese salts. Any variation from this color indicates the pres ence of some impurity; blue indicates an excess of lime; dark green, a large percentage of iron; brown, an excess of clay; a yellowish shade indicates an underburned material.
Fineness. It should have a clear, almost floury feel in the hand; a gritty feel denotes coarse grinding.
Specific Gravity is between 3 and 3.05. As a rule the strength of Portland cement increases with its specific gravity.
Tensile Strength. When moulded neat into a briquette and
placed in water for seven days it should be capable of resisting a ten sile strain of from 300 to 500 pounds per square inch.
Setting. A pat made with the minimum amount of water should set in not less than three hours, nor take more than six hours.
Expansion and Contraction. Pats left in the air or placed in water should during or after setting show neither expansion nor con traction, either by the appearance of cracks or change of form.
A cement that possesses the foregoing properties may be con sidered a fair sample of Portland cement and would be suitable for any class of work.
Overlimed Cement is likely to gain strength very rapidly in the beginning and later to lose its strength, or if the percentage of free lime be sufficient it will ultimately disintegrate.
Blowing or Swelling of Portland cement is caused by too much lime or insufficient burning. It also takes place when the cement is very fresh and has not had time to cool.
Adulteration,. Portland cement is adulterated with slag cement and slaked lime. This adulteration may be distinguished by the light specific gravity of the cement, and by the color, which is of a mauve tint in powder, while the inside of a water-pat when broken is deep indigo. Gypsum or sulphate of lime is also used as an adulterant.