Cements are either natural or artificial.

Roman Cement is the best-known natural cement in this country. It is made by burning nodules containing some 30 to 45 per cent of clay, found in the London clay. This cement is of a rich brown colour, and weighs about 75 lbs. a bushel. It sets in about 15 minutes, and is valuable for tide-work, or stucco, but its ultimate strength is very small.

Other quick-setting Cements - For the names and uses of other somewhat similar cements, see p. 178. They are not used for mortar or concrete, but chiefly for plasterers' work.

Portland Cement is an artificial compound made by mixing chalk and clay in water in the proportion of about 75 per cent chalk to 25 per cent clay, drying and burning the mixture in kilns, and grinding the resulting " clinker " to such a fineness that 90 per cent of it will generally pass a sieve of 2500 meshes to the square inch, and it will weigh about 115 lbs. per bushel.

The result is a fine powder of greenish gray colour, which when mixed into a pat will set either in the air or under water, becoming hard in twenty-four hours, attaining considerable tensile strength in seven days, and in course of time a strength far greater than that of any other cement.

Testing

The tensile strength of samples of Portland cement is ascertained by forming the cement into briquettes or blocks, of the. form shown in Fig. 406, the section at A being generally 1 inch square. These are broken in a machine which applies slow tension upwards and downwards at K and K.

A good cement after setting seven days under water is expected not to break under a less weight than of about 800 lbs. on the area A (2 square inches), i.e. 355 lbs. per square inch.

Cooling

It is of the utmost importance that Portland cement should be thoroughly cool when used - all the lime in it thoroughly air-slaked - otherwise it may swell in the work when used, and cause much damage. In order to cool it, it should be spread out on a floor protected from the weather, and turned over daily for some weeks, so that every part of it may become thoroughly air-slaked.