The rock from which natural cements are made differs greatly in character in the same locality, and in different strata in the same quarry. In some of the mills the nature of the product is regulated by mixing, in proper proportions, the clinker obtained by burning rock from different strata. Each portion of the rock must be burned in such degree as is suited to its composition, and hence, as the material is not pulverized before burning, it must be burned sepa rately and mixed afterward. To produce uniformly good cement, therefore, requires close and careful attention; for this reason there is often considerable difference in the quality of cement made by works in the same locality and from very similar materials.
Mixed Cements.—In localities where both Portland and natural cements are made by the same works, mixtures of the lower grades of Portland with natural cements are sometimes made. These are usually sold as natural cements under the name Improved Cements. The effect of the mixture is to make the setting slower, and to somewhat increase the strength of the natural cement.
It is very important, in making slag cements, that the slag be ground very fine, and be very intimately mixed with the lime. The lime is slaked and bolted and then ground mechanically with the slag so as to insure thorough incorporation into the mixture. In some of the European plants the slag is finely ground and bolted through fine sieves before being mixed with the lime, but more com mon practice is to slake and bolt the lime and mix with the granular slag before grinding, or to do the pulverizing of the slag in two stages and make the mixture between the first and second grinding.
Puzzolan cement is usually very finely ground, and is slow in setting. It is sometimes treated with soda to quicken the set. When allowed to harden in dry air, it is likely to shrink and crack. When used for under-water work, mortar of puzzolan cement fre quently gives nearly the same strength as good Portland cement. It is essentially a hydraulic material, and it is specially important that it be kept damp during the early period of hardening, in order that the water necessary to proper hardening may not evaporate.
The composition of slag cement usually differs from that of Portland cement in having a less quantity of lime, more silica and alumina and more alumina in proportion to silica.
16. Sand cement is the name given to material formed by grinding together Portland cement and silica sand to extremely fine powder and a very intimate mixture. It is claimed that a considerable amount of sand may be thus mixed with the cement without materially reducing the strength of mortar made by mixing the resulting cement with the usual proportions of sand. The additional grinding reduces all of the cement to impalpable powder, thus increasing the amount of active material.
Sand cement as ordinarily made contains equal proportions of Portland cement and silica sand. Cement of this character has recently been made in California by grinding volcanic rock, or tufa, with Portland cement. The tufa used is a puzzolan, and it is claimed that it reacts with the lime of the cement. The results of tests indicate that mortar made from this cement is equal in strength to that of the original Portland. Cement of this kind is now being made by the U. S. Reclamation Service in some of the Western States to reduce the cost of concrete work where Portland cement is expensive and difficult to get.
Similar methods arc employed in Germany where a puzzolan called trass is used, and in Italy where volcanic lava is ground with the cement. These cements are used for work in sea water to les sen the action of the sea salts upon the lime salts of the Portland cement.
Sand cement has frequently been used for the purpose of secur ing impermeable mortar where waterproof work is needed. It is useful for this purpose on account of its extreme fineness.
17. Soundness of Cement.—The permanence of any structure erected by the use of cement is dependent upon the ability of the cement, after the setting and hardening processes are complete, to retain its strength and form unimpaired for an indefinite period. Experiment has shown that mortars made from cement of good quality frequently continue to gain strength ami hardness through a period of several years, or at least that there is no material diminu tion in strength with time; and that changes of temperature, or in the degree of moisture surrounding it, produce no injurious effects upon the material. This durability in use is commonly known as the permanence of volume or soundness of the cement.