Page:EB1911 - Volume 18.djvu/908

 slaking and increases that of the setting processes. Cements containing magnesia are pronounced both by Vicat and Chatoney to resist the dissolving action of sea-water better than those in which no magnesia is present, and it is pretty well established by experience that cements derived from argillo-magnesian limestones furnish a durable cement for construction in the sea.

The old mortar of the Romans, which proves its great property of endurance by many of their works still remaining, was in all probability composed of lime mixed with pozzolana or “trass.” These materials are similar in character and are obtained from extinct volcanoes—in the case of the Romans from the Italian volcanoes, but also from extinct volcanoes in the valleys of the Rhine and in Holland. Good as these mortars undoubtedly were, it may be safely asserted that no cement or mortar has been discovered to excel in strength, or in durability in all climates, the Portland cement of the present day. The best varieties of this material are made in England, the country of its origin, much of the continental and American product being deficient in the qualities which combine to make a good cement. (For the properties of Portland cement and the method of its manufacture see .)

The comparative strengths under tensile stress of grey-lime mortar, Portland-cement mortar, and Portland-cement mortar with the addition of lime, are given in the following table, which is the result of a series of tests by G. R. Redgrave.

It is a good plan, where the question of cost precludes the use of mortar made entirely of cement, to add to lime mortar mixed in the usual proportions a small quantity of Portland cement. This is termed) “gauged” lime mortar. By this addition the strength is greatly increased and the extra cost is but slight.

The following table shows the force required to tear apart common stock bricks bedded in mortar, mixed in proportions commonly used, and left to set and harden for four weeks.

These results show clearly that the adhesive strength of mortar varies according to the proportion of sand used, the power of resistance of the mortar to the force brought to bear upon it decreasing as the proportion of sand is increased.

The primary cause of the premature decay which sometimes takes place in mortars and like material is due to the presence of mud and decayed vegetable and animal matter in the sand, or possibly in the lime or cement itself. It is therefore of great importance to use a perfectly clean sand for the aggregate, and to select a lime or cement of good quality for the matrix,

care being taken that no foreign matters detrimental to the mortar be introduced during the processes of preparation.

The effect of salt in mortars as a preventive of the destructive effects of frost has not as yet been thoroughly determined, and the few experiments that have been carried out show varying results. In some German experiments, cubes of stone were joined together with cement mixed with water of different characters, ranging from pure rain-water to

water containing from 2 to 8% of salt. Before the cement was set the blocks were exposed in air at a temperature varying from 20° F. to freezing-point, after which they were kept for seven days in a warm room. The samples were then examined with these results: The cement mixed with pure water was quite crumbled, having lost all its tenacity. The cement made with water containing 2% of salt was in rather better condition, while that containing 8% of salt had not suffered from its exposure to frost. The use of salt causes much efflorescence on the face of the work, and should therefore not be used where this would be undesirable. Nor should salt be employed for work that is to be subsequently painted. The mortar for the brick facing of the Forth Bridge below water was composed of one part of Portland cement and one part of sand mixed with salt water in a mill. Briquettes made from this compound withstood a tensile stress of an average of 365 ℔ per square inch when a week old, and of 510 ℔ at five weeks after mixing. Salt has no effect upon the strength of a mortar, although it retards the setting process somewhat.

Cement mixed with a percentage of sugar (usually 2% and under) has been used with varying success. In India sugar is a frequent ingredient in mortar, probably because it has the effect of preventing too rapid setting; it also retards the drying of the material. The sugar must be dissolved in the water used for gauging, as the results obtained when the sugar is mixed

with the other ingredients in a dry state are not good. The addition of sugar to water enables it to take up about fourteen times more lime than pure water. It is supposed by many writers who have studied the methods of the ancients that old Roman mortars contained strong ale, wort or other saccharine matter, and it is probable that the use of sugar with lime passed from India to Egypt and Rome. The following is an extract from the Roorkee Treatise on Engineering, a work of reference published in India: “It is common in this country to mix a small quantity of the coarsest sugar, ‘goor’ or ‘jaghery,’ as it is termed, with the water used for mixing up mortar. Experiments were made with bricks joined together by mortar consisting of one part of common shell lime to one and a half parts of sand, one pound of ‘jaghery’ being mixed with each gallon of water. The ricks were left for thirteen hours and after that time the average breaking weight of the joints in twenty trials was 6 ℔ per square inch. In twenty-one specimens joined with the same mortar without the ‘jaghery’ the breaking weight was 4 ℔ per square inch."

Of the saccharine matters used in mortar treacle seems to give the best results, rough cane sugar being next in effectiveness; beetroot sugar is not a good material to use.

The by-laws made by the London County Council in 1891 under sec. 16 of the Metropolis Management and Buildings Act Amendment Act 1878 require that “the mortar to be used in the construction of walls must be composed of freshly burned lime and clean, sharp sand or grit without earthy matter, in the proportions of one of lime to three of sand

or grit.” The cement to be used must be Portland cement or other cement of equal quality to be approved by the district surveyor, mixed with clean, sharp sand or grit in proportions of one of cement to four of sand or grit. Burnt ballast or broken brick may be substituted for sand or grit, provided such material be properly mixed with lime in a mortar mill.

The varieties of lime and cement chiefly used for mortar in the British Isles are set forth below:—

Pure or fat limes should not be used for mortar. Grey stone lime, feebly hydraulic, makes a good mortar, but should not be employed for work below ground or in other damp situations. It is obtained chiefly at Dorking, Halling, Lewes and Merstham. It is used in the proportion of one part to two or three parts of sand. An analysis of the lime

from Castle Bytham gives the following composition:—

Blue lias lime is eminently hydraulic and should be used in good class work. Its use is a necessity for foundations and work in damp situations where Portland cement is not employed. It is used in the proportions of one part to one or two parts of sand. The best-known varieties are obtained from Watchet in Somersetshire, Barrow-on-Soar in Leicestershire, Rugby in Warwickshire, and Lyme Regis in Dorsetshire. A typical lias lime shows on analysis the following composition:— Portland cement is the best matrix known, since it is the most powerful and the most durable. It is used for mortar wherever great strength, hard-wearing properties, and resistance to damp are required. It should weigh 112 ℔ per striked bushel and be ground fine enough to pass through a sieve having 2500 meshes to the square inch and leave not more than 10% residue. Test briquettes after setting under water for seven days should stand a tensile strain of 350 ℔ on a square inch. It is used in the proportions of one part of cement to from one to five parts of sand.

Portland cement of a similar character to the English cement, but somewhat less powerful, is largely made in America. The principal seat of manufacture is Coplay, Pa., where the first