1911 Encyclopædia Britannica/Mortar
MORTAR, the name (1) of a vessel in which any material may be crushed or pounded, and (2) given to various compositions used in building and consisting of lime and cement with sand or other fine aggregate, well mixed by manual labour or machinery with a proper quantity of clean water (see below, and also Brickwork). The Latin name both for such a vessel and for the material as mixed in it, is mortarium. The earlier English form mortar, from Fr. mortier, has been in modern English more closely adapted to the spelling of the Latin original. As applied to a vessel, the name is chiefly used for one employed in the preparation of drugs, which are pounded or triturated in the “ mortar ” by means of a pestle (Lat. pistillum; pinsere, to pound). The name has also been given, from a resemblance in shape to the vessel, to a short thick piece of ordnance, resting on a “ bed ” formerly used for high-angle fire. The barrel was always very short, normally even shorter than it was wide, and sometimes even resembled a bowl in shape. The place of the mortar in artillery is now taken by the howitzer. In modern times the name “ mortar ” is occasionally used for a particularly short howitzer. (See Ordnance.)
Building Mortar.—The sand forming the aggregate is placed on the mixing platform and formed into a ring within which lime in the required proportion is placed; it is then gently but thoroughly sprinkled with clean water through the rose of a watering-can or hose-pipe. The lime is covered with the sand and left undisturbed for a day or two to slake, and the whole mass is then turned over and well mixed with the larry. The mortar is often used immediately the materials are thoroughly incorporated, but it should rather be kept covered over with sacks until well tempered. For large works a mortar mill working by hand, steam, or other power effects a considerable economy. Stone chippings, clean, hard, broken bricks or furnace clinkers may take the place of sand when the mill is employed, as the action of grinding reduces any large pieces to small sandlike particles.
The remarks above apply to ordinary lime mortar. Mortar of hydraulic lime, cement mortar, or mortar gauged with cement, must be mixed up in quantities sufficient only for immediate use. Any material not used at the time, or at least the same day, will be wasted; cement cannot be reworked after it has begun to set as its setting properties are destroyed.
Slaking is a most important part in the process of making mortar. There are three methods of slaking lump lime—the first by immersion, the second by sprinkling with water, and the third by Slaking. exposing the lime to the atmosphere and leaving it to absorb moisture. Different qualities of lime require varying amounts of water, but the average quantity is about a gallon and a half to every bushel of lime. It should be all added at one time and the mass then left to slake undisturbed. Hot limes are often used for mortar. These are unsuitable for plastering unless slaked for along period. It will at once be seen that when mortars composed of these limes are used immediately after mixing, slaking must continue for a long time, drying up the moisture necessary for setting, and causing the mortar to crumble to dust in the joints of the brickwork. This fact gives us the reason for the old Roman enactment which set forth that lime should be slaked for three years before using. In the south of Europe it is the custom to slake lime the season before it is used.
The practical application of mortar to building work, and the methods of pointing the joints of brickwork and stonework, are described and fully illustrated in the article on Brickwork.
The results of many careful tests and experiments serve to show
that the hardening of mortar is due to several causes acting collectively.
With ordinary lime mortars the chief causes of Hardening
of Mortar. hardening are the absorption of carbonic acid from the air and the combination of part of the water with the lime, which unites with some of the silica of which the sand is composed and forms silicate of lime. The initial setting is due to the evaporation of the excess of water and to the production of minute crystals of hydrate of lime which slowly absorbs carbonic acid gas from the air. With mortar of rich lime an outer crust is thus formed on the exposed parts which prevents ready access of air to the interior and retards setting. In illustration of this peculiar property of lime to remain soft, some remarkable cases may be mentioned. One of the bastions erected by Vauban in 1666 was removed by General Treissart, in 1822, a hundred and fifty six years after erection. The lime in the interior of the masonry, where it was inaccessible to the action of the atmosphere, was found to be quite soft. Dr John of Berlin mentions that in removing a pillar 9 ft. in diameter in the church of St Peter, Berlin, eighty years after erection, the mortar in the interior was found to be quite soft. Sir C. W. Pasley, in removing the oldwharf wall at Chatham dockyard in 1834, found that the work executed in lime mortar was easily removable, the mortar being in a state of pulp. The brickwork, built with Roman cement, it was found necessary to blast.
The Romans were convinced that it was owing to prolonged and thorough slaking that their works in plaster became so hard and were not defaced by cracks. L. B. Alberti mentions in his writings that he once discovered in an old trough some lime which had been left there five hundred years and that it was quite soft and fit for use. The setting and hardening of hydraulic limes and cements are due mainly to crystallization brought about by the action of water on the silicate of lime, and not by mere absorption of carbonic acid gas from the atmosphere. As a consequence we find that this variety of limes and cements has the valuable property of setting hard while immersed in water and in many cases growing increasingly hard with the lapse of time.
Opinions differ very widely on the question of the suitability for
building purposes ofrlimes or cements which contain an appreciable
proportion of magnesia, many experts holding the view Magnesia
in Mortar. that the expansion which often occurs in floors and other works of concrete from one to four years after laying may be justly attributed to the presence of this substance. For mortars, however, it may be assumed that the presence of magnesia is not detrimental to the value of the matrix, but on the contrary may be a source of strength, for experiments show that it reduces the energy of 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 oly 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 Cement.)
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.
|Properties by Measure.|
|Sand.||Cement.||Lime.||Water.||Breaking Weight per sq. in. in ℔.|
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
of Mortar. commonly used, and left to set and harden for four weeks.
|Adhesive Strengths of Lime and Cement Mortars.|
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 Decay.
of Mortar. 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
Mortar.few experiments that have been carried out show varying results. In some German experiments, cubes of stone were joined together w1th 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 Sugar in
Mortar. 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 ' jaggery,' 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 ' jaggery ' 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 6½ ℔ per square inch. In twenty-one specimens joined with the same mortar without the ' jaghery ' the breaking weight was 4½ ℔ 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 By-Laws
of Mortar. the construction of walls must be composed of freshly aged 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 Limes and
Mortar. 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 American Portland cement was manufactured in 1874 by Mr. David O. Taylor.
The chief works of reference on this subject are G. R. Burnell, Limes, Cements, Mortars; Rivington, Notes on Building Construction; F. W. Taylor and S. E. Thompson, A Treatise on Concrete, Plain and Reinforced. (J. Br.)