Mortar in Building

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. As a consequence we find that good hy draulic limes and cements have the valuable property of setting hard while immersed in water and in many cases growing increas ingly hard with the lapse of time.

Extra Constituents.

Magnesia.—Opinions differ on the ques tion of the suitability for building purposes of limes or cements which contain an appreciable proportion of magnesia, many ex perts holding the view 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 limestone furnish a durable cement for con struction in the sea. Recent specifications for Portland cement provide that magnesia contents shall not exceed 2% of the whole and modern writers express the view that magnesia in excess of 4% is dangerous as it reduces the tensile strength of the cement under normal conditions and makes mortar or concrete unsound. (See CEMENT.)' It is a good plan, where the question of cost pre cludes the use of mortar made entirely of cement, to add to lime mortar mixed in the usual proportions a small quantity of Port land cement. This is termed "gauged" lime mortar. By this addition the strength is greatly increased at little extra cost.

Adhesive Strengths of Lime and Cement Mortars Adhesion of Mortar.—The above table shows the force re quired 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 indicate that the adhesive strength of mortar varies with the proportion of sand used, the resistance 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 impurities or of decayed vegetable and animal matter in the sand, though the lime or cement itself may be defective. 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.

Salt.—In some German experiments, cubes of stone were joined together with cement mixed with waters of different character, 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 crumbled, having lost all its tenacity. The cement made with water containing 2% of salt was in a better condition, while that containing 8% of salt had not suffered from its ex posure to frost. The use of salt causes 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 Port land 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 lb. per square inch when a week old, and of sio lb. at five weeks after mixing. Salt has no serious effect upon the strength of a mortar, although it definitely retards the setting process.

Sugar.—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 very much 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.