Mortar in Building

MORTAR IN BUILDING. This consists of granular and siliceous material, such as sand, crushed brick, or stone, chippings, or crushed clinker with a cementitious material, such as lime or cement, mixed and reduced to a plastic state with water. The purpose of mortar is to bed uniformly and to unite blocks of building material in order to provide continuous contact of sur faces for the transmission of loads and stresses, and also to render the joints weatherproof.

Lime mortar is in common use for buildings such as dwellings and for industrial structures of moderate height, but cement mortar is much better adapted for the transmission of great stresses and, therefore, more suitable for use in high buildings, foundations, piers, etc. Cement mortar is also used for pointing or filling of external joints in all buildings where weathertight beds and vertical joints are required. Lime mortar is much weaker than most building materials; cement mortar more nearly ap proaches the strength of bricks and stones and enables these to withstand higher stresses than if bedded in lime mortar. Where the latter is used for bedding building blocks it should be of the character known as "hydraulic," especially for work at or below the ground level. Pure or fat limes are most suitable for the process of internal wall plastering. Grey stone lime which has slight "hydraulic" properties makes a good mortar for general work above ground.

Mixing.

Lime mortar may be mixed by hand or by machinery. Well slaked lime must be thoroughly mixed and incorporated with fine granular material, using sufficient water to make it plastic for manipulation with the trowel. Where a mill is used, stone chippings, furnace clinker or other substitute for sand may first be ground under the heavy rollers, in water, to a fineness approach ing that of sand ; the lime may then be added and the mixing accomplished by a similar operation which must not be prolonged otherwise there is a danger of reducing the particles to a much finer state which results in a fattier mass and reduces the proba bility of setting in the interior of the mass. Mortar made from hydraulic lime or cement or mortar gauged with cement must be mixed in quantities sufficient only for immediate use; cement cannot usefully be reworked after its initial set has begun because its setting properties are reduced, and the resulting mortar is weak and unsound.

Slaking.

This is a most important operation 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 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 all be 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 a long period. When mortars composed of these limes are used immediately after mixing, slaking must continue for a long time absorbing the moisture necessary for setting, making the work liable to serious damage by expansion of the slaking particles and often causing the mortar to crumble to dust in the joints of the brickwork. There was an old Roman enactment which set forth that lime should be slaked for three years before using and in the south of Europe it is still the custom to slake lime the season before it is used.

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 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 combine with carbon dioxide 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 confines the setting of the interior of the mass to a very slow and often incomplete formation of particles of silicate of lime.