Mortars

A mortar is usually a mixture of lime or cement with sand and water, often with additional pozzolanas and accessory agents added, used as a binder for different types of mineral building components: slabs, tiles, bricks, blocks and in certain circumstances roof tiles (Table 17.1). Mortars can also be based on gypsum, clay and sulphur. Fine or coarse sand is used as aggregate, according to the smoothness of finish required. In lime mortar, fine sand is usually chosen, preferably beach sand where the salt is washed out. Successful trials have also been made with mortars using a portion of crushed recycled glass which also has pozzolanic properties (Fragata, 2007). Small amounts of fibre can be added to increase the strength of the mortar. Mineral fibres or organic alternatives such as hemp, sisal, jute or animal hair can be used. Fine aggregates of granulated and foamed recycled glass, perlite, vermiculite or similar materials can also be added to increase the thermal insulation value. In certain modern mortar mixtures extra additives provide elasticity, watertightness, etc.

Table 17.1 Mortars in masonry

Mortar

Mixtures, parts per volume

Properties

Areas of use

Lime

Lime 1; Sand 2-3

Elastic, medium strength, sensitive to moisture/frost, quick-drying

All types of internal masonry; brick floors

Hydraulic lime

Hydraulic lime 1; Sand 2-4

Elastic, medium strength, frost-resistant, quick-drying

All types of internal and external masonry; brick floors

Anhydrite and gypsum

Gypsum 1; Sand 1-3

Elastic, weak, sensitive to moisture/frost, quick-drying

Internal masonry

Portland cement

Cement 1; Sand 3-4

Less elastic, low moisture diffusivity, frost-resistant, quick-drying

Internal and external tiling

Lime Portland cement

Lime 1-2; Cement 1-2; Sand 7-11

Elastic, medium strength to strong, frost-resistant, quick-drying

All types of internal and external masonry

Sulphur

Sulphur

Elastic, medium strength, moderate resistance to moisture/frost, watertight

Walling of sulphur blocks and bricks

Clay

Clay 5; Sand 1

Elastic, weak, sensitive to moisture/frost, quick-drying

Walling of earth blocks (adobe) and low-fired bricks

Aggregates must not react chemically with any other materials in the mortar, nor take an active part in the solidifying or curing. Water used in lime and cement mortars should be fresh and must not contain salt, sulphur or other substances that can break down the mixture.

Blocks or bricks are usually laid with mortar joints. But a fibre-reinforced mortar can also be sprayed on both sides of a wall built completely dry (see Figure 13.4).

Mortars have different elasticity coefficients and strengths. This is critical for the tasks they perform, but is also important for any later dismantling of components. Pure Portland cement mortar is, for example, twice as strong as pure lime mortar; hydraulic lime mortar is somewhere between these. The use of lime mortars, hydraulic lime mortars and lime-cement mortars rich in lime makes it quite easy to dismantle walls of bricks and concrete blocks for re-use. They are also more tolerant to movements due to thermal and moisture action. Lime-based mortars are also capable of self-healing of microscopic cracks by plastic flow and diffusion (Yates etal., 2007).

Mortar products are based mainly on materials with rich reserves. The energy consumption in production is considerable; as are emissions of carbon dioxide, nitrogen oxides, sulphur dioxide and dust. The environmental load is less for mortars rich in pozzolanas or based on lime and hydraulic lime than for mortars based on Portland cement. Lime mortars will also subsequently reabsorb more of the carbon dioxide emitted during production than is the case with Portland cement mortars.

Mortars were once entirely mixed on site with local aggregates; it is more normal these days to use ready-mixed mortars. Centralized production means an increased use of transport energy, since even the aggregate has to be transported great distances. However, the aggregates used are often light and give better thermal insulation in the finished structures.

Mortars cause no problem once in place, as long as no volatile organic compounds have been added.

Sulphur mortars can be recycled. This is also true for pure lime mortars, in theory, because they can be re-burned, but this is difficult to achieve in practice. Most mortars can be ground into aggregate for low quality concrete structures.

As waste, mortars are normally inert and can be used as fill. Ground lime mortars can be used for soil improvement. Sulphur pollution can develop from gypsum waste because of microbial decomposition. Sulphur waste should be deposited at special dumps, preferably neutralized by adding lime.

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