Chemical binders

Mortars, adhesives and fillers are important binders in the building industry. Mortar and adhesives are used to bind together different or similar components; fillers are a sub-group used to fill cracks and stick to the surfaces that surround them.

Mortars

A mortar is usually a mixture of lime or cement with sand and water, sometimes with additives, used as a binder for different types of mineral building-stones, slabs, tiles, bricks, blocks and in certain circumstances roof tiles. (See also ‘Hydraulic binders’ and ‘Non-hydraulic binders’, pp. 94-97.) Fine or coarse sand is used, according to the smoothness of finish required. In lime mortar, fine sand is usually chosen, preferably beach sand. Small amounts of fibre can be added to increase its strength. Mineral fibres or organic alternatives such as

Table 17.1: Mortars used for masonry

Mortar

Materials and proportions

Properties

Areas of use

Lime

Lime: 1 Sand: 2/3 Water

Elastic, medium strength, not very resistant to water and frost, quick­drying, balances relative humidity well

Internal laying of bricks, stone, expanded clay blocks, brick floors, render

Hydraulic

lime

Hydraulic lime: 1 Sand: 2/4 Water

Hydraulic, medium strength, elastic, frost-resistant, balances relative humidity, quick-drying

All types of internal and external masonry, render

Portland

cement

Cement: 1 Sand: 3/4 Water

Hydraulic, strong not so elastic, frost-resistant, low moisture absorption, slow-drying

Internal and external laying of tiles, render

Lime cement

Lime: 1І2 Cement: 2/1/1 Sand: 10/7/11 Water

Hydraulic, medium strength to strong, elastic, frost-resistant, medium moisture absorption, medium-slow-drying

All types of internal and external masonry, render

Anhydrite and gypsum

Gypsum: 1 Sand: 1/3 Water

Elastic, weak, not very resistant to water and frost, balances relative humidity well, quick-drying

Smaller internal walls, internal render/plaster, and external render

Clay

Clay: 5 Sand: 1 Water

Elastic, weak, not very resistant to water and frost, balances relative humidity well, quick-drying

Laying of earth blocks and low-fired brick

Sulphur

Sulphur

Elastic, medium strength, medium resistance to frost, watertight

Laying of sulphur blocks and bricks

hemp, sisal, jute or animal hair can be used, with a fine aggregate of granulated and foamed recycled glass, perlite, vermiculite or similar materials, added to increase the insulation value. In certain modern mortar mixtures extra additives provide elasticity, watertightness, etc. Lime cement mortar is often made using additives that bring air into the mix, giving it a waterproofing quality. (See also ‘Additives in cement’, p. 97.)

Aggregates must not react chemically with any other materials in the mortar, nor take an active part in the solidifying or curing of the mortar. 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 between them. The Southwest Research Institute in Texas has developed a fibre-reinforced sulphur mortar which can be sprayed onto both sides of a wall built completely dry.

Mortars have different elasticity coefficients and strengths. This is critical for the different tasks they perform, but also important for any later disman­tling of components. Pure 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 possible to dismantle walls of bricks, concrete blocks and lightweight concrete blocks, etc for re-use. Lime cement mortars must contain a minimum of 35 per cent cement, partly because a smaller percentage does not strength­en the mortar and partly because the cement slows down the curing of the lime.

Mortar products are based mainly on materials with rich reserves. Their consumption of primary energy lies somewhere between that of timber and steel. Pollution during the production of binders is mainly in the form of dust and the emission of a considerable amount of nitrogen oxides, sulphur diox­ide and carbon dioxide. Binders containing pozzolana create the least pollu­tion.

Mortars were once entirely mixed on site with local aggregates; it is more nor­mal today to use ready-mixed mortars. Centralized production means an increased use of transport energy, since even the aggregate has to be transported greater distances. However, the aggregates used are 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 true for pure lime mortars, in theory, because they can be reburned, but it is difficult to achieve in practice. Cement 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.

Updated: September 27, 2015 — 6:13 am