Cement products

Cement can be used as insulating material in three forms:

1. As foamed concrete.

2. As aerated concrete.

3. As binder for light mineral and organic aggregates.

Foamed concrete has considerably better thermal insulation properties than normal concrete, but still a lot less than conventional insulation materials. The lambda value is 0.1W/mK for densities of approximately 650 kg/m3. It consists of Portland cement and fine sand in proportions of about half and half. The foaming agent is either tenside or protein substances. The latter can cause considerable problems in the indoor climate if they react with cement. Foamed concrete is seldom used in building construction because of its relatively low thermal insulation and low loadbearing capacity. It is used nowadays mostly for the levelling of floors, sprayed onto horizontal surfaces from mobile tanks transported by lorry.

Protein-based foaming agents are biologically degradable whilst this is only the case for about 80% of tenside products. Additional environmental aspects of this concrete are the same as for in situ concrete (see The composition of concrete, page 195).

Aerated concrete is produced by reacting finely powdered quartz (about 50% by weight) with lime, gypsum and cement. Fly ash can replace some of the lime. A yeast constituent such as aluminium powder is added to a proportion of about 0.1%. Aluminium reacts to release hydrogen. When the substance is almost stiff, it is cut into blocks and slabs that are hardened in an autoclave at 180 °C. Prefabricated slabs are usually reinforced with steel. Aerated concrete is the only commercial pure mineral block with good structural properties combined with high thermal insulation value. The material is very porous, and needs a surface treatment which is open to vapour, hydraulic lime render, for example. If the water content becomes too high the material will easily be cracked by frost. The production of aerated concrete is dependent upon aluminium. The total contribution of aluminium in the external walls of an average sized dwelling is 10 to 20 kg. Often this is recycled aluminium.

Quartz dust can be a problem during production. Aerated concrete normally has good moisture regulating properties and does not have any negative effects on the indoor climate, although the steel reinforcement can increase the electromagnetic field in a building. Aluminium will have completely reacted in the finished product, and in practice aerated concrete can be considered inert and problem free as waste. Both prefabricated slabs and blocks can be re-used, depending on how they were laid and the mortar used. Strong mortars are generally used which make it difficult to dismantle the components without damaging them. More appropriate mortars are weak lime cement mortar and hydraulic lime mortar. Crushed aerated concrete can be used as insulating granules for road building, and also as aggregate in lime sandstone, different light mortars and light concretes.

Concrete with light aggregate is usually produced as blocks, slabs or floor beam units and is relatively strong. There is a difference between products that have an organic and a mineral aggregate.

Mineral insulating aggregate in concrete can be pumice, fossil meal and light expanded aggregates of clay, vermiculite, perlite, recycled glass or blast furnace slag. Pumice, expanded clay, perlite and glass are the products best suited, as the others have a higher moisture absorption.

Sawdust and chopped straw can be used as organic constituents in concrete. Blocks are also produced using broken up waste polystyrene, and it is possible to produce lightweight concrete mixed with waste paper. With the exception of woodwool slabs (discussed later in this chapter in the 'Timber'-section), concrete with organic constituents generally has a poor thermal insulation value compared to rival products such as aerated concrete.

Raw material for concrete with light aggregate is widely available. The pollution and energy use caused by the production processes are quite similar as those for constructional concrete, see Chapter 13.

Except for possible pollution from the additional use of insulation based on fossil oils in some products, the use of concretes with light aggregate usually causes no problems.

Lightweight concrete blocks can easily be re-used if they are held together by weak mortars, as can larger concrete units that have been bolted or placed without fixing.

Concretes with most light aggregates are initially inert and the waste from them can be used as a filler for road building, as ground insulation or as insulating aggregate in smaller concrete structures, light mortars and plaster.

0 0

Post a comment