The composition of concrete

Binders

Air-curing binders and hydraulic cements can be used. Among air-curing binders, slaked lime and gypsum are the most important ingredients. Hydraulic cements include lime and pozzolana mixtures and Portland cement, often with pozzolanas added. Another hydraulic alternative is geopolymeric cement which is manufactured without lime. Sulphur, see page 185, is a binder in a group of its own because it cures when cooling, having passed through a melting phase.

Cements based on lime, including Portland, which is by far the most common type, cause very high emissions of carbon dioxide during

13.4

Standard concrete precast units for walls and floors.

13.4

Standard concrete precast units for walls and floors.

production. If lime based ingredients are replaced by pozzolanas this can be somewhat reduced. Some of the carbon dioxide emitted during production will, during the building's lifetime, be reabsorbed through the process known as carbonatation (Fossdal, 2003). Geopolymeric and sulphur cements have low associated climate emissions.

During construction, contact with lime products can cause serious damage to the skin and eyes, so these products should be used with care. Portland cement contains chrome that can lead to a skin allergy, even though current products are usually neutralized, mostly with ferrous sulphate.

Pure mineral binders usually have no effect on the indoor climate. There can, however, be cement dust from untreated concrete surfaces. This can irritate the mucous membranes. Problems also occur if cement dust is left behind when a building is completed, for example in ventilation ducts. If the cement is not completely hydrated, due to insufficient watering, it is capable of reacting with other materials such as fillers with organic additives and plastic coatings.

As a waste product, Portland cement with fly ash releases soluble sulphurs into the environment.

Generally speaking lime cements and geopolymeric cements give the least environmental problems but they are slightly weaker than Portland cement.

Aggregates

In ordinary concrete the aggregates are divided into three groups: sand, gravel and crushed stone. In lightweight concrete there are also air-filled, thermally insulating aggregates which are discussed in Chapter 14 (See also summary in Table 13.3).

13.5

Mortar free construction with concrete lightweight blocks. Source: Maxit.

13.6

Mobile equipment for producing recycled aggregate.

13.6

Mobile equipment for producing recycled aggregate.

Little attention has been paid to the fact that different types of crushed stone make different demands on the concrete mix. The decisive factor is the tensile strength, and paradoxically a low tensile strength is more favourable. Crushed stone with a tensile strength of 200 kp/cm2 needs much less cement than that with an ultimate strength of 500 kp/cm2. Up to 10% of the world's cement production could probably be saved if this was considered (Shadmon, 1983).

In places with no sand, gravel or crushed stone, several types of building waste that do not attack lime can be used. Clay tiles and clay bricks crushed to a size of 1-40 mm can be used, but the material must be of good quality. Brick waste cannot be used if it contains nitrate residue from artificial fertilizers, as this increases the decay rate of the concrete. Artificial fertilization of agricultural land became widespread in the 1950s and affects many sites where raw material for brick-making is extracted.

In many European countries, Portland cement-based concrete is now being recycled to some extent (Figure 13.6). The concrete is crushed back to normal aggregate size and can be used up to a certain proportion in new concrete, depending on the strength required; for example, in concrete slabs for foundations of small houses and parking blocks, where they can replace up to 20% of the gravel.

There have also been successful test projects using recycled glass as aggregate in concrete. This has also been shown to reduce the need for plasticizers. If the recycled glass is used in foamed form this creates a lightweight and thermally insulating concrete. Other industrial residues such as fly ash and some types of slag can also be used. Another experiment has been to solve a major waste problem by adding used car tyres in ground up form, a product named Rubcrete (Lu etal., 2007).

In some countries where deposits of gravel and sand are scarce, sand is sometimes removed from beach zones and even from the sea.

Table 13.2 Concrete mixes, their properties and areas of use

Type

Mixtures, parts by volume

Properties

Areas of use

Lime sandstone

Lime 1; Quartz sand 9

Durable, sensitive to moisture/frost

Internal and external structures, cladding, moisture buffering

Lime concretes

Lime 1; Sand 2-4; Aggregate 4-6

Elastic, sensitive to moisture/frost

Internal structures, moisture buffering

Lime pozzolana concretes

Lime/pozzolana 3; Sand 1; Aggregate 2

Medium strength, elastic, resistant to moisture/frost

Internal and external structures

Gypsum concretes

Gypsum 1; Sand 1; Aggregate 2

Sensitive to moisture/frost

Internal structures, moisture buffering

Portland concretes

Cement 1; Sand 3-6; Aggregate 3-5

Strong, durable, not particularly elastic, resistant to moisture/frost

Internal and external structures, foundations

Portland-pozzolana concretes

Cement/pozzolana 1; Sand 3; Aggregate 3

Strong, durable, little to moderate elasticity, resistant to moisture/frost

Internal and external structures, foundations

Sulphur concretes

Sulphur 1; Sand/Aggregate 3

Waterproof but still sensitive to frost

Internal and external structures, foundations

|Table 13.3 Lightweight concretes, their properties and areas of use

i

Type

Materials

Properties

Areas of use

DC <

Aerated concretes

Cement, sand, lime, gypsum, aluminium powder

Moderate thermal insulation, low resistance to moisture/frost

Internal and external construction

Û.

Concretes with light aggregate

Cement, expanded clay or similar lightweight aggregate, sand

Low thermal insulation, resistant to frost

Internal and external construction, foundations

Pumice concretes

Cement, pumice, sand

Moderate thermal insulation

Internal and external construction

Concretes with wood chip

Cement, impregnated wood chip

Low thermal insulation, low resistance to moisture/ frost

Internal construction

Woodwool cements

Cement, impregnated woodwool

Moderate thermal insulation

Light internal and external construction

This disturbs the shore and can be damaging to marine ecological systems.

Some concrete aggregates may contain a substantial amount of crystalline silica, which when the concrete is cut or broken can expose the worker, and people in the vicinity, to potentially toxic dust. Aggregates may also contain varying amounts of radioactive material. The levels are low and usually have no effect on the indoor climate. Exceptions to the rule are some slates and industrial aggregates, which can affect the level of radioactivity quite strongly.

In foundations and buildings made largely of concrete, the aggregates will by weight be the single largest component. To reduce transport associated energy use and pollution it is always important to use local sources. Prefabricated concrete elements may involve long distance transport compared to preparing concrete in situ.

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