Aerated Concrete-1: Manufacture and
Properties
The group of building materials described collectively as “lightweight concretes" comprises two main types — aerated concretes and
concretes made with lightweight aggregates.
Over the last ten years, the knowledge and use of these materials has extended considerably and the production of lightweight concretes is now increasing rapidly. In this digest, the information about aerated concrete is brought up-to date. This report is a reproduction of the British Research Station Digest 16 (second series).
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HE term lightweight' as applied
to concrete is somewhat arbitr- ary: although formerly it was used to denote concrete having a dry density not exceeding 100 lb/ft3, the more recent adoption of light ma- terials for reinforced units has led to the inclusion of concretes with a density of 110-115 or even 120 lb/
fts.
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Concrete is made lighter by introduc- ing air into its composition and this can be done in three main ways. single-size aggregate, either dense lightweight, can be used to give air voids, as in 'no-fines' concrete; ог a porous aggregate can be used, which will retain some air within itself, as in lightweight-aggregate concrete.' The type of concrete with which this Digest is concerned, however, is produced in a third way, namely, by introducing air (or other gas) into a cement or lime mix so that when the mass has set, or has been hardened by steam treatment, a uniform cellular product is formed. This is known in Britain as 'aerated con- crete,' and in other countries is known variously as 'cellular concrete,' 'gas concrete' or 'foamed concrete.'
Aerated concrete is a more
or less homogeneous fine-grained silicate mass enclosing a large number of small non- communicating cells. It is composed of cement (or in some instances lime) together with a fine siliceous material such as ground sand, pulverized-fuel ash, ground burnt shale or ground slag, or a mixture of these, with certain minor additions. The aeration can be produced in various ways as will be shown later. Thus, although aerated concrete is a concrete in the functional sense, it has little resemblance to ordinary concrete It also differs from the other forms of lightweight concrete, which are modified forms of conven- tional concrete.
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By adjustment of its composition and method of manufacture, aerated con- crete can be produced in a range of densities between 25 and 90 lb/ft 2, with a correspondingly wide
range of other properties.
Aerated concrete is made in two forms: precast units and in situ con- crete. The former are usually cured in high-pressure steam (autoclaving), while in situ material is necessarily air-cured. This distinction is important, since only by autoclaving is it possible to obtain really light material with strength and drying shrinkage characteristics accept- able for load-bearing purposes; air- cured aerated concrete possesses only about half the strength obtainable from autoclaved products of the same den- sity, and its drying shrinkage may be four or five times as much. However, in situ aerated concrete is used for insulation roof-screeds, pipe-lagging and other similar purposes, for which high strength and low drying shrinkage are not essential.
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Formerly, production of autoclaved aerated concrete in Britain fined to
masonry blocks, but more recently large reinforced units have become available; these offer the ad- vantages of low density (thus reducing the dead load of the structure as well as the handling loads), speed of erection, and high thermal insulation.
Although the basic principles of aerated concrete manufacture were well- known as early as the beginning of this century, full-scale production did not begin until 1929, in Sweden. Since then, very great progress has been made, and in Sweden to-day aerated concrete almost eclipses all other building materials, whilst licenses of the Swedish companies operate in nearly twenty countries from the Arctic to the Tropics. In the United Kingdom, nine factories now produce autoclaved aerated concrete, and several more are being planned. In addition, several firins make in situ aerated concrete.
Building
MANUFACTURE
In making aerated concrete there are two distinct methods for introducing air or other gas into a cement or lime mix, so as to impart a cellular struc- ture to the product. In the first, gas is generated by chemical reaction within the mass during the liquid and plastic stages, usually through the addition of fine aluininium powder to the mix. The aluminium produces bubbles of hydro- gen by reacting with lime ог other alkaline substances that are either added deliberately or formed by the setting of the cement.
In the second method, air is intro. duced from without, either by mixing- in a stable foam (such as is used in fire- fighting) or by whipping-in air with the aid of an air-entraining agent added to the mix. The foam and air-entraining methods are used chiefly for producing aerated concrete in situ. For the pro duction of precast aerated concrete units,
the gas-generating method is used almost exclusively in Britain at the present time and it will there- fore be described in more detail.
When the mix is based on Portland cement, the product gains its strength partly by the normal setting of the cement, and partly by the subsequent autoclaving. With lime as the binder, however, the strength development depends entirely upon the reaction be- tween the lime and the siliceous filler, and this takes place only under auto- clave conditions; when lime is used. therefore, autoclaving is essential to make a product at all. Whether cement lime is used, the products after autoclaving are substantially alike in composition.
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In addition to the cement (or lime) and aluminium powder, various other ingredients are often included in the mix. For example, a wetting agent may be used to assist the wetting of the cement and the metal powder; sugar is sometimes added to increase the
THE HONG KONG & FAR EAST BUILDER-VOLUME 16, NUMBER 5
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