Manufacture of Aerated Concrete Blocks
Aerated blocks are made from cement, lime, sand, pulverized fuel ash (PFA, a by-product from power stations) and water. The PFA, sand and water are mixed together into a slurry which is then heated and mixed with the cement and lime. Next, a small quantity of aluminium powder is evenly dispersed within the mixture, chemically reacting with the other constituents to form millions of tiny
bubbles of hydrogen. The mixture is then poured into elongated moulds.
The hydrogen subsequently diffuses out from the material as it sets, to be replaced by air, creating an internal micro-cellular structure of millions of air pockets (see Fig 38). When the concrete has partially set, the long strips of aerated concrete are wire-cut into blocks of the correct size. The cut blocks are then transferred to a high-pressure autoclave for steam-curing, during which calcium silicates are formed, which permanently bind all the ingredients together into a solid mass.
Aerated concrete blocks are more expensive than more traditional concrete blocks (by approximately 50 per cent) and break easily if handled carelessly, but they have numerous advantages:
• they are easy and light to handle;
• they facilitate faster construction;
• they have a high ratio of strength to density;
• they are available in a variety of thicknesses, from 75mm to 230mm;
• they can be cut, drilled, chased and fixed to easily;
• they offer good heat insulation and low thermal conductivity;
• their sound transmittance is low;
• their fire resistance is good;
• their frost resistance good;
• their sulphate-resistant properties are good;
• the load on foundations is reduced.
Within their wide range of aerated blocks, manufacturers are always ‘tinkering’ with the constituents and
manufacturing process to produce blocks that are enhanced in certain key areas such as strength, thermal
transmittance, sound insulation or fire resistance; the last two criteria are particularly important for party walls.
Due to the large number of benefits offered by aerated concrete blocks, they are put to a wide range of uses and applications, with a variety of blocks available to suit different purposes
Fig. 38 Inside an aerated concrete block.
Standard blocks are available in a range of crushing strengths – 2.8N/mm2, 3.5N/mm2, 7.0N/mm2 and 8.4N/ mm2 – and are used for internal partitions (load-bearing and non-load-bearing), internal skins of cavity walls, external skins of cavity walls (when rendered), external solid boundary walls (provided they are rendered or clad for weather resistance) beam and block floors, and sub-structure work below ground level. The higher-strength blocks tend to get used where greater strength is required, such as for two – and three-storey construction. 10.4N/mm2 blocks are available for four-storey construction
Wide blocks are available for use as ‘trenchblocks’ below ground, which eliminates the need for two skins of a cavity wall, plus cavity in-fill, to be built from the foundation to ground level. This greatly speeds up construction. Construction speed is also enhanced by some manufacturers of trenchblocks incorporating a tongue and groove moulding into the blocks, which enables them to essentially ‘slot together’.
While aerated concrete blocks will receive paint and tiles directly, there are a number of block types available with a smooth, paint-grade surface. These are suitable where a higher standard of internal decorative finish is required without plastering.
Mortar mixes should not be too strong as aerated blocks do not accommodate movement very well. The recommended mortar mix, however, is much the same as for dense concrete blocks: cement:lime:sand at 1:1:6 or 1:2:9. Below ground level a slightly stronger mortar should be used, such as 1:1/2:4.