Materials Science In Construction: An Introduction by Ahmed Arshad Sturges John

Author:Ahmed, Arshad, Sturges, John
Language: eng
Format: epub, pdf
ISBN: 9781135138400
Publisher: Taylor and Francis

13.3 Production of aircrete

Cement or cement and lime with pulverised fuel ash (PFA) and/or ground sand form the calcareous and siliceous ingredients, which are mixed together with water to form a low-viscosity slurry. The mix receives a predetermined small dose of finely divided aluminium powder just before it is poured into pre-oiled large steel moulds. The mould is of a size and dimensions suitable for the volume required to make a specific number of blocks or slabs. Approximately one-third of the mould is filled with the slurry. Therefore, the mould is kept in a warm and humid environment and the slurry is left to hydrate. The period of time depends on the process used. The aluminium powder soon reacts with the alkaline environment in the mix to generate tiny bubbles, which stabilise to form the aircrete cellular structure. During this process the mix rises in a similar way to cake in an oven, hence giving the material the porous structure. After the initial set has taken place it is cut into block size. The final process involves autoclaving for approximately 9–12 hours at 180–200 °C, which provides the product with its final strength. It is ready for despatch usually after cooling. In the UK most aircrete is manufactured using PFA, which confers environmental benefits as PFA is a waste product from coal power stations.

Aircrete consists of 60–85 per cent air by volume, depending on density. The solid material part is a crystalline binder, which is called tobermorite. Besides the binding phase, tobermorite, grains of quartz and small quantities of other minerals also exist. The chemical composition of tobermorite comprises of silicium dioxide, calcium oxide and H2O (water). It is tobermorite that provides the relatively high compressive strength and stability of aircrete in spite of the high proportion of pores and lack of course aggregate in this material.

During manufacturing, gas-forming tiny aluminium flakes within the powder produce millions of small bubbles of hydrogen in the slurry. The mixture rises in its moulds until all aluminium has reacted and the desired volume is reached. Bubbles formed are mostly about 1 mm in diameter, which gives the porous structure. The production process is modified accordingly to attain the appropriate level of porosity. The production process is simplified and illustrated in Figure 13.1.

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