AAC Introduction

Autoclaved aerated concrete (AAC) is a popular building material in use all over the
world. It has a 50-year history of successful use in all environments for all types of buildings (Wittmann, 1983, 1992). Two types of AAC are commonly being manufactured. Both use combinations of lime and Portland cement mixed with either ground quartz sand or Class F fly ash. AAC offers a wide array of positive attributes vis à vis concrete block and insulated wood 2 frame construction for residents of North America. However, the final decision to use AAC instead of more conventional building materials will ultimately hinge on the cost of making, transporting and assembling AAC at its point of use. To date, even after intensive lobbying by the AAC industry, the North American consumer has not made the switch.

In fact two of the original German manufacturers have pulled up stakes and gone home.
Environmentally speaking, AAC is less material and energy consumptive than many
construction materials. It is estimated that the total amount of energy consumed during
production (including energy invested in making Portland cement, lime and rising agent) is on the order of 1000 MJ/m3 (Aroni et al., 1993). It achieves this status because some AAC manufacturers already use fly ash and other industrial waste in their formulations coupled with the traditional low energy processing in an autoclave..

The introduction of a cellular structure during initial formation of the AAC allows one to produce 3-4 m3 of AAC from 1 m3 starting materials (Aroni et al., 1993). When making lime-silica AAC, ground quartz flour is mixed with lime. Portland cement and water to make a thin slurry. At the last moment, rising agent is added to the mixture. The mixture is then transferred to a massive steel mold wherein the slurry begins to foam. After 45 minutes or so, the AAC “cake” occupies approximately 3-4 times as much space as it did when it was first poured. It has also developed enough green strength to be demolded, cut with wire saws to a variety of shapes (blocks and/or panels), and placed in autoclave. After treating in autoclave the block or panel is removed and ready for use. Densities are on the order of 400-600 kg/m3 and compressive strengths range from 2 to 5 MPa (Aroni et al, 1993).

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