Aerated concrete (AAC) block wall

A single-leaf wall of autoclaved aerated concrete (AAC) blocks: a mineral conglomerate expanded in an autoclave, where millions of closed air micro-cells lower the conductivity to insulant-grade values while it remains a load-bearing, non-combustible material. Extremely light and workable, it is laid in a thin bed and combines structure, weather seal and thermal insulation in a single layer.

Aerated concrete - or autoclaved aerated concrete (AAC) - comes from a mix of cement, lime, very fine sand and water, to which an expanding agent (aluminium powder) is added that develops bubbles of hydrogen. The mix «rises» like bread, is then cut into blocks and steam-cured in an autoclave. The result is a mineral material more than eighty per cent made of air, dense with closed cells, that carries load and insulates at the same time.

The conductivity of a solid drops sharply if it is filled with still air. In AAC the micro-cells, closed and evenly distributed, continually interrupt the path of heat through the solid matrix and prevent convective motion: the conductivity falls to 0.09-0.13 W/mK, panel-insulant values in an element that is also structure. The very low density (300-500 kg/m³) makes the block light and easy to handle, cut and rout with hand tools.

The blocks, ground to flat parallel faces, are laid with a thin-bed adhesive-mortar joint a few millimetres thick combed on, often with dry tongue-and-groove interlocks on the vertical joints. The thin joint is essential: a traditional thick-bed mortar, far more conductive than the block, would sew back a grid of thermal bridges. Lintels, ring beams and reveals are solved with «U»-shaped blocks used as permanent formwork and with insulating elements that restore continuity.

AAC is non-combustible (Euroclass A1) and, for the same thickness, offers high fire resistance: it is the material of choice for compartments. It is, however, very porous and absorbs water readily, so it must be protected from driving rain and rising damp with breathable plasters, water-repellent plinths and a proper detachment from the ground. Fixings also need care: the low density calls for specific large-surface anchors, while concentrated loads must be spread.