AAC and LECA Lightweight Blocks

AAC, autoclaved aerated concrete, is made from cement, lime, silica sand and aluminium powder. Its porous structure brings the weight down to roughly a quarter of solid clay brick. Lighter internal and perimeter walls reduce dead load and therefore the seismic force acting on the frame, which matters greatly in renovations and added storeys. LECA blocks are made from expanded clay aggregate: heavier than AAC but far more moisture-tolerant, making them the better option for yard walls, parapets and damp environments.

The second advantage is thermal. AAC has a conductivity of roughly 0.11 to 0.16 W/m.K, against more than 0.7 for solid brick. That gap allows an AAC wall to meet part of the Part 19 energy requirements without a separate insulation layer. Fire performance is also strong, as AAC is non-combustible.

The main misconception is treating these blocks as load-bearing. In framed buildings AAC and LECA are infill only and must be tied to slabs and columns with angles, bed reinforcement or steel studs so they do not fail out of plane during an earthquake. The second error is laying AAC with ordinary sand-cement mortar in thick joints; it is designed for thin-bed adhesive with 2 to 3 mm joints, and thick mortar creates thermal bridges and cracking. The third is ignoring AAC's high absorption: the surface must be dampened and primed before plastering, otherwise the plaster loses its water and turns powdery.

In permanently wet locations such as bathrooms or walls in contact with soil, AAC without full waterproofing is a poor choice, and LECA or clay brick serves better.