Masonry facades

Airborne carbon dioxide reacts with sodium, potassium and calcium hydroxide in cement based products gradually converting the product from a strongly alkaline material to a weakly acid state, a process known as carbonation. While carbonation has more serious effects in reinforced concrete (Deterioration of concrete), the loss of alkalinity in mortars, for example, can permit corrosion to occur in ferrous metals that may be embedded.

The effects of carbonation on concrete and calcium silicate bricks can be difficult to distinguish from normal shrinkages but nonetheless, carbonation induces volume changes in porous components such as lightweight blocks (autoclaved aerated products) within the first few years of the component's life.

Dense concrete blocks are affected by carbonation, but in these materials the rate of carbonation is much slower.