Why Cement Sealing Is Banned for GIS Wall Penetrations？

Indoor GIS equipment typically involves wall-penetrating installations, except in cases with cable in/out connections. In most cases, the main or branch bus duct extends from indoors through a wall to the outdoor side, where it connects to porcelain or composite bushings for overhead line connections. The gap between the wall opening and the GIS bus enclosure, however, is prone to water and air leakage and therefore usually requires sealing. This article discusses why cement-based sealing is not allowed.

The 2015 edition of China Southern Power Grid’s Anti-Accident Measures explicitly prohibits the use of cement to seal wall-penetrating sections of GIS bus ducts.

The primary reason for this prohibition is the risk of chemical reactions between alkaline components in cement and the aluminum alloy used in GIS enclosures. When wet (unhardened) cement or rainwater-soaked cement comes into contact with the aluminum surface, corrosion can occur, potentially leading to gas leakage. Note that the reaction only occurs when the cement is damp—once fully hardened, the risk is significantly reduced. However, this issue requires special attention during the construction phase.

When cement particles come into contact with water, hydration begins on the surface layer by layer. The main hydration products include: calcium silicate hydrate (C-S-H) gel, calcium ferrite hydrate gel, calcium hydroxide (Ca(OH)₂), calcium aluminate hydrate, and ettringite. Among these, alkaline substances such as calcium hydroxide and calcium aluminate hydrate can react with aluminum alloy, causing damage to the GIS enclosure.

Besides cement, other sealing materials like asbestos boards or waterproof sealants may also be used at wall-penetrating points. However, asbestos boards often contain cementitious components, and improperly selected sealants—especially alkaline types—can similarly corrode the aluminum alloy enclosure, risking gas leakage.

Although aluminum alloy is naturally corrosion-resistant in air, another possible corrosion mechanism exists: cement may first degrade the protective paint layer on the enclosure surface, since paint is less corrosion-resistant than aluminum. Once the coating is compromised, the underlying metal becomes vulnerable. In the construction industry, this is why a primer or putty layer is typically applied before painting over cement.

This explains the origin of the prohibition against using cement to seal wall-penetrating GIS bus ducts.