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Cyclopedia Title: 

Tiling on bonding coat

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Why tiling on gypsum based plaster is not a good idea

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Cyclopedia Introduction: 

While manufacturers provide clear guidance on whether tiling on gypsum-based substrates is permissible, the debate surrounding this topic persists. Given gypsum's prevalence as the primary plaster material in the UK, the issue is widespread and often cited as a leading cause of tile failure. This concern extends to any surface composed of calcium sulfate, including anhydrite, hemihydrate, alpha-hemihydrate, gyvlon screeds, gypsum-based bonding coats, plasters, and similar materials.

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The underlying reason for caution when tiling on gypsum stems from its chemical composition.

Gypsum contains calcium sulfate, which reacts with the calcium aluminate present in Portland cement, forming ettringite or calcium sulfoaluminate crystals. These crystalline formations, appearing as small white or yellow crystals, can accumulate excessively and lead to adhesive failure.

While gypsum is deliberately introduced into cement to serve as a retardant, preventing premature setting, the proportion of gypsum in Portland cement typically remains low, never exceeding 4-5%. Elevating the gypsum content beyond this threshold can compromise the integrity of the concrete, increasing the risk of failure.

In circumstances where tiling on a gypsum-based substrate is unavoidable, several mitigation strategies can be explored.

Firstly, utilizing gypsum-based adhesive may offer improved compatibility and adhesion.

Additionally, establishing a barrier between the substrate and the adhesive is advisable. This barrier can be achieved through the application of a specialized polymer-based primer or the installation of a membrane, effectively minimizing the risk of adhesive failure.