First Water-Soluble, then Water-Repellent

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The biggest disadvantage of gypsum-bound construction materials is their low moisture resistance. Thanks to SILRES® BS Powder S, a highly efficient, powdered water repellent is now available for the first time for treating gypsum-based dry-mix mortars.



Dr. Daniel Schildbach, in charge of an applications lab run by the Construction Chemicals business team, with lab employee Theresia Genzinger.


Gypsum is a general-purpose material, with applications well beyond drywall construction. Construction elements and construction materials based on gypsum are environmentally friendly to manufacture and can be rapidly and inexpensively processed. They create a pleasant indoor climate, regulate interior humidity and always feel pleasantly warm and dry to the touch.

When set, gypsum has two weaknesses, however: it has relatively poor hardness, and is slightly – yet noticeably – soluble in water. The solubility of calcium sulfate dihydrate is roughly 2.6 g/L at 20 °C. Compared to silicate-based building materials, which are practically insoluble in water, this is quite high. Furthermore, gypsum-based construction materials lose their compressive strength when saturated with moisture. Plus, prolonged exposure to water irreversibly damages the material structure.


Drywall Construction in Damp Locations

Gypsum-based construction elements and construction materials should therefore only be used in damp rooms if they have been made effectively hydrophobic, i.e. water-repellent. However, all the materials currently available for rendering gypsum construction materials water repellent have the disadvantage that they are only available in liquid form. Organosilicon water repellents, based either on what are known as H-siloxanes (polymethylhydrosiloxanes) or on alkali alkyl siliconates, have proven highly effective for gypsum building panels – such as gypsum plasterboard, fiberboard or block – and established themselves on the market accordingly. The former are used as oils or aqueous emulsions, while the latter are in aqueous solution – they are therefore not suitable for use as additives for formulating dry-mix mortars.

“WACKER has now designed a completely new gypsum hydrophobizing agent, SILRES® BS Powder S, which can be used in the dry-mixing process,” says Dr. Rudolf Hager, head of the Construction Chemicals business team at WACKER. “This additive allows manufacturers to produce gypsum-based wall plasters, joint fillers, top coat materials and adhesives that can be used throughout the interior of a building.”