First Water-Soluble, then Water-Repellent - Wacker Chemie AG


First Water-Soluble, then Water-Repellent

Fast and dust-free: the SILRES® BS Powder S hydrophobizing agent is used in the in-plant dry-mixing process before mixing water is added on the building site.

The aim of the WACKER developers was to reduce the water absorption of the set dry mixtures to less than five weight percent, as has been specified for damp-room gypsum board for several years by the standard EN 520:2004. According to the experts, below this limit, joint fillers can be used generally in all interior rooms, including damp rooms. The EN 13963 standard, which was updated and revised in 2014, confirms this. Accordingly, only Type H1 jointing compounds are approved for challenging damp-room applications. These materials absorb no more than five percent by weight of water, even after immersion in water for two hours.

“The tests on different gypsum test specimens showed that our new hydrophobizing agent suppresses the penetration of water extremely effectively,” explains product developer Dr. Daniel Schildbach from WACKER’s Technical Service in Burghausen. “After two hours’ immersion in water, the water absorption remains below five percent – assuming that the dosage rate is correct.

Requirements to Be Met by Water Repellents for Gypsum-Based Dry-Mix Mortars

  • Water absorption < 5%
  • Quantity used < 0.5%
  • Mixing characteristics (when mixing the dry-mix mortar with water): excellent
  • Dust formation (when mixing the dry-mix mortar with water): none

The starting point for Dr. Schildbach’s study was a commercially available gypsum joint filler to which SILRES® BS Powder S had been added at various concentrations. According to Dr. Schildbach, “Unlike traditional powder-form water repellents, our new additive can be used in gypsum construction materials to keep water absorption very low over long periods of time. SILRES® BS Powder S additionally imparts a pronounced beading effect to the construction material surface.

Outdoor weathering tests conducted near WACKER’s Burghausen site also confirm that SILRES® BS Powder S provides long-lasting protection against water. Large, uncoated gypsum-plaster test objects still displayed pronounced beading after two years of weathering. When they were again immersed in water, they absorbed 70 percent less water than the test objects without water repellent. “However, the driving rain typical of Western Europe had eroded the soft surface of the gypsum plaster considerably,” reports Dr. Schildbach. He therefore does not recommend using gypsum for outdoor renders in such climatic regions. In regions that experience little rainfall, such as Southern Europe or South Asia, on the other hand, SILRES® BS Powder S can increase the durability of existing outdoor gypsum plasters.