A new water-repellent agent for gypsum-based drymix mortars

1 Traditional powdered gypsum water repellents (left and center) are available in carrier or encapsulation systems. Silres (right), however, is a completely new, powdered water repellent in which the active agent itself is used

2 In traditional powdered gypsum water repellents, the actual active agent – an alkyl silicic acid – is formed in situ via alkaline hydrolysis of an alkyl alkoxysilane. M stands for an alkali metal or alkaline earth metal. The monomeric structure of the alkyl silicic acid (and/or alkyl siliconate if the pH is elevated) shown here on the right-hand side of the equation is only an example. Further structures could be formed as well

Table 1: Requirements stipulated by manufacturers of gypsum construction materials for water repellents used in gypsum drymix mortars

3 The structure of the potassium methyl siliconate in the new Silres, M symbolizing potassium. The potassium-silicon ratio can be adjusted and optimized with respect to the additive’s interaction with gypsum crystals by varying parameter x

4 The effect of traditional, commercially available water repellents in a gypsum-based joint filler (pH 9, water-to-solid factor = 0.50)

5 The effect of traditional, commercially available water repellents in a lime-gypsum plaster for machine application (pH 12, water-to-solid factor = 0.60)

6 The effect of the new Silres in a gypsum-based joint filler (pH 9, water-to-solid factor = 0.50)

7 The effect of the new Silres in a lime-gypsum plaster for machine application (pH 12, water-to-solid factor = 0.60)

8 The effect of the new Silres in a lime-gypsum plaster for machine application consisting of 30 % stucco and 70 % carbonate filler (pH 12, water-to-solid factor = 0.35)

9 Water absorption in test objects that had been immersed in water for long periods. The test objects consisted of gypsum plasters that had been modified with various water repellents. Addition of the new Silres at a concentration of 0.3 % is enough to keep water absorption below 5 % in test objects immersed in water for seven days. MR stands for moisture-resistant

All photos: Wacker Chemie AG

Gypsum drymix mortars formulated with SILRES^® BS Powder S are water repellent, making them suitable for universal use in interior applications

All photos: Wacker Chemie AG

1 Introduction

Builders appreciate gypsum-based construction materials for more than just economic and environmental reasons – they also value their favorable processing characteristics, as well as the physical and biological properties they exhibit in construction applications. These materials, for instance – especially plasters that use gypsum as a binder – create a comfortable indoor climate, because they regulate interior humidity and always feel pleasantly warm and dry to the touch.

Set gypsum has two weaknesses that limit its potential applications, however: it has relatively poor...

Dr. Daniel Schildbach, Technical Marketing Manager Construction Chemicals, Wacker Silicones, Munich/Germany

A new water-repellent agent for gypsum-based drymix mortars

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ZKG 03/2014