Hydrophobizing of gypsum by silanes (Part 1)

1 Relative reaction rate v_rel for hydrolysis and condensation reaction of silanes as a function of pH [15, 20]

2 Water uptake reduction expressed by water drop absorption time (water drop test) of gypsum surface as a function of the content of the alkyltriethoxysilanes PTES and OTES

3 Hydrophobicity (water drop test) of gypsum surface with varying content of a- and b-hemihydrates with 0.165 wt% PTES based on the dry mix

4 Hydrophobicity (water drop test) of gypsum surface as a function of the hydrolysis time of 0.165 wt% propyltriethoxysilane (PTES). Hydrolysis time = pre-hydrolysis time of silane in alkaline suspension + 15 min. workability time of fresh mortar

5 Comparison of the ¹³C-NMR-spectra of propyltriethoxysilane (0,5 wt%) at pH 12.5 after various times of hydrolysis

6 Comparison of the ²⁹Si-NMR-spectra of propyltriethoxysilane and reaction species at pH 3 (left) and 12.5 (right) after 35 min time of hydrolysis

Water drop test on a gypsum surface

The main disadvantage of gypsum products in the building industry is the poor water resistance. While it is known that the water resistance can be increased by adding special silanes, only very little is known about the working mechanism of such silanes. The mechanism of the hydrophobizing effect of propyltriethoxysilane as a representative for a short chain alkylalkoxysilane in an alkaline gypsum matrix is studied in detail.

1 Introduction

Gypsum products are used in a wide range of applications, primarily in the building material sector. The most important are gypsum and gypsum-lime plasters, plaster boards, gypsum fiber boards, standard wall boards or plaster floors. Despite a low solubility of 2.75 g gypsum/L water gypsum products have a high water sensitivity. This limits the use to indoor applications. Gypsum can absorb water reversibly via the pore system, whereas swelling caused by water mass transport can result in significant reduction of strength and surface damage. The increase of the water resistance of...

TEXT Dipl. Chem. Andrea Winkler¹, Dipl. Nat. Susanne Fritz¹, Dr. Thomas Aberle², Dr. Daniela Freyer¹*, Prof. Dr. Wolfgang Voigt¹
¹TU Bergakademie Freiberg, Institute of Anorganic Chemistry, Freiberg/Germany
²Akzo Nobel Chemicals AG, Sempach-Station/Switzerland

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