INTRODUCTION

Fixing tiles to anhydrite or gypsum based surfaces requires checks & additional preparation measures to be undertaken to avoid the adverse reaction which can often occur between cement based adhesive mortars and this type of substrate, which invariably results in lifting & failure of the bond if the following action is not undertaken.

DESCRIPTION & IDENTIFICATION

Anhydrite screeds are also referred to as Calcium Sulphate or gypsum based screeds and are a mix of fine and coarse aggregates with a calcium sulphate (gypsum) binder, and therefore look similar to a cement sand screed. Often a light colour, almost white, will indicate that the screed is likely to be calcium sulphate based. Other descriptions to identify the screed as anhydrite based can be made by referring to the flooring contractors’ supplier or manufacturer, where names such as Lafarge GYVLON or Tarmac TRUFLO identify a commonly used source of this type of underlayment.

Anhydrite screeds are not suitable as a wearing surface or for external or internal locations where they can become damp, frequently wet or in saturated areas. The low shrinkage properties of this system make it a preferred choice for deep bed floor levelling specifications whereby the thickness of an anhydrite screed can be typically in the range of 25mm to 80mm, ideal for embedded sub-floor heating systems.

SURFACE PREPARATION

Before considering fixing tiles to an anhydrite screed there are several essential preparatory steps to consider.

The removal of a loose friable layer of surface laitence must be carried out usually 4-6 days after application using appropriate equipment. This will also assist the drying of the screed.

This will be carried out using Diamond floor grinder depending on the laitence and how hard it has dried.

Also Sealing the floor prior to using Suitable Adhesive that is not cement based so as it does not react with the gypsum based floor.

G Taylor Tiling would also recommend decoupling it with a matting system – DURABASE CI++. With the DURABASE CI++ multi-purpose decoupling mat as a stress-alleviating intermediate layer, damaging shear stresses are absorbed by the special TRIPLE-LAYER MAT STRUCTURE keeping tiled surfaces permanently free of cracks.

TESTING FOR RESIDUAL MOISTURE LEVEL:

Screed drying time is approximately 1mm/day up to 40 mm thickness under ambient temperatures and drying conditions. This will increase for screeds thicker than 40 mm deep and in poor drying conditions.

A typical example is for a screed thickness of 30mm, with ambient temperature of 20’C and with good ventilation that should reach a moisture content of 0.5% in approximately 30 days.

The British Standard BS8203-1:2001 method for measuring the moisture condition of a screed is to use an electronic meter or hair hygrometer. This non-destructive test method gives a figure which should equate to approximately 75% relative humidity (the required limit for floor finishes).