what  chemical  powder  is  used in  mixing  cement   for tiles ?

Mix 3 parts of sand to 1 part of cement mortar. Once the dry
ingredients are thoroughly mixed, heap into a 'molehill' shape and
create a well in the centre. Slowly pour into the well a small amount
of water and using an old spoon or trowel mix from the centre
outwards.

The cement mix should always be firm. If any water is visible after
mixing, too much has been added and the bonding properties weakened.

To strengthen the cement mixture you may wish to add P.V.A. adhesive
to the water in a ratio of 3 parts P.V.A. to 1 part water.

This is an advantage to designs on a vertical base or those using
added mosaic materials such as pebbles etc. Leave cement to cure for
approximately 3 days before grouting.

PVA Adhesive 
A synthetic resin adhesive suitable for bonding most materials with
the exception of plastic and rubber. It has been designed specifically
for use in the building industry as a general purpose bonding agent,
an additive for concrete and plasters and as a general purpose
adhesive