Kerakoll H40 Gel adhesive is a new product which has been perfectly formulated for laying natural stone tiles.  It can be mixed to a wider range of useable consistencies than standard adhesive, so you can vary its fluidity according to your needs.  This is really useful - if you're working vertically, when the adhesive keeps its shape and holds your tiles really effectively with no vertical slip.    Another strength of H40 gel is that it will not leave water stains on natural stone tiles, something which can be a problem with other standard adhesives. 

You can find a full technical data sheet for Kerakoll H40 here.

The product's 'gel' properties are very useful if you are laying heavy stone tiles.  The adhesive maintains its shape even under pressure. If you are interested in the science, this is because the adhesive, once mixed to a gel, is thixotropic, ie it is thick or viscous under static conditions but will become more fluid when agitated or stressed, and then returns rapidly to its more solid state once the agitation (ie you placing your tile) stops.   As you put tesserae in place, the instantly-fluid adhesive makes contact with all available surface area, filling any voids, and then rapidly returns to a more solid form.  Sounds like magic (and it is!).

Kerakoll H40 is suitable for all substrates and can be used indoors and out, and in wet conditions.  I'm recommending it for use with natural stone because of its gel consistency and because it doesn't stain stone (as some standard adhesives can) but it is also perfectly suited to a whole range of other materials, including glass and porcelain.

Because one of the strengths of the product is the possibility of varying its consistency, it's not possible to give firm rules about how much water should be added to the powdered adhesive, but approximately 2.5:1  powder to water by weight gives you a general idea.

Kerakoll H40 should be used at a thickness of between 2mm and 15mm.  You should not work in extremes of heat or cold; ensure the ambient temperature is between +5°C to +35°C, and that your materials have not been kept somewhere very cold.  The pot life of the mixed up adhesive (ie the time, from mixing up, that you can still use it) is around 4 hours at +23°C.  Don't be tempted to try and add more water and remix after this!  Open time at +23°C (ie the time you have to place your tiles into the adhesive) is around 45 minutes.  

You can grout your work around 20 hours after laying tiles, and the area will be ready for light foot traffic after 1 to 2 days.