We do not recommend using grout in freeze/ thaw climates. Travertine Pavers can be laid right up against each other leaving no gap. However, if you do choose to use grout it is best to use one that has low water absorption. Additives can also be mixed with the grout to make it less susceptible to water penetration and increase grout flexibility. Natural stone sealants can also block moisture during installation allowing further protection from the harsh weather.
Travertine needs to be able to expand and contract in freeze/ thaw climates. The best option is to use a flexible mortar (such as Flex-Bond) on the pool coping and around the edges of your project, or anywhere that you would normally use traditional mortar
If you are going to be mud-setting the pavers, it is recommended to use Flex-Bond (or similar) adhesive instead of traditional cement products to prevent cracking during the freeze/thaw cycle.