W.R. Meadows Co. makes a product called Deck O Seal that will do the job of filling the gap between pool coping and cement deck.
Yes, you can use cement board outside for your project. It is a durable and weather-resistant material commonly used for outdoor applications such as siding, decking, and tile underlayment. Be sure to follow manufacturer guidelines for installation and maintenance.
If the decking shifts,rises and moves it will push on the coping of the pool. Also, this helps keep water from under the pool and if the ground has moisture until it and freezes then it cause the deck to rise.
To set untreated timber in cement is asking trouble. Only pressure treated lumber should be used as a foundation support for outdoor decking. Greenwood can also lose its original shape.
Cement board
Cement board
Cement board
To allow floor flange bolts to have a place to fit and to anchor the flange to the cement /wood decking
Cement with a rough textured finish or paver's. Paver's are probably a slightly better choice because if anything goes wrong with the plumbing you don't have to do any concrete cutting.
Asbestos cement
That would depend on what type of pool you are installing and how you want it set up. Pool decking is most necessary for above ground pools, this would typically be of wood. For an in ground pool you would most likely use cement.
cement?
Cement is a commonly used binding material in construction, made from limestone and clay that hardens when mixed with water. Geopolymer is a newer alternative material that uses industrial by-products like fly ash or slag, activated by an alkaline solution, to create a similar binding effect. Geo polymers tend to have lower carbon emissions and higher strength compared to traditional cement.