If you want an official position on the use of the HK/HKC with Hardie, I would recommend tagging Tyler.
I think exclusions #05 and #06 in the warranty, particularly the bit about "damage caused by misuse" are relevant to this discussion. Misuse is using a tool for something other than it's intended use. An extreme example would be using your Carvex to hammer in nails. [scared] If Festool deems cement board not to be an intended use, that could be grounds for warranty exclusion.
The manual that was linked to by Tom, as far as I know, is not for the U.S. market. It may be changed for this market, that's yet to be seen since I have not seen it officially published by Festool USA. For what it's worth, the TS 55 manual also references in multiple places how to use the saw with cement board. That said, I've only seen Festool's marketing show the HK/HKC being used for wood.
Yes, we sell a blade for fiber cement. However, Festool does
not have a fiber cement blade. That could be interpreted as them saying it's not for that use because they don't have a suitable blade.
If you read Festool's warranty, it does not state that using a non-Festool accessory will void your warranty. In #09, it states that
damage resulting from the use of a non-Festool accessory is excluded from warranty coverage. The way
I interpret that statement is that you can use non-Festool accessories as long as they do not damage the tool. Not that using a non-Festool automatically voids the warranty. However, see the statements above about misuse, which may or may not apply to the use of this blade as it pertains to warranty coverage.
Reference:
http://www.festoolusa.com/service/warranty/
Basically, I think it's unclear and there are some mixed messages between the manual and the lack of a blade for this application. Again, I would suggest contacting Festool if you want clarification.
Shane
