Initially reported on https://github.com/mautic/mautic/issues/2524
Description:
The CAPTCHA version of mautic form is quite basic. As you may know, Google has developed an open source reCAPTCHA which is quite powerful.
recaptacha
It would be very nice that this feature is integrated instead (or furthermore) of the existing CAPTCHA.
I found a piece of doc there 
https://developers.google.com/recaptcha/old/docs/php
See also existing plugin. I think we should consider adding it in the mautic core. https://github.com/KonstantinCodes/mautic-recaptcha