We do epoxy injection work and sometimes need to slow the hardening rate of the epoxy. Can we do this by adding less hardener to the resins?
No. Reducing the amount of hardener doesn't change the hardening rate of an epoxy adhesive. It simply reduces the number of chemical linkages within the hardened epoxy. Ask your epoxy supplier for a formulation that sets more slowly. You could also use an ice bath to lower the temperature of the components before mixing. Cooled epoxy, however, is more viscous and won't penetrate cracks as readily.