The role of lime activation on the strength development in concrete with very high fly ash replacement of cement is explored. Quicklime is shown to be very effective in providing increased quantity of lime in the mix, which provides significant enhancement in strength particularly at later ages. Concrete strengths of 30 MPa and higher were achieved with a cement content of 100 kg/m3. The efficacy of quicklime increases with increasing temperature. At 40oC, the rate of strength gain is significantly increased and 30 MPa is achieved within 15 days. It is known that strength gain in concrete is related to the depletion of lime in the system, which is directly influenced by silica provided by fly ash. An investigation of the underlying reactions reveals the potential for further strength enhancement through activation of all the reactive components of fly ash.