There has been a great demand of using the recycled aggregate and fly ash in the concrete structure. It is known that the compressive strength will increase in the early age when the mineral fine powder, such as fly ash, is mixed into the concrete with a part of the fine aggregate, under the condition of constant unit cement content. However, its mechanism has not been clearly understood until now. In this paper, we made experiments to explain the mechanism of the early strength development by using recycled aggregate mortar. In addition, we examined the influence of the mineral fine powder, which is mixed into the mortar, which extended the strength development of the recycled aggregate mortar from 3 days to 180 days. From the results obtained, we found that in response to the influence of calcium hydroxide in the adhesion cement paste of recycled aggregate, the fly ash had brought forward the early pozzolanic reaction for 28 days. However unlike the pozzolanic reaction of the usual fly ash, the early pozzolanic reaction did not contribute to strength development. By mixing the mineral fine powder, the early strength development had strongly influenced the filler effect of physical action. The mixture of the recycled aggregate mortar using the mineral fine powder as a part of the fine aggregate had brought much larger strength increment ratio than the normal aggregate mortar. The results indicate that the combination of fly ash and recycled aggregate would have a good affinity.
