Hydration process modeling of ITZ between new and old cement paste

• An ITZ between new and old cement paste is modeled by a software of CEMHYD3D.
• The second hydration effect of active ingredient in old cement paste is discussed.
• Hydration process for ITZ between new cement and old paste is simulated and studied.

An interfacial transition zone (ITZ) model between new and old cement paste in recycled aggregate concrete (RAC) is presented base on the software of CEMHYD3D. This model is obtained by replacing gravel with hydrated cement paste in ITZ models between aggregate and cement paste. By varying the water–cement ratio of new and old cement paste, series ITZs are numerically simulated. The effect of active ingredient in old cement paste to second hydration is discussed. By CEMHYD3D the microstructure and compounds distribution of this ITZ are compared with those of conventional ITZ and cement paste. Porosity and Calcium–Silicate–Hydrate (C–S–H) are also analyzed in this paper. Results indicate that the ITZ between new and old cement paste has a fast hydration rate, possesses a lower porosity than the ITZ between gravel and cement paste. And this ITZ generates C–S–H embedded in old cement paste, thus could strengthen the bonding of ITZ.