Cement paste content and water absorption of recycled concrete coarse aggregates

In this paper, we first show that the amount of residual cement paste in recycled concrete aggregates (RCA) depends on the initial paste content, on the mechanical properties of the initial cement paste and on the mechanical quality of the transition zone at the interface between the initial natural aggregates and cement paste. Our results suggest that water absorption of RCA at 24 h can be seen as the simple sum of the capillary absorption of both residual cement paste and initial natural aggregates. The absorption kinetics is similar to the capillary absorption of traditional uncrushed concretes but, on the industrial time scale of interest, is strongly affected by the size and configuration of the residual cement paste patches at the surface of the initial natural aggregates. We finally propose a tentative frame for a classification of RCA based on water absorption rate and water absorption capacity at 24 h.