Hardened behavior of mortar based on recycled aggregate: Influence of saturation state at macro- and microscopic scales

Abstract : The influence of saturation state of recycled concrete aggregate (RCA) on the hardened behavior of mortar was investigated at macroscopic and microscopic scales. At the macroscopic scale, a good adhesion between the RCA and the new cement matrix at 28 days of hydration is observed. However due to its intrinsic strength; a limiting effect of the RCA is found in comparison to natural aggregates (NA). This effect limits the strength of mortars for small values of effective water to cement ratio. In parallel, a low influence of moisture condition of used RCA on the mechanical properties is observed. The microstructure of interfacial transition zone (ITZ) is characterized by image analysis of backscatter scanning electron microscopy (SEM) images at 2 and 28 days of hydration and nano-indentation test under SEM at 28 days of hydration. For mortars based on RCA, the average porosities in the ITZ of mortars with dried and over-saturated aggregates are identical. But the saturation state of aggregates has a significant influence on the distribution of porosity in the ITZ. The porosity in the ITZ of mortars containing RCA is larger than that of mortars made with NA. This difference can be explained by a higher effective water to cement ratio in the mortar based on RCA. This excess may be due to a lower value of absorption of RCA than that provided by the absorption estimated on RCA. The nano-indentation test under the SEM shows that the initial saturation state of recycled sand (dry or over-saturated) does not influence the micro-hardness of the new cement matrix.