Interfacial Transition Zone of Cement Composites with Recycled Concrete Aggregate of Different Moisture States

Past research suggests that the moisture state of recycled concrete aggregate (RCA) at the time of concrete mixing can dramatically affect the concrete properties. This study employed image analysis of backscattered electron micrographs to monitor the interfacial transition zone (ITZ) development in coarse mortars with a highly absorptive RCA at oven dry (OD), saturated surface dry (SSD), and sub-SSD (i.e., 80 % SSD) moisture conditions. The batching water was not adjusted to account for the aggregate moisture condition to explicitly evaluate the effect of the RCA moisture condition. The 28-day compressive strength was found to be greatest for the sub-SSD RCA mix, followed by the OD RCA and SSD RCA mixes. No difference was found in the ITZ microstructure between mixtures with sub-SSD and SSD RCA. The OD RCA appreciably affected the ITZ by demanding water, which reduced the local water-to-cement ratio, yielded an increase in the unhydrated (UH) cement content, reduced the bulk microstructure porosity, and increased the estimated ITZ size. Because these ITZ trends do not directly correlate with the compressive strength trends, it is postulated that the sub-SSD will absorb some of the cement slurry during mixing, allowing for a stronger interfacial mechanical bond to form. This does not occur to the same extent for the SSD RCA, but will likely also occur for the OD RCA; however, the OD RCA absorbs so much water that it deters hydration, yielding a larger ITZ with a higher UH content, so the bulk strength of the mortar with OD RCA is less than that of mortar with sub-SSD RCA.