Microstructure of four-graded roller compacted concrete

•Concrete microstructure cannot be represented by paste microstructure.•Characterize the microstructure of paste and ITZ by means of image analysis.•Aggregate size and free water content affect four-graded RCC microstructure. This paper presents an experimental study on microstructure of four-graded RCC. Two temperatures of 20 °C and 40 °C were picked to represent interior and surface layers of four-graded RCC. The most used fly ash contents of 50% and 70% were selected, and two ages of 28 days and 56 days were tested. Three levels of paste, mortar and concrete were used to simulate different microstructural regions in four-graded RCC. 3D digital microscope experiment, micro-hardness test and back-scattered electron image analysis were carried out sequentially. It was found that four-graded RCC microstructure can be divided into three characteristic parts: bulk paste, interface transition zone around fine aggregate (ITZ-f) and interface transition zone around coarse aggregate (ITZ-c). By comparing the average wear width and depth, the strength and bonding ability of bulk paste was the highest, followed by ITZ-f and ITZ-c. Compared to the surface layer, the micro-mechanical strength of bulk paste and ITZ-f was higher in the interior layer, but that of ITZ-c was opposite. At early age, 50% fly ash content was more conducive to microstructure than 70%. The micro-hardness value of ITZ-c presented a decreasing trend with age and that of ITZ-f did not grow. In terms of product structure, Ca(OH)2 content in ITZ-c gradually decreased from aggregate surface to paste part, and Ca(OH)2 in ITZ-f had been partially converted to C-S-H gel. The proportions of hydration product and porosity were highest in ITZ-c, followed by ITZ-f, with bulk paste having the lowest. Microstructure of four-graded RCC was significantly affected by the combination of aggregate size and free water content on aggregate surface.