Microscopic characterizations of pervious concrete using recycled Steel Slag Aggregate

Abstract Pervious concrete using Steel Slag Aggregate (SSA) is a promising Low Impact Development (LID) and sustainable pavement material solution. This study investigated the enhancing mechanism using SSA instead of limestone in pervious concrete from the perspective of microscopic characterizations. Nanoindentation, Backscatter Electrons - Scanning Electron Microscope (BSE-SEM), and Energy Dispersive Spectroscopy (EDS) tests were conducted to obtain the micro-mechanical, micro-morphological and element composition of the interfacial transition zone (ITZ) and adjacent cement paste. The nanoindentation tests showed that the modulus and hardness of the ITZ in SSA pervious concrete were 44% and 68% higher than those of limestone concrete, and the contour plots showed that ITZ thickness of SSA pervious concrete was around 18 μm, narrower than the 25 μm thickness of limestone samples. The BSE-SEM tests showed that the ITZ of SSA pervious concrete had more hydration products and were more compacted. Specifically, denser flower-shape polyhedral crystals were observed, which was coalesced hydration products including calcium silica hydrates (CSH), calcium hydroxide (CH) and ettringite. The EDS results showed that not only were the contents of silicon and calcium in ITZ of SSA pervious concrete were higher, the silicon and calcium (S/C) ratio of SSA pervious concrete was also 42% higher, indicating that more CSH gel was formed. Based on the change of element composition, the estimated width of the ITZ in SSA pervious concrete was also smaller than that of the limestone sample. In all, the ITZ enhancing mechanism of using SSA includes improved strength, reduced width and densified structure.