Experimental study on 3D-printed cementitious materials containing surface-modified recycled crumb rubber

Abstract

Surface modification on recycled rubber particles, such as pre-coating of cementitious paste, has been considered a practical way to improve the mechanical properties of rubberised concrete with the rubber particles partially replacing natural aggregates. Although research on this strategy has been widely carried out on cast concrete, the properties of 3D-printed cementitious materials containing surface-modified rubber particles remain unexplored. In this study, the compressive and microstructural properties of 3D-printed cementitious mortar with cement-coated crumb rubber were investigated. The crumb rubber had a 15% replacement of river sand by weight. The compressive loads were applied in two different directions, including the Z direction (layer deposition) and the Y direction (printing). Compared to the reference specimen containing rubber particles without modification, the compressive strength of 3D-printed specimens with cement-coated ones increased by 25.7% and 14.0% in the Y and Z direction, respectively. Scanning electron microscopy (SEM) images showed the interfacial bonding between cement matrix and rubber particle was effectively improved due to the formation of the hardened cementitious shell located outside the rubber surface. X-ray micro-computed tomography (μCT) analysis of the pre-test and post-test specimens suggested the different increment of compressive strength along two loading directions could be attributed to several factors, including the internal elongated pores with their alignment relative to the loading direction and the rubber-matrix interface bonding.