Nanocharacterisation on the cement paste-recycled lump interface in recycled lump-filled concrete using reverse casting method

Recycled lump-filled concrete is a sustainable concrete material produced by stacking large-sized recycled lumps in moulds and subsequently filling the gaps between the lumps with self-compacting concrete or mortar. However, the homogeneity of the interfacial transition zones (ITZs) and cement paste matrix surrounding the recycled lumps is critical in determining the overall mechanical properties and permeability resistance. In this study, two specimen preparation methods were used including the sequential casting method (directly pouring self-compacting mortar onto the recycled lump stacks) and the reverse casting method (extruding recycled lumps into self-compacting mortar). The results show that both casting methods adequately fill the voids in the recycled lump-filled concrete and form an effective bond between the recycled lumps and self-compacting mortar. However, the sequential casting method relies more strongly on the relationship between the yield stress and shear stress of self-compacting mortar. Particle accumulation occurred mainly at the top and upper sides of the recycled lumps due to sedimentation, while rough surface of recycled lumps and the viscosity of the mortar led to more voids at the bottom of the specimens fabricated by the sequential casting method. In contrast, the ‘wall effect’ was less significant and a more homogeneous interface was achieved in specimens prepared by the reverse casting method, although the bottom interface was slightly denser than the top. Overall, the reverse casting method was more conducive to achieving a homogeneous distribution of micro-pores and phases, thereby stabilising the mechanical properties and permeability resistance of recycled lump-filled concrete.