Compressive behavior and water permeability of recycled lump-aggregate concrete with recycled sand from weathered residual soil of granite

The disposal of massive amounts of demolished concrete and excavated soil, and the consumption of large amounts of cement and natural aggregates for concrete have restricted urban development. Recycled lumpaggregate concrete (RLAC), containing recycled lumps (RLs), recycled coarse aggregates (RCAs), and recycled sand (RS), is a potential solution. However, the higher crushing index and clay content, along with the lower fineness modulus and apparent density of RS derived from weathered residual soil of granite (WRSG) compared to natural sand, have raised concerns regarding the performance of RLAC. In this study, the compressive behavior and permeability of RLAC were investigated. The positive effect of WRSG-derived RS on RLAC permeability was quantitatively revealed for the first time, and the underlying mechanisms were elucidated. The quantitative effect of RS on the compression behavior and its engineering acceptability were also clarified. The results showed that: (a) The use of WRSG-derived RS had a slight effect on the compressive strength of RLAC. However, the presence of clay minerals with poor elastic modulus in RS led to a reduction in the elastic modulus of RLAC, with a maximum decrease of 6.1%, which is acceptable for practical applications. (b) The promotion of RS to cement paste hydration, adequate filling of RS in the cement paste, and tight mechanical interactions between RS and the cement paste contributed to a denser mortar microstructure. The permeability coefficient of RLAC and the porosity of harmful pores (>100 nm) in fresh mortar were reduced by 86% and 55%, respectively, with 70% RS utilization. (c) When the water-cement ratio of fresh concrete was 0.36 or 0.45, although the impermeability grade of the RLs was only P4, the RLAC still had an impermeability grade of P12, which can satisfy almost all the engineering impermeability requirements.