Strength development of MgO recycled aggregate concrete with recycled sand from WRSG and compressive behavior of recycled lump-aggregate concrete

The existing literature suggests that natural aggregate concrete demonstrates the least shrinkage, followed by recycled aggregate concrete (RAC) prepared using natural sand, with RAC prepared using recycled sand (RS) from the weathered residual soil of granite demonstrating the greatest shrinkage. Internal incorporation of a MgO expansion agent (MEA) effectively compensates for the excessive shrinkage of the latter; however, the influence of the MEA on the strength development of RAC prepared using RS after natural curing, rather than accelerated carbonation curing, remains unclear. In this study, compression tests of RAC prepared using RS at different stages of natural curing were performed and the corresponding material compositions of RAC were determined and quantified via X-ray diffraction and thermogravimetry-differential thermogravimetry. The soluble carbonate content in RS was determined by ion chromatography, and the morphology of RAC was observed using scanning electron microscopy. The mechanism of strength development of RAC during aging was determined. Furthermore, compressive tests of recycled lump-aggregate concrete (RLAC) were performed to investigate the influencing degree of RAC as fresh concrete on the compressive properties of RLAC. The following key results were noted: (a) the MEA impairs the compressive strength of concrete, but the degree of impairment decreases with curing, and this is attributed to the transformation of Mg(OH)2 to MgCO3. (b) The presence of soluble carbonates in RS (7.2 %) is the main source of carbonate in the conversion of Mg(OH)2 to MgCO3. Mg(OH)2 particles adhere to the surface of RS particles and react with soluble carbonate to generate MgCO3. (c) At 56 days of curing, the addition of 6 % MEA or increasing the replacement ratio of RS impaired the compressive strength of RLAC to a certain extent. However, even with 100 % RS, the compressive strength and elastic modulus of RLAC were impaired by only 7.4 % and 5.8 %, respectively. With 6 % MEA, the impairments were even smaller and negligible. •Compressive and micro tests were conducted on RAC and RLAC with RCA, RS, MEA and DCL.•Impairment of RAC's compressive strength due to 6 % MEA decreased with curing age.•The strength development of RAC was partly due to the conversion of Mg(OH)2 to MgCO3.•RS provided carbonate for conversion of Mg(OH)2 to MgCO3 in RAC under natural curing.•Using 6 % MEA or increasing RS content impaired RLAC's compressive behavior limitedly.