Effect of the combined limewater pre-treatment and carbonation treatment on the properties of recycled concrete

Compared with ordinary concrete, recycled concrete exhibits higher drying shrinkage and lower resistance to chemical attack, which limits its engineering applications. In this study, recycled aggregates (RA) with different particle sizes were used to investigate the effects of limewater pre-treatment combined with varying carbonation parameters on the durability of recycled concrete (RC), including drying shrinkage, chloride ion penetration resistance, carbonation resistance, and sulfate attack resistance. For smaller aggregates, after optimization of the carbonation parameters following pre-treatment, the water absorption and crushing value of the RA were lower, while the apparent density was higher. When the objective was to reduce drying shrinkage and improve resistance to chloride ion penetration, carbonation for 12 h at 2 bar produced the best results, with both the 70 d drying shrinkage and the chloride migration coefficient reduced to their lowest values. When the objective was to suppress carbonation depth, under the 12 h condition, 2 bar and 3 bar produced similar results, and the incremental benefit became limited when the pressure exceeded 2 bar. When the objective was to improve sulfate attack resistance, carbonation for 12 h at 3 bar yielded the best compressive strength retention and mass loss resistance; after 150 wet-dry cycles, the compressive strength and mass loss rate improved by 31.2% and 97.7%, respectively, compared with the untreated group. For most indicators, treatment for 6 h at 3 bar already achieved results close to those of 12 h. In summary, all performance conclusions are based on accelerated carbonation of RA after limewater pre-treatment and reflect the relative effects of carbonation parameter variation on the properties of RC and RA. The selection of carbonation parameters should therefore be made according to specific durability requirements.