Water absorption of recycled concrete aggregates immersed in fresh cement paste: The capillary compressibility limit

The use of Recycled Concrete Aggregates (RCA) in fresh concrete presents challenges due to their high porosity, which affects water absorption and, consequently, the fresh and hardened properties of the material. In this study, we developed a Time-resolved Low Field Nuclear Magnetic Resonance (NMR) method based on T₁ relaxation time to quantify water absorption in situ during the immersion of RCA in a fresh cement paste. This technique enables real-time monitoring of water redistribution between the cement paste and the aggregates, overcoming the limitations of conventional gravimetric methods. Our results indicate that water absorption in cement paste is significantly lower than in water, a reduction attributed to the balance between the cement paste compressibility and the capillary suction of the aggregates. This indicates that standard water absorption tests may overestimate the actual water uptake of RCA in a cementitious environment. As a result, this overestimation increases the effective water-to-cement ratio in the constitutive paste of a RCA-based concrete, affecting rheology, mechanical and durability performances. These results highlight the need to improve both the assessment and our understanding of water absorption to optimize mix design of RCA-based concretes and promote recycling.