Effect of moisture conditioning of expanded clay aggregates on fresh and hardened properties of structural lightweight self-compacting concrete

This study addresses the influence of the initial moisture condition of lightweight expanded clay aggregates (LECA), an important parameter that remains insufficiently quantified, particularly regarding how controlled pre-saturation duration influences the interfacial transition zone (ITZ) and, consequently, the fresh and hardened properties of structural lightweight self-compacting concrete (LWSCC). LECA moisture conditions, from dry to pre-saturated states, were investigated for their effects on fresh properties, mechanical performance, and capillary water absorption. Results reveal a pronounced non-linear response to aggregate moisture. Dry LECA increased compressive strength and stiffness due to local water absorption and a reduced effective water-to-cement ratio, but impaired workability and increased blockage risk. High moisture contents led to excess free water, promoting segregation and a fragile, porous interfacial transition zone. The 5-min pre-saturation condition (PS-5min) provided the optimal balance, improving slump flow by over 30%, halving T500 time, reducing capillary absorption, and maintaining compressive strength within 8% of dry aggregates. SEM observations indicate that these macroscopic behaviours are closely related to the evolution of the ITZ, which becomes denser and more homogeneous under controlled pre-saturation conditions. This highlights the critical role of controlled LECA moisture in achieving balanced fresh and hardened properties.