Lanthanum-modified calcined layered double hydroxides for enhanced chloride adsorption and durability of ultra-high performance concrete

Chloride ingress and immobilization in highly alkaline cementitious environments are governed by complex interfacial adsorption and reconstruction processes, which remain a major challenge for the long-term durability of ultra-high performance concrete (UHPC). In this study, lanthanum-modified Ca-Al layered double hydroxides (LDHs) and their calcined derivatives (CLDHs) were rationally designed to regulate surface chemistry and structural memory effects for efficient chloride capture. Lanthanum incorporation combined with calcination significantly increased the specific surface area, mesoporosity, and active adsorption sites of CLDHs. Chloride adsorption experiments conducted in simulated concrete pore solution revealed that the adsorption behavior followed a pseudo-second-order kinetics model and the Langmuir isotherm model, indicating a dominant chemisorption mechanism associated with structural reconstruction. The La-modified CLDH exhibited a high maximum chloride adsorption capacity of 3.61 mmol·g −1 , outperforming the undoped counterpart. When incorporated into UHPC, CLDHs enhanced both hydration kinetics and chloride immobilization through synergistic effects of interlayer anion exchange, structural memory reconstruction, and promoted formation of chloride-binding hydration products. An optimal addition of 1.0 wt% La-modified CLDH increased the 28-day compressive strength to 143.4 MPa and reduced the chloride migration coefficient by 34.4% compared with the reference mixture. This work elucidates the structure-adsorption-performance relationship of lanthanum-modified CLDHs in alkaline environments and provides a material-design strategy for developing high-efficiency chloride adsorbents toward durable cementitious systems.