Strength development and microstructural evolution of lithium slag-based cementitious materials: Influence of the slag type and microwave curing

Lithium slag (LS), an industrial by-product of lithium extraction, has potential as a supplementary cementitious material (SCM) to reduce carbon emissions. However, its use as a cement substitute lowers early strength, and the relationship between microstructure and mechanical properties, particularly under microwave curing, is not well understood. This study compares the reactivity of acid-extracted lithium slag (A-LS) and alkali-extracted lithium slag (K-LS) as SCM and investigates the effect of microwave curing on LS hydration in cement-based materials. COMSOL Multiphysics (R) finite element simulations were used to optimize the microwave curing process and examine its impact on material properties and microstructure. Hydration was analyzed using thermogravimetric analysis (TG/DTG) and X-ray diffraction (XRD), while microstructure and pore structure were characterized using scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). The results show that, under standard curing, K-LS achieved 53.5 MPa at 28 days, a 20 % increase over A-LS, due to its pozzolanic reactivity and pore-filling effects. Microwave curing accelerated hydration, with K-LS samples reaching 34.5 MPa early strength (a 55 % increase) and 60.8 MPa late strength (a 50 % increase), approaching pure cement strength. Finite element analysis revealed that 300 W microwave power ensures uniform heating and optimal curing, while higher power causes uneven electromagnetic distribution, local overheating, and potential material damage. Microstructural analysis confirmed that microwave curing promotes dense C-S-H gel formation, reduces porosity, and significantly improves material performance. This study demonstrates that LS, combined with microwave curing, not only enables efficient resource use and reduces environmental impact but also enhances cement-based material performance, providing key support for low-carbon construction.