Synergistic effects of limestone calcined clay cement on alkalinity, mechanical performance, and vegetative compatibility of ecological concrete

Ecological concrete is essential for sustainable construction, yet its application is limited by the high alkalinity of ordinary Portland cement (OPC), which inhibits plant growth, and by conventional alkalinity-reduction methods that often compromise mechanical performance. This study explores the use of limestone calcined clay cement (LC³) as a sustainable alternative, aiming to achieve a synergistic balance among low alkalinity, adequate compressive strength, and enhanced vegetative compatibility. LC³ ecological concretes with varying metakaolin and limestone contents and porosities of 22, 26, and 30% were prepared at a water-to-binder ratio of 0.24. The materials were characterized in terms of pH, compressive strength, hydration products (XRD, TGA/DTG, SEM, LF-NMR), pore structure, and plant growth performance using tall fescue over 60 days. Results indicate that LC³ significantly reduces 28-day pH to a plant-compatible range of 8.4–8.8, while mixes such as M20L15 and M30L15 achieve compressive strengths up to 13 MPa at 22% porosity. Microstructural analyses revealed that metakaolin’s pozzolanic reaction consumes portlandite, and its synergy with limestone promotes the formation of dense C-A-S-H gels and carboaluminate phases, refining the pore structure and hindering alkali ion migration. Vegetation tests confirmed vigorous plant growth on LC³ concrete, particularly at 30% porosity, whereas OPC controls caused plant mortality. Overall, LC³ ecological concrete effectively addresses the limitations of conventional formulations, simultaneously ensuring mechanical integrity, controlling alkalinity, and supporting sustainable vegetative growth, highlighting its potential for green infrastructure applications.