Assessment of particle packing approach for design of low-clinker, LC3 binders at low water-to-solids ratio

Limestone calcined clay cements (LC3) are a promising class of low-clinker cementitious binders. While LC3 concrete is typically formulated with a water-to-solid (w/s) ratio of 0.40 or higher, more recent research has explored using a low w/s ratio. Many advanced cementitious systems like ultra-high-performance concrete (UHPC) and engineered cementitious composites (ECC) require lower w/s for optimal performance. This study explores the hydration and strength development of LC3-based binders with a lower w/s ratio using particle packing approach as a design guide. Hydration was assessed through isothermal calorimetry, thermogravimetric analysis, and X-ray diffraction. The results show strong correlations between compressive strength and the initial particle packing index (PPI), particularly at early ages up to 7 days when physical effects are more influential. Additionally, an Environmental Performance Indicator (EPi) revealed a high correlation between PPI and the environmental efficiency of the mixtures. However, these correlations diminish at later ages as the differences in hydration kinetics and products that alter the microstructure become more significant. The presence of unhydrated cement grains after 90 days highlights incomplete hydration at low w/s ratios. In addition, the results show that while the inclusion of the fly ash did improve hydration of the system, the full benefit from the pozzolanic reactions may not be realized at the low w/s examined here. These findings demonstrate the potential of using particle packing to design LC3 with low w/s ratios, particularly for predicting early strength, while addressing factors that affect long term performance.