VALIDATION OF PORTLANDITE AVAILABILITY AS A GOVERNING PARAMETER IN HIGH-KAOLINITE LOW-CLINKER LC3 BINDERS

The combination of the hydration of clinker, calcined clay, limestone, and gypsum in limestone calcined clay cement (LC3) facilitates significant clinker reduction. However, when the clinker factor is lowered below the standard LC3-50, the portlandite generated from clinker hydration may not be enough to support the pozzolanic reaction of metakaolin, especially in systems containing high-kaolinite calcined clay. This study uses reagent-grade calcium hydroxide as a controlled chemical probe in LC3-35, LC3-25, and LC3-15 formulations to validate portlandite availability as a governing parameter in high-kaolinite low-clinker LC3 binders. Mechanical performance was linked to early-age heat development and hydration kinetics using mortar compressive strength and isothermal calorimetry. According to the compressive strength results, the addition of calcium hydroxide becomes more significant as the amount of clinker decreases. After 28 days, the strength of LC3-25-1:1 increased from around 18.7 MPa to 32.6 MPa, and LC3-15-1:1 increased from roughly 10.7 MPa to 30.2 MPa due to the addition of calcium hydroxide. On the other hand, the improvement in LC3-35-1:1 was relatively smaller, suggesting that in very low-clinker systems, portlandite limiting becomes more crucial. The observed improvement is explained by greater binding hydrate production through the pozzolanic reaction and the synergy between limestone and calcined clay, as well as improved metakaolin utilization. The results offer a useful strength-hydration validation framework for future chemically optimized low-carbon cement systems as well as for the design of high-kaolinite low-clinker LC3 binders.