Development of sustainable low carbon preplaced aggregate concrete with Portland limestone cement, Metakaolin, silica fume and fly ash: Evaluating binary and ternary binder systems for strength and microstructural properties

This study investigates the mechanical and microstructural behavior of Preplaced Aggregate Concrete (PAC) incorporating Portland Limestone Cement (PLC) with binary and ternary Supplementary Cementitious Material (SCM) systems comprising fly ash (FA), metakaolin (MK), and silica fume (SF). Despite extensive research on PLC-SCM synergy in conventional concrete, its application and mechanistic understanding within PAC systems remain limited. This work addresses that gap by correlating mechanical performance with microstructural evolution using compressive, tensile, and elastic modulus testing supported by SEM, EDX, and XRD analyses. Binary and ternary binder systems demonstrated up to 18% higher compressive and 22% higher tensile strengths than the control mix at 90 days, driven by pozzolanic reaction-induced matrix densification. EDX analysis revealed a reduction in Ca/Si ratio from 1.51 (control) to 0.77 (MK system), while XRD confirmed a 30-40% decrease in portlandite peaks, indicating extensive CH consumption and secondary C-S-H formation. The improved microstructure was characterized by refined pore networks, a denser interfacial transition zone, and reduced microcracking. The study provides new mechanistic insights into PLC-SCM interactions in PAC, establishing foundational knowledge for designing durable and environmentally optimized binary and ternary binder systems.