Microstructure-resolved analysis of aggregate packing and morphology effects on stress transfer in magnetite heavyweight concrete

This experimental-numerical study investigated magnetite-based heavyweight concrete through aggregate-grading optimization, quantitative packing analysis, and scanning electron microscopy (SEM)-informed finite difference modelling (FDM). Six mixtures within EN 206 grading envelopes had constant binder composition and varying aggregate grading. The optimized grading achieved hardened density exceeding 4000 kg/m3, 28-day compressive strength of 75.1 MPa, and dynamic elastic modulus of 75.6 GPa. Two-dimensional SEM masks were extruded into quasi-3D (2.5D) voxel domains to examine elastic stress and strain redistribution. Under identical displacement-controlled uniaxial elastic loading, two selected domains differed by approximately 30% in domain-averaged axial stress. The rounded quartz-containing domain showed higher response and more coherent stress-transfer pathways, indicating higher effective elastic stiffness under the adopted modelling conditions. Local geometry, phase identity, stiffness contrast, and spatial arrangement collectively influenced elastic stress redistribution. Integrating grading analysis with SEM-informed modelling provides a microstructure-sensitive framework linking mixture design to local elastic response in high-density structural concrete.