Long-Term Mechanical Properties and Microstructure of High-Performance Concrete with Waste Foundry Sand and Coal Bottom Ash

This study investigates the long-term mechanical and microstructural behavior of high-performance concrete (HPC), incorporating hybrid mixtures of coal bottom ash (CBA) and waste foundry sand (WFS) as sustainable mineral additives. The experimental program included evaluation of physical parameters (porosity, water absorption, and density), mechanical properties (compressive, splitting tensile, flexural strength, and elastic modulus), ultrasonic pulse velocity (UPV), and microstructural observations by scanning electron microscopy (SEM). The incorporation of CBA and WFS up to 30 wt% modified the pore structure and densified the matrix, leading to improved long-term strength and durability. The BA25FS5 and BA20FS10 mixtures exhibited the most balanced performance, showing compressive strengths up to 86 MPa at 730 days and UPV exceeding 4.5 km/s (measured at 730 days). SEM analysis confirmed a dense C–S–H network and strong ITZ bonding in hybrid concretes. Empirical models, including Ryshkevich, Balshin, and ACI–fib correlations, accurately described the relationships between porosity, density, and mechanical properties, achieving coefficients of determination above 0.9. The results demonstrate that the combined use of CBA and WFS enhances microstructural refinement, stiffness, and long-term performance while promoting sustainable utilization of industrial by-products in high-performance concrete.