Advanced ultrasonic characterization and life cycle assessment of waste carbon black mortar for sustainable cementitious materials development

Construction materials face growing pressure to balance environmental sustainability with structural performance requirements. This investigation examines waste carbon black (WCB) as a partial cement replacement. Multiple ultrasonic evaluation approaches are deployed to monitor periodic strength development during curing. Mortar specimens containing 2.5% and 5% WCB by weight of cement were tested using conventional and advanced ultrasonic methods. Testing included ultrasonic pulse velocity (UPV), higher-harmonic β parameter, Sideband Peak Count Index (SPC-I), and Spectral Dissipation Index (SDI). Results demonstrate that 2.5% WCB incorporation yields a marginal improvement in both compressive strength (39.3 ± 1.96 MPa versus 37.46 ± 1.87 MPa for control, approximately 5% higher) and flexural strength (4.43 ± 0.22 MPa versus 4.25 ± 0.21 MPa for control, approximately 4% higher) changes largely within experimental variability. Scanning electron microscopy (SEM) reveals a slightly denser microstructure and reduced pore coverage in 2.5% WCB sample compared to control sample. However, increasing WCB content to 5% causes particle agglomeration, compromising mechanical properties with compressive strength dropping to 23.12 ± 1.16 MPa. Ultrasonic monitoring revealed distinct microstructural evolution patterns, with UPV values reaching 3333 ± 100.05 m/s for optimal 2.5% WCB mixtures. The β parameter effectively detected early-stage particle agglomeration induced micro-cracking at 5% WCB prior to strength degradation, while hybrid parameters, i.e. SPC-I and SDI, provided sensitive indicators of internal structural changes throughout curing. Life cycle assessment confirmed environmental benefits, showing modest 2–3% reductions in global warming potential and resource depletion. This integrated approach establishes a comprehensive framework linking mechanical performance monitoring with environmental impact assessment, advancing sustainable concrete technology through industrial waste utilization.