Mechanical and Microstructural Properties of Cement Pastes with Partial OPC Replacement by Copper Slag Flotation Tailings

Abstract

The growing need to reduce the environmental impact of cement production has intensified interest in alternative supplementary cementitious materials derived from industrial by products. In this study, copper slag flotation tailings (CSFT) obtained from the reprocessing of copper smelting residues were evaluated as a partial replacement of Ordinary Portland Cement (OPC) in cement pastes. Six mixtures were prepared by replacing OPC with CSFT at levels of 5–30% by mass, using a constant water-to-binder ratio of 0.30. Mechanical performance was assessed through compressive and flexural strength testing at curing ages up to 180 days. Complementary nondestructive techniques, including ultrasonic pulse velocity and electrical resistivity, were employed to monitor microstructural development, while thermogravimetric analysis (TGA) was used to quantify hydration products associated with C–S–H and portlandite. Results show that CSFT replacement above 20% leads to reduced early-age mechanical performance of hardened pastes, primarily due to clinker reduction and the low initial reactivity of the CSFT. However, progressive strength recovery was observed from 56 days onward. In particular, mixtures with low CSFT contents (5–10%) approached or matched the OPC mixtures in mechanical–strength by 90–180 days. TGA results revealed that the mass loss associated with C–S–H and chemically bound water increased with curing age and converged across all mixtures at 180 days, whereas portlandite content remained consistently lower with increasing CSFT replacement and did not exhibit recovery at later ages. These findings indicate that CSFT behaves predominantly as a filler material with possible low intrinsic reactivity, while potentially contributing to delayed secondary hydration processes at extended curing ages. The decoupled evolution of C–S–H formation and portlandite content highlights the importance of extended curing evaluations when assessing low reactivity cementitious additions. Overall, CSFT shows potential as a sustainable clinker reducing component in cementitious systems when applied at moderate replacement levels and evaluated beyond standard 28-day performance criteria.