Comparative evaluation of limestone calcined clay cement and portland cement binders in rubberized self-compacting concrete

Cement production releases significant CO₂, making it a major contributor to global warming. Although alternative binder research has explored the use of rubberized self-compacting concrete (RSCC) to mitigate the depletion of natural resources and reduce CO₂ emissions, studies comparing the influence of binder type, particularly ordinary Portland cement (OPC) versus limestone calcined clay cement (LC³), on RSCC performance are limited. This study addresses the existing gap by presenting a comparative evaluation of OPC and LC³ as binders in RSCC. Ten mixtures were produced with each binder system, incorporating crumb rubber (CR) as a partial volumetric replacement of fine aggregates at 0%–40%. The experimental program assessed the fresh properties (slump flow, T500, V-funnel, L-box, and U-box), mechanical performance (compressive strength, split tensile strength, elastic modulus, and Poisson’s ratio), permeable voids, microstructural characteristics (scanning electron microscopy, SEM; X-ray diffraction, XRD; and energy dispersive X-ray, EDX), and environmental performance. The results demonstrated that increasing the CR content reduced the flowability in both systems. LC³-based RSCC exhibited comparable workability to OPC mixtures but required higher superplasticizer dosages; however, the finer particles of LC³ enhanced the flowability within the desired self-compacting range. At 28 days, OPC- and LC³-based mixes with 0% CR achieved comparable compressive strengths (∼68 MPa). However, at 40% CR, LC³ retained 25.2 MPa versus 12.4 MPa for OPC. Similarly, the splitting tensile strength at 40% CR was 2.53 MPa in LC³ versus 1.47 MPa in OPC (72% higher). The permeable voids at 40% CR were 9.36% in LC³, versus 11.36% in OPC mixes, indicating better pore refinement. Microstructural analyses confirmed that LC³ promotes the formation of high-density C–S–H and C–A–S–H gels with stronger interfacial transition zones around rubber particles. XRD results verified the dehydroxylation of kaolinite and formation of amorphous metakaolin, and EDX confirmed higher Al incorporation in LC³ hydrates, leading to enhanced microstructural densification. Sustainability assessment revealed that LC³ reduced CO₂ emissions by ∼41% and energy demand by 18%–21% compared with OPC. Overall, LC³ significantly improved the mechanical, microstructural, and environmental performances of RSCC, demonstrating its potential as a low-carbon binder for sustainable and circular-economy concrete applications.