The growing environmental burden of cement manufacturing and natural-aggregate extraction has driven interest in concrete that simultaneously valorises agricultural and construction wastes. This study investigates the mechanical performance and life-cycle environmental impact of sustainable concrete incorporating rice husk ash (RHA) and recycled coarse aggregate (RCA). Three mixtures were evaluated: a control mixture, a mixture with 50% RCA and 20% RHA (50%RCA+20%RHA), and a mixture with 100% RCA and 20% RHA (100%RCA+20%RHA). The compressive strength, axial stress–strain behaviour, peak stress and strain, static modulus of elasticity, and toughness were determined and compared with the Popovics and Carreira & Chu constitutive models, while the environmental performance was quantified through a cradle-to-gate life-cycle assessment (LCA) covering embodied energy (EE), global warming potential (GWP), acidification potential (AP), eutrophication potential (EP), and photochemical ozone creation potential (POCP), each normalised by compressive strength and analysed using multi-criteria decision-making (MCDM). The compressive strength decreased from 27.35 MPa for the control to 25.93 and 22.55 MPa at 50% and 100% RCA, and the modulus of elasticity fell more sharply than the strength, while moderate (50%) RCA combined with 20% RHA produced the highest toughness and toughness index. Because the Popovics and Carreira & Chu models share the same three-parameter formulation, fitting the peak-strain and shape parameters to each curve makes the two models coincide; the resulting unified model reproduced the measured stress–strain response of all three mixtures with a coefficient of determination R² ≥ 0.993 and a root-mean-square error of 0.23–0.78 MPa. The 100%RCA+20%RHA mixture achieved the largest reductions in EE, GWP, AP, EP, and POCP (about 18.3%, 18.5%, 30.7%, 6.8%, and 85.1%, respectively) relative to the control, whereas the 50%RCA+20%RHA mixture offered the most balanced impact-to-strength ratios. The MCDM analysis ranked the 100%RCA+20%RHA mixture highest under the adopted weighting scenario, confirming that the combined use of RHA and RCA can deliver structurally viable, markedly lower-impact concrete when the RCA replacement level is appropriately selected. As the assessment is limited to 28-day mechanical and cradle-to-gate environmental performance, durability and cost validation remain necessary before structural deployment.