Sustainable Lightweight Concrete from Cinder Waste: Optimization of Nano-CaCO3 and Carbon Fiber Reinforcement Using Response Surface Analysis

Growing demand for sustainable construction materials has increased interest in industrial by-products as replacements for conventional aggregates. This study develops eco-friendly lightweight concrete (LWC) using cinder as a complete replacement for natural coarse aggregate, combined with nano-calcium carbonate (Nano-CaCO3, 0–5% by mass of cement) and carbon fiber (0–1.2% by mass of binder) to enhance mechanical performance. Twenty-four mix combinations were evaluated for compressive, split tensile, and flexural strength at 7, 14, and 28 days, and the results were analyzed using quadratic response surface regression to quantify the individual and interactive effects of Nano-CaCO3 and carbon fiber. The regression models explained 82–84% of the variance in 28-day strength (R2 = 0.82–0.84, p < 0.0001), with carbon fiber content the strongest predictor of tensile and flexural performance. The mix containing 2% Nano-CaCO3 and 1% carbon fiber (98% cement + 2% Nano-CaCO3 + 1% CF) consistently outperformed all other combinations, achieving a 28-day compressive strength of 36.89 MPa (a 19.9% increase over the plain-cinder control), split tensile strength of 5.42 MPa (+64.2%) and flexural strength of 5.32 MPa (+37.1%). Beyond this dosage, both compressive strength and, to a lesser extent, tensile and flexural strength declined, consistent with fiber agglomeration and nanoparticle over-packing at higher dosages. The concrete achieved a unit weight of 1548 kg/m3, well below the 2000 kg/m3 threshold for lightweight concrete, confirming a genuine density reduction relative to conventional M25 concrete (≈2400 kg/m3). Microstructural analysis (SEM, EDAX, XRD) of the optimum mix showed a visibly densified matrix and well-adhered interfacial transition zone between cinder and cement paste, consistent with the mechanical trends. To the authors’ knowledge, this is the first study to jointly optimize Nano-CaCO3 and carbon fiber dosage in cinder-based lightweight concrete using a statistically validated response surface model, identifying a single dosage combination at which compressive, split tensile, and flexural strength simultaneously peak. These results identify a statistically supported optimum dosage for cinder-based lightweight concrete and provide a regression-based design tool for similar Nano-CaCO3–carbon fiber systems, while directly supporting waste-derived, resource-efficient construction practice.