ارزیابی چرخه حیات بتن ژئوپلیمری حاوی متاکائولین و پودر کاشی ضایعاتی

The construction sector is under increasing pressure to reduce its environmental footprint, especially the carbon emissions associated with ordinary Portland cement (OPC). OPC production accounts for approximately 8% of global CO₂ emissions, primarily due to limestone calcination and high-energy combustion processes. In response, geopolymer concrete has emerged as a promising low-carbon alternative. When metakaolin is used as a precursor, geopolymer concrete eliminates the need for clinker production, thereby reducing overall carbon emissions. However, resource consumption during the production of metakaolin and the environmental burdens of alkaline activators remain concerns that require further investigation. This study investigates the use of ceramic waste powder (CWP) as a partial substitute for metakaolin in geopolymer concrete. The primary aim is to address the dual challenges of reducing environmental impacts and optimizing resource consumption. Ceramic waste powder is a by-product generated during tile cutting, polishing, and finishing processes, and is often landfilled, leading to soil and water pollution. By repurposing this waste as a precursor, the study not only reduces the demand for virgin metakaolin but also offers a viable pathway for waste valorization, aligning with circular economy principles. A cradle-to-gate life cycle assessment, in accordance with ISO 14040, ISO 14044, and ISO 21930 standards, was conducted to evaluate five mix designs with different CWP replacement ratios (0, 25, 50, 75, and 100%) and sodium silicate to sodium hydroxide (SS/SH) ratios of 1, 1.25, and 1.5. The total binder content was kept constant at 400 kg/m³, of which 100 kg/m³ consisted of ground granulated blast furnace slag (GGBFS). The aggregate content was fixed at 1680 kg/m³ to maintain consistency across all mixtures. The analysis primarily focuses on global warming potential (GWP) as the key environmental indicator, while also examining trade-offs between waste utilization and embodied carbon. This comparison examines how CWP replacement and the associated activator proportions influence embodied carbon; mechanical performance and durability were outside the scope of the present LCA. This study covers the following life cycle product stages: A1 – Raw Material Supply, including extraction, handling, and processing of materials and fuels; A2 – Transportation of materials from suppliers to the ready-mix plant gate; and A3 – Manufacturing, covering energy used to operate the plant. Excluded stages include production of capital goods, infrastructure, equipment, delivery vehicles, and personnel-related activities. Energy and water use associated with management and sales activities are excluded, and electricity impacts are calculated based on Iran's national average grid, providing a realistic baseline for the local construction industry. The highest total GWP, calculated as A1 + A2 + A3, was 441.2 kg CO₂e/m³ for G1.1; the corresponding value for the OPC reference G6 was 403.2 kg CO₂e/m³. Among the geopolymer mixtures, G4.3 and G5.3 had the lowest baseline value of 418.7 kg CO₂e/m³. Stage A1 represented 92.4-96.2% of total GWP. In the baseline model, CWP carried zero upstream burden and affected results primarily through transportation; sensitivity and Monte Carlo analyses identified A1 background-data uncertainty as the dominant source of variation. Therefore, the net environmental benefit depends on the allocation rule, activator data, transport distance, and mixture formulation. The findings highlight the dual challenge of reallocating waste streams while simultaneously reducing life cycle emissions, requiring careful optimization of material selection and proportions. To address these challenges, it is essential to modify mix designs, particularly by replacing these chemicals with more sustainable options. Exploring innovative activators derived from industrial by-products or bio-based sources, or reducing sodium silicate and sodium hydroxide amounts, can further lower the environmental footprint; the associated mechanical and durability effects should be verified experimentally in future work. This study emphasizes optimizing materials rather than operational changes, given the relatively small contribution of transportation and production to overall GWP. This suggests that environmental improvements should prioritize raw material selection over logistical efficiency. Future research should focus on identifying alternative activators with lower environmental impacts, evaluating the long-term durability and performance of CWP-based geopolymer concretes under various exposure conditions and assessing scalability for widespread adoption. By addressing these aspects, geopolymer concrete containing waste tile powder can offer a transformative solution for sustainable construction, reducing life-cycle environmental burdens and accelerating the transition to greener infrastructure. This research ultimately contributes to decarbonizing the built environment while promoting resource efficiency and waste reduction.