Life cycle assessment of coal gangue composite cements: From sole OPC towards low-carbon quaternary binder

Cement production is responsible for substantial anthropogenic CO2 emissions, and various solid wastes have been explored as replacements for cement clinker to produce composite cements in lowering the carbon footprint. The composite cements are claimed to be more environmentally friendly than ordinary Portland cement (OPC) in most Life Cycle Assessment (LCA) studies but overlooking the environmental burden from upstream material and strength degradation after replacing the cement may cause significant inequities between LCA models. This paper aims to present a comprehensive LCA, starting from sole OPC through binary and ternary, and followed by a high-volume replacement of OPC with alternative materials, known as quaternary composite cements. The latter incorporated coal gangue, limestone, and ground granulated blast furnace slag as cement replacements. The uncertainty caused by the variation of calcination temperature and kaolinite content of coal gangue were assessed by Monte-Carlo analysis. Results show that ternary composite cement is more recommended in comparison to binary composite cement, considering the very minimal emissions from limestone production, and a calcination temperature of 600 °C is the most recommended owing to the wider selection kaolinite content range of CG. The quaternary composite cement was determined to achieve the most satisfying performance from both mechanical and global warming potential perspectives. However, after considering the environmental burdens of upstream processes as allocation, additional attention should be paid to the deterioration of other environmental impacts, such as freshwater quality and human carcinogenic toxicity.