Multidimensional Life-Cycle Assessment of Cement Clinker Production Using Solid-Waste Alternative Raw Materials

The introduction of carbon peaking and carbon neutrality targets has made a green, low-carbon transition the prevailing trend in the cement manufacturing industry. Solid-waste valorisation, which involves substituting natural minerals and fossil fuels with suitable solid waste, can effectively reduce emissions from carbonate decomposition and fuel combustion. It is therefore regarded as a key technological pathway for achieving the carbon reduction goals. The existing studies on single dimensions such as energy or material saving, or carbon mitigation in clinker production using solid waste are relatively well-developed. However, valorisation can generate both environmental benefits and drawbacks, there is a clear need for a comprehensive assessment framework based on multiple indicators that can capture trade-offs across impact categories, rather than drawing conclusions from a single metric.

A multidimensional assessment framework was developed based on standard product carbon footprint and life-cycle assessment models. This framework could comprise three primary indicators, i.e., carbon, resources and environment and twelve secondary indicators, enabling the structured and transparent evaluation of benefits as well as potential burden shifting. A consistent system boundary was defined to cover the entire clinker production value chain, including solid waste inputs, raw material production, waste pre-treatment, transportation, clinker production and credits for avoiding conventional disposal methods (landfilling or incineration). Within this boundary, five substitution scenarios involving the use of solid waste were constructed and benchmarked against a mineral-only baseline, i.e., carbide slag (S1), manganese slag (S2), coal gangue (S3), fly ash (S4) and sewage sludge (S5). We then systematically quantified the potential benefits and burdens of solid waste utilisation in terms of carbon emissions, resource consumption and environmental impacts. This could provide a decision-relevant evidence to support the cement industry’s transition to a low-carbon economy.

In terms of carbon emissions, all scenarios achieve net reductions, albeit with significant variation in magnitude. Scenario 1 demonstrates the most significant mitigation, reducing emissions by 35.2%, compared to the baseline, resulting in a total reduction of 351.3 kg CO₂eq. By contrast, Scenario 2 involves pre-treatment requiring additional Ca(OH)₂ and anhydrous Na₂SO₄, which increases raw-material production emissions by 9.8 kg CO₂eq relative to the baseline. Scenario 5 results in an increase of 4% in fuel-combustion emissions due to the high moisture content of the waste, leading to the weakest overall mitigation with a net reduction of only 38.9 kg CO₂eq. Overall, these results indicate that although carbon benefits are generally achievable across different types of waste, their magnitude depends on waste composition and the intensity of the pre-treatment. In terms of resources, overall consumption decreases across all scenarios, but there is a trade-off between “material savin” and “energy saving”. Scenario 1 achieves the greatest overall saving of resources, with a reduction of 19.4% relative to the baseline. This is primarily due to its high substitution rate and avoidance of waste landfilling. However, its energy-intensive pre-treatment increases fossil resource consumption by 27.2%, offsetting the net energy benefit. Scenario 3 exhibits the most pronounced energy-saving effect, reducing fossil resource consumption by 18.9% via substituting approximately 20% of coal with coal gangue. However, although the substitution rate in Scenario 5 is 20%, a need to add bauxite to satisfy mineralogical requirements offset some of its material-saving potential. These findings highlight that the outcomes of resource use depend on substitution ratios and on the additional inputs required to meet clinker quality constraints. In terms of the environmental impact, the avoided disposal process has a significant effect on the results. When this process is included in the assessment, Scenario 1 demonstrates the most significant reduction in overall environmental impact at 84.7%, compared to the baseline. This is primarily due to the prevention of heavy metal emissions (i.e., Cd, Se and As) linked to landfill. Scenario 2 shows a similar pattern, but improvements in human toxicity indicators are limited because mercury (Hg) emissions associated with producing anhydrous sodium sulphate (i.e., anhydrous Na₂SO₄) offset part of the benefits. If avoided disposal is excluded, Scenario 3 exhibits the optimum overall environmental performance. In the comprehensive evaluation, Scenario 1 consistently ranks first under equal weighting and across all three dual-priority schemes, establishing itself as a core pathway with optimal overall benefits. Scenario 3 is suitable as a priority enhancement option under both “carbon emissions–resource priority” and “resource–environment priority” orientations. Scenario 2 can serve as an alternative under the objective of “carbon emissions–environment priority”.

The five solid waste valorisation scenarios for clinker production had different advantages and constraints with regard to carbon mitigation, material conservation and environmental improvement. Scenario 1 delivered the optimum overall performance, but its high energy demand and associated pollutant emissions during pre-treatment could be limiting factors. Scenario 2 achieved considerable carbon reduction and environmental benefits, but it was limited by heavy metal emissions and energy consumption during pre-treatment. Scenario 3 could be characterised as “low risk, steady benefit”, offering significant savings in fossil energy without apparent adverse environmental impact. Scenario 4 yielded the modest gains across the board without any negative effects, making it suitable for routine implementation. Scenario 5 showed the greatest advantage in terms of photochemical smog control, but it had the weakest overall performance. To advance the valorisation of solid waste in the cement sector, we could recommend optimising the pre-treatment of high-substitution waste (i.e., carbide and manganese slag) to reduce energy use and air pollutant emissions arising from energy-intensive steps. Large-scale substitution could be encouraged for wastes with a substantial valorisation potential (such as carbide and manganese slag) to further enhance integrated environmental benefits.