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.

Cement production accounts for approximately 12% of China' s total CO₂ emissions, having a significant challenge to achieving the national "dual carbon goals". Carbon capture, utilization, and storage (CCUS) represent a pivotal innovative technology for mitigating these emissions. However, conventional amine-based CO₂ capture requires an energy-intensive high-temperature desorption, hindering its industrial implementation in cement plants. Also, the limited utilization pathways for captured CO₂ pose another challenge for cement CCUS. Diethanolamine (DEA) offers a promising solution as it functions both as a CO₂ absorber and a cement additive. This dual capability enables a potential carbonation utilization of CO₂ absorbed DEA solutions without requiring the desorption step within cementitious systems. This study was thus to investigate the effect of CO₂-absorbed diethanolamine (DEAC) on the early hydration behavior and strength development of cementitious systems. The findings could propose a novel approach for low-energy CO₂ capture coupled with efficient in-situ utilization within cement industry.

Cement mortars with a water-to-cement ratio (W/C) of 0.50 were prepared with P·I 42.5 Portland cement (GB 8076) and ISO standard sand. The specimens were designated as REF, D0.1%C0%, D0.1%C0.02%, D1.0%C0%, and D1.0%C0.22%, respectively. DEA and its equivalent CO₂ admixture were added as percentages of cement mass. All associated cement paste mixtures were prepared at a W/C ratio of 0.3. DEAC was prepared by continuously bubbling CO₂ gas (≥99% purity) at a flow rate of 200 mL/min through a 5 mol/L DEA solution maintained at 40 ℃ until saturation was achieved. An eight-channel microcalorimeter recorded the hydration heat of cement paste specimens at 25 ℃ for 72 h. The phase composition of hardened cement pastes was determined by X-ray diffraction (XRD). The contents of bound water, CH, and CaCO₃ were analyzed by thermogravimetric analysis (TGA). The cumulative porosity and pore size distribution of hardened paste samples at 7 d were characterized by mercury intrusion porosimetry (MIP). The compressive strength was measured on mortar specimens at 1, 3 d and 7 d of curing in accordance with the standard GB/T 17671.

The hydration calorimetry results demonstrate that D0.1%C0.02% and D0.1%C0% both accelerate the hydration rate of silicate phases, as evidenced by an increased second exothermic peak rate, while leaving the induction period duration unaffected. Conversely, D1.0%C0% and D1.0%C0.22% significantly reduce the second exothermic peak rate. D1.0%C0.22% extends the hydration induction period to 240 min, while D1.0%C0% has a negligible effect on its duration. The XRD patterns and TG analyses reveal that the impact of DEAC on the cement hydration depends critically on its specific DEA and CO₂ dosage. At a low dosage (i.e., D0.1%C0.02%), a mild carbonation promotes a concurrent hydration of silicate and aluminate phases. However, a high dosage (i.e., D1.0%C0.22%) substantially inhibits early hydration of silicates. The MIP results indicate that DEAC and DEA both refine the pore structure of hardened cement paste. The pores below 20 nm are significantly reduced in D0.1%C0% and D0.1%C0.02% systems, aligning with their enhanced early hydration kinetics. This refinement also occurres in D1.0%C0% and D1.0%C0.22% systems despite inhibited silicate hydration. The results of compressive strength tests show that D0.1%C0.02% and D0.1%C0% can enhance mortar strengths at 1, 3 d, and 7 d, respectley. The strengths of D0.1%C0.02% systems can be increased by 8.4%, 10.2%, and 16.8% at these ages, respectively, primarily due to the DEA component with the weak carbonation contributing minimal additional enhancement. The 3-day and 7-day strengths of D1.0%C0.22% and D1.0%C0% systems both are increased (more significantly in the carbonated system), indicating a synergistic hydration-carbonation effect. However, the 1-day strength of D1.0%C0.22% system drastically is reduced by 52.2%, with silicates hydration inhibition by D1.0%C0% identified as a primary factor underpinning early strength reduction. According to the analysis of bound water content, calcium hydroxide (CH) content, porosity, and compressive strength relationships, a linear correlation between CH content and mortar strength is proposed. This demonstrates that silicate phase hydration kinetics can be modulated differently by DEAC and DEA formulations-fundamentally governed compressive strength development.

The addition of 0.1%DEA with 0.02% CO₂ (D0.1%C0.02%) as DEAC enhanced the flexural and compressive strengths of cement mortar at 1, 3 d, and 7 d. In contrast, the addition of 1.0% DEA with 0.22% CO₂ (D1.0%C0.22%) significantly reduced the 1-day strength. In D0.1%C0.02% system, the CO₂ component reacted with dissolved Ca²⁺ released from cement minerals to precipitate CaCO₃. This reaction promoted cement hydration, refined the pore structure of the hardened paste by reducing the volume of harmful pores, and facilitated a synergistic enhancement of hydration and carbonation. D1.0%C0.22% addition significantly retarded cement hydration within the first 24 h, primarily by inhibiting the dissolution of silicate phases and extending the induction period, leading to the reduced early strength. Although carbonation exacerbated the retardation of silicate phase hydration via DEA interaction, the hydration process recovered normal kinetics after 7 d.

In the field of industrial denitration, Selective Catalytic Reduction (SCR) technology is highly favored for its efficiency. However, SCR catalysts inevitably lose activity over time and are difficult to regenerate through conventional means, resulting in a significant amount of spent catalysts. These spent catalysts contain valuable components such as vanadium (V), tungsten (W), and titanium (Ti), and their effective recovery can enable resource recycling.

This article focuses on the deactivation mechanisms and recycling pathways of commercial vanadium-titanium-based catalysts currently in use. It provides a detailed summary and comparison of the two main recycling methods: hydrometallurgical and pyrometallurgical processes, and discusses the reaction mechanisms involved and the advantages and disadvantages of each. Faced with the challenges of full element recovery in typical methods, this article collates cooperative treatment recycling processes, finding that current work needs to focus on reducing process costs and simplifying procedures while further improving recovery rates. Additionally, the high-value utilization of valuable components is also worthy of in-depth research. This article aims to provide a reference for the future recycling and utilization of spent catalysts, which is crucial for resource recovery and green low-carbon development.

Selective Catalytic Reduction (SCR) technology, due to its high efficiency and applicability, has become a core technology for NO_x emission reduction in industries such as steel, cement, and glass. As these industries increasingly rely on SCR technology, the recycling and high-value utilization of spent catalysts have become an urgent issue that needs to be addressed. When spent catalysts are difficult to regenerate, the recovery of valuable components becomes the preferred option.

Thermal treatment methods dominate the recycling process due to their high extraction rates but also bring about issues of high energy consumption and environmental pollution. In contrast, hydrometallurgical processes are favored for their simplicity, low cost, and high single-metal extraction rates, yet their development is limited by the low extraction rates of leaching agents for multiple valuable elements and the leaching of many impurities. Therefore, the integration of multiple processes and the development of high-value-added products represent the future direction for SCR catalyst recycling.

In the recycling process of SCR catalysts, the key lies in the efficient extraction of vanadium (V), tungsten (W), and titanium (Ti) while reducing energy consumption and environmental harm. Both acid and alkali methods can effectively recover over 90% of V, but given the toxicity of V, the use of a reducing acid leaching method is recommended to prioritize vanadium extraction, facilitating the preparation of high-purity V₂O₅. The recovery of W and Ti is more challenging, especially for W, whose leaching rate is limited by the choice and concentration of solvents, with low-concentration alkali leaching rates typically around 70%. Ti is only soluble in concentrated alkali solutions, and due to the high proportion of titanium in spent catalysts, its extraction would consume a large amount of reagents, leading to high costs. Thus, the most reasonable method is to use alkali to leach W while controlling the alkali concentration to retain as much Ti as possible in the residue. The alkaline leachate contains high levels of silicon and aluminum impurities, which need to be purified before the recovery of products. Additionally, the presence of heavy metals in spent catalysts requires targeted adjustments to the recycling process to enhance recovery rates and purity.

Currently, research on the recycling and utilization of spent vanadium-titanium catalysts focuses on the impact of operating conditions and reaction mechanisms on recovery rates, with insufficient evaluation of energy consumption and environmental impacts during the recycling process. Future studies could employ life cycle assessment and material flow analysis to evaluate the environmental impacts and resource efficiency of spent catalyst recycling. Moreover, the development of new and efficient leaching agents, optimization of thermal and hydrometallurgical process conditions, and exploration of high-value utilization pathways for spent catalysts, such as the preparation of new materials or as raw materials for other industrial processes, will be key to advancing SCR catalyst recycling technology.