硅酸盐学报

在“双碳”战略背景下，具备低碳属性的碱激发胶凝材料备受关注，但其生产生命周期碳排放评估体系尚未完善。针对碱激发胶凝材料低碳定义的模糊性及碳足迹核算方法缺失的问题，提出了一种涵盖原材料处理、运输及生产生命周期的碳足迹核算方法。以赤泥为主要硅铝质前驱体，系统探究了激发剂种类(Na_2CO₂、NaOH、Na_2O·2SiO₂)及掺量对碳排放与力学性能的影响规律。结果表明，采用Na_2CO₃为激发剂可显著降低碳排放，其低碳优势源于低能耗生产工艺。通过响应面法优化设计，开发出赤泥基碱激发胶凝材料最优配比(Na_2CO₃质量掺量5.54%、赤泥35.56%、矿渣57.22%、水泥7.22%)，其碳排放为228 kg CO₂/1 t碱激发胶凝材料(AACM),28 d抗压强度达58.5 MPa。该方法为碱激发胶凝材料的低碳化设计提供了理论依据与技术支撑。

Introduction As the second largest global carbon dioxide emission source, the cement industry contributes approximately 8% of total anthropogenic carbon emissions. Under the “dual-carbon” strategic framework, developing novel low-carbon cementitious materials emerges as a research priority in civil engineering. Alkali-activated cementitious materials(AACM) are regarded as a promising alternative to conventional Portland cement due to their advantages of eliminating high-temperature calcination and utilizing industrial solid wastes. However, the existing research predominantly focuses on material performance optimization, while the definition of their "low-carbon" attributes remains limited to raw material substitution, lacking a systematic life cycle carbon emission accounting framework. No scientific evaluation methods or quantitative models are established to assess carbon footprint disparities among different activator systems(i.e., carbonate, hydroxide, and silicate). Furthermore, red mud-a massive solid waste generated by the alumina industry with global annual emissions exceeding 150 million tons exhibits natural compatibility with alkali-activated systems due to its high alkali metal content. Nevertheless, the existing studies predominantly emphasize its reactivity activation, while neglecting the development of integrated low-carbon processing technologies. Establishing systematic carbon accounting methodologies and elucidating the coupling mechanisms among activators, carbon emissions, and mechanical properties become critical scientific challenges for advancing the engineering applications of alkali-activated materials.Methods In the AACM production framework, the study defined a “cradle-to-gate” system boundary spanning from raw material acquisition to final product formation. This boundary was divided into three distinct phases, i.e., raw material acquisition stage(Am), encompasses processes such as raw material production, mining, and extraction, and raw material transportation stage(At). Accounts for carbon emissions generate during the transportation of all required raw materials. Production stage(Ap) includes operational steps such as drying, homogenization, grinding, storage, and packaging. The investigation specifically employed red mud as an intrinsic alkali source, focusing on elucidating the influence patterns of both red mud and external alkalis(i.e., Na_2CO₃, Na OH, and Na_2O·2Si O₂) on the carbon emissions of AACM. Response surface methodology(RSM) with the Box-Behnken design was employed to optimize mix parameters(i.e., Na_2CO₃ dosage, red mud content, and cement content), conducting multi-factor interaction analysis with dual objectives, i.e., 28-d compressive strength and carbon emissions. To compare the life cycle carbon emissions of AACM production in the same production line with industrial solid waste(i.e., red mud), granulated blast furnace slag, and ordinary Portland cement clinker as precursors, as well as Na_2CO₃, Na OH, and Na_2O·2SiO₂ as activators and to design low-carbon AACM, the following parameters were established. For the raw material transportation phase, heavy-duty diesel trucks(46-ton payload) with a carbon emission factor of 0.057 kg CO₂/(t·km) were selected, assuming a default transportation distance of 1000 km. For the production phase, the carbon emissions from drying red mud and granulated blast furnace slag were estimated based on electricity consumption of drying equipment and exhaust fans. The field data indicated that drying granulated blast furnace slag with 10%–13% moisture content required approximately 11.5 k W·h per ton of wet material. For high-moisture red mud drying(using a Φ3 m×28 m rotary dryer with 2.45–4.20 r/min rotation speed and 3.5% slope), the energy consumption reached 35.0 k W·h per ton of wet material(data from Shanghai Bai Ao Heng New Materials Co., Ltd., China). Carbon emissions from material batching, grinding, and homogenization processes were estimated using reference power consumption data(i.e., ≈33 k W·h per ton of cement) from a plant of Southwest Cement Co., Ltd., China. The average carbon emission factor of China's regional power grids was adopted as 0.5366 kg CO₂/(k W·h)(2022 national average electricity CO₂ emission factor). Results and discussion This study proposes a life cycle carbon footprint accounting model for AACM production, reveals the synergistic regulation mechanism of activator types and mix proportions on the material low-carbon characteristics and mechanical performance through response surface methodology-based optimization design and establishes a cradle-to-gate carbon footprint accounting system covering raw material acquisition, transportation, and production stages. The contribution weights of red mud-based AACM precursors and activator types to carbon emissions are quantified, providing a quantitative basis for evaluating low-carbon cementitious materials. As an endogenous alkali source, red mud can effectively reduce the dosage of external activators. Coupled with the low-carbon characteristics of Na_2CO₃(carbon emission factor: 0.79 t CO₂/t), this significantly reduces the system's carbon emissions. At a red mud content of ≤50% and a cement clinker dosage of ≤10%, the material's carbon emissions can reduce to below 280 kg CO₂ per ton of AACM, validating a feasibility of the synergistic pathway of solid waste recycling–low-carbon process. The low-carbon design based on the RSM demonstrates that in the optimal mix proportion of the red mud-slag system(with 5.54% Na_2CO₃ dosage, 35.56% red mud, 57.22% slag, and 7.22% cement), the cementitious material achieves a 28-d compressive strength of 58.5 MPa and a carbon emission of 228 kg CO₂ per ton of AACM. This system exhibits both high strength and low-carbon characteristics, with performance comparable to Portland cement P·O 42.5R. In this mix proportion, red mud with a high alkali content provides abundant endogenous alkali to the system, reducing reliance on exogenous alkali. The amorphous phase structure of blast furnace slag endows it with a high alkali reactivity, enabling a rapid release of Ca²⁺ and SiO₄^4– under Na_2CO₃ activation to promote gel formation and enhance strength. The hydration of minerals such as C_3S and C_2S in cement generates additional Ca(OH)₂, further elevating the p H value of the system. This accelerates the depolymerization of slag vitreous phases and enhances the compactness of the matrix interfacial transition zone, thereby ensuring the mechanical performance of the AACM. Conclusions This study could pioneer the integration of red mud's intrinsic alkali properties with low-carbon activators, proposing a holistic “low-carbon design–carbon accounting–performance optimization” methodology. The framework provided a theoretical support for industrial applications of alkali-activated materials, offering dual benefits, i.e., large-scale red mud consumption in the alumina industry and a technical pathway for achieving the “dual carbon” goals in construction materials.