Modulating red mud for the fabrication of cementitious material by analyzing the thermal evolution of hydrogarnets

This work aims to develop a modulation strategy for converting red mud (RM) into cementitious material based on elucidating the phase transformation of hydrogarnet. The results show that cementitious minerals 2CaO·SiO2 (C2S), 12CaO·7Al2O3 (C12A7), and 4CaO·Al2O3·Fe2O3 (C4AF), as well as the free iron minerals Fe and FeO, are formed by integrating calcification dealkalization and reduction roasting treatment of RM. During the reduction roasting process, CaO is preferentially combined with SiO2 and Al2O3 to form cementitious minerals, and the Fe(III) compounds in hydrogarnet and hematite can be directly reduced to free iron minerals without intermediate ferrites. By optimizing the reduction roasting parameters and eliminating the useless minerals 2CaO·Al2O3·SiO2 (C2AS), and FeO, the reduction roasting product is mainly composed of C2S, C12A7, C4AF, and Fe. Therefore, cementitious material is obtained after the magnetic separation of Fe, which possesses both early and late hydration properties. In addition, 75% Fe in RM can be recovered, and the reduced iron powder (RIP) is also useful in the cement clinker production or steel smelting process. The findings in this work lay the foundations for understanding the phase transformation of RM-derived hydrogarnet in the reduction roasting process and also provide a new reference for the modulation and utilization of RM in the cement and concrete field.


Introduction
Red mud (RM) is a hazardous waste deriving from the alumina industry. With difficult digestion and complex composition, more than 85% of RM can only be accumulated (Liu et al. 2020), thereby causing environmental and ecological issues (Song et al. 2021). The comprehensive utilization of RM has attracted much attention across various fields, such as construction, environment, catalyst, and metallurgical industry. As the bulk materials in the construction field, cement-based materials are considered a suitable solidification and stabilization substrate for solid wastes with large-scale consumption (Siddique 2010;Yang et al. 2019;Zhang et al. 2021b), thus the usage of RM in cement-based materials is an economical and environmental strategy (Liu and Zhang, 2011;Wang et al. 2021a). Up till now, the utilization of RM in the cement and concrete field is mainly in three approaches (Liu and Zhang, 2011). In the aspect of cement production, RM can be treated as the main source of iron or aluminum corrective material (Hertel et al. 2021). RM also serves as supplementary cementitious material or mineral additive in cement and concrete materials (Anirudh et al. 2020). With abundant aluminosilicate minerals, RM has the potential to become a geopolymer by activation (Ye et al. 2016). However, due to its high alkali content and poor cementitious activity, the usage of RM in cementbased materials is seriously limited. For example, the added dosage of RM in cement mortar is usually less than about 30% (Anirudh et al. 2020). Therefore, many researchers have developed modulation strategies of RM to enhance the cementitious activity and applicability in cement-based materials, in terms of modulating the chemical composition and mineral phases of RM.

3
The high alkali of RM is mainly attributed to the presence of insoluble sodium aluminosilicate phases, such as sodalite and cancrinite, which pose significant challenges to the utilization of RM in various applications. Luo et al. 2017;Wu et al. 2022). Apart from the alkali elements, sodium aluminosilicate phases account for a large proportion of silicon and aluminum elements in RM. It is worth noting that silicon and aluminum are the basic components of cementitious minerals, and superfluous alkali elements such as sodium and potassium is not advantageous for the hydration process. Thus, a suitable method to achieve chemical composition modulation is urgently needed, in order to enable the transfer of silica and aluminum elements from sodium aluminosilicate to the cementitious mineral phase. Calcification is an effective method for recovering alkali and alumina from RM while also retaining silicon elements to meet the requirements of chemical composition modulation. During the calcification process, sodium aluminosilicate phases are converted into alkali-free insoluble phases such as calcium hydrogarnet and sodium calcium silicate (Lyu et al. 2021). Among the two kinds of calcified products, hydrogarnet is the more appropriate one with no alkali elements, so the conversion of the sodium aluminosilicate phase to hydrogarnet is a favorable way for RM disposal in the cement and concrete field, the specific reactions in the calcification approach are as follows (Dilnesa et al. 2014;Lu et al. 2019): Besides the favorable chemical composition, the hydrogarnet derived from RM can also be transformed into cementitious minerals and obtain application potential. For example, in the calcification-carbonization strategy proposed by Lü et al., C 2 S is obtained by the carbonation process of calcified red mud (CRM), predominantly composed of silicate hydrogarnet (Lu et al. 2017;Lü et al. 2019).
Hematite is another significant component of CRM that lacks hydration capacity besides hydrogarnet. . To further increase the proportion of silicate and aluminate minerals and enhance the application potential of CRM in the cement and concrete field, the iron content in CRM should be further reduced. Considering the simplification of the process and the reduction (1) of energy consumption, it would be a very effective way to integrate the iron separation process into the fabrication of cementitious material from CRM (Wang et al. 2019a(Wang et al. , 2021b. Of the various iron separation routes, the reduction roasting process not only converts ironcontaining phases to magnetic ones for subsequent separation but also provides a thermal transformation opportunity for hydrogarnet, allowing the formation of silicate and aluminate minerals (Zinoveev et al. 2019;Wang et al. 2021b;Liu et al. 2021). From the study of Wang et al., iron recovery of 97.6% is achieved through the reduction roasting of CRM ). In addition, considering that thermal activation is an effective means to improve the reactivity of pozzolanic materials, cementitious mineral systems involved in C 4 AF and C 3 S can be produced by 1450 °C calcination of hydrogarnet-based RM (Wang et.al. 2019b) Based on the above literature research, the reduction roasting process of the CRM is bound to a promising strategy for the mineral phase modulation of RM, and boosting the utilization of RM as a cementitious material in the cement and concrete field. However, the thermal decomposition and activation characteristics of hydrogarnet and the formation regulation of mineral phases by thermal activation remain unclear, eliminating the development and application of reduction roasting treatment with hydrogarnet-based RM. In addition, there are few systematic studies on the fabrication of cementitious materials from RM-derived hydrogarnet, which mean that cannot guarantee the comprehensive utilization of hydrogarnet-based RM. Therefore, the phase transformation of RM-derived hydrogarnet and the relevant conversion path from CRM to cementitious material during the reduction roasting process needs to be investigated.
In this work, a strategy integrating calcification dealkalization and reduction roasting process (calcification-reduction roasting strategy) is proposed for the chemical composition and mineral phase modulation of RM facing in the field of cement and concrete. The possible realization of cementitious material converted from hydrogarnet-based RM via a reduction roasting process is also investigated. By analyzing TG-DSC, XRD, and Mossbauer spectra, the thermally induced phase transformation of RM-derived hydrogarnet and the relevant conversion path is illustrated. The comprehensive influences of CaO dosage and reduction roasting temperature on the phase composition of calcined calcified red mud (CCRM) are elaborated for designing the non-magnetic component to be the cementitious mineral system. Finally, the hydration properties of the non-magnetic component and the relevant Fe recovery rate are measured. This work can give a feasible approach for the utilization of RM in the cement and concrete field. 1 3 Experiment Material RM was obtained from an aluminum factory in Shandong Province. CRM was prepared by calcification treatment of RM. As depicted in Fig. 1, CRM is mainly composed of hydrogarnet Ca 3 AlFeSiO 4 (OH) 8 (C 3 (A,F)SH 4 ), CaCO 3 , and Fe 2 O 3 . The chemical compositions of RM and CRM are shown in Table 1. It can be found that, after the calcification treatment of RM, the alkali content of CRM was less than 0.5 wt.% with an alkali removal rate of 94%. The chemical composition of CRM can meet the requirement for cement-based materials (GB/T 21372-2008).

Experiment
The experimental procedure of the calcification-reduction roasting strategy is shown in Fig. 2. As is displayed, CRM was obtained by hydrothermal reaction of RM in the 40%NaOH 40α K hydrothermal solution at 200 °C for 2 h. Then, the prepared CRM was mixed with a certain dosage of CaCO 3 and placed in the tube furnace (NBD-O1200-50IT) for the reduction roasting process. The reduction roasting was carried out at the setting temperature (800~1000 °C) with a heating rate of 5 °C/min under a reduction atmosphere (H 2 :Ar 2 = 3:7). After holding for 2 h, the CCRM was cooled to room temperature with the cooling rate of 5 °C/min. The fabricated CCRM samples were named after the reduction roasting temperature; e.g., 800CCRM was the sample prepared with the temperature of 800 °C. Then, CCRM samples were ground for magnetic separation to separate the magnetic component (reduced iron powder, RIP). The three-step magnetic separation procedure was implemented by Davis magnetic separation tube (XCGS-Ф50) with a working magnetic field intensity of 0.2, 0.15, and 0.1T, respectively. After magnetic separation, the residue after extraction of iron components (REIC) was obtained, among which the one

Characterization analysis
The phase composition of CCRM samples was measured by an X-ray diffractometer (XRD, Bruker D8 Advance) with Cu (Kα) radiation. The testing rate was 5 °/min with a step size of 0.02 °. X-ray fluorescence (XRF, Bruker D8 Advance) analysis was performed to determine the chemical composition. The thermal properties of CCRM were tested by thermolysis (TG-DSC, Netzsch STA 449 F5). Mossbauer spectrometer (SEE Co W304) was conducted to investigate the iron phase compositions and content at room temperature with a 57Co/Rh radioactive source, using α-Fe° as a reference. Surface morphology and elemental mapping analysis of CCRM were analyzed by scanning electron microscope (SEM, ZEISS Gemini 300) equipped with an energy dispersive spectrometer (EDX, Smartedx). The magnetic measurements were implemented by a vibrating sample magnetometer (VSM, LakeShore7404), and the iron content was measured using inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent 720ES). The hydration heat evolution rate and cumulative hydration heat of REIC was measured by an eight-channel isothermal calorimeter (TAM air) within 72 h at the test temperature of 20 °C.

CRM phase transformation during the reduction roasting process
The thermal performance of CRM and the phase transformation of hydrogarnet C 3 (A,F)SH 4 were characterized through TG-DSC analysis. As illustrated in Fig. 3, the TG-DSC curve exhibited four distinct stages. The initial endothermic peak in the temperature range of 100~230 °C is mainly ascribed to the evaporation of free water and interlayer water in hydrogarnet. Upon raising the temperature from 230 to 400 °C, the dehydration of hydrogarnet leads to a remarkable weightless course. Till this period, C 3 (A,F) SH 4 is converted to C 3 (A,F)S 3 (Song et al. 2018). The mass loss occurs at 570~700 °C accounted for the decomposition of CaCO 3 (Chen et al. 2022). Subsequently, as the temperature increased from 700~1200 °C, the endothermic rate increased significantly, while the sample weight remains stable as presented by the DTG curve. This phenomenon is mainly related to the phase transformation of C 3 (A,F)S 3 , leading to the changing heat flow and stable weight of the sample (Klimesch and Ray 1999). The subsequent decrease in the endothermic rate may be induced by the deceleration in phase transformation of C 3 (A,F)S 3 or the occurrence of the exothermic reactions in the CRM. In the present work, the temperature range of the reduction roasting process was set to 800~1200 °C, aiming to investigate the phase transformation of CRM and design the mineral phase composition of cementitious material. The thermal-induced phase transformation of CRM during the reduction roasting process was analyzed via XRD patterns. As depicted in Fig. 4, two types of mineral phases coexisted in the CCRM, namely aluminum-siliceous minerals (C 2 S, C 2 AS, C 12 A 7 ) and iron-containing minerals (Fe, FeO, ferrites).
The correlation between phase transformation and roasting temperature is revealed in Fig. 4. The diffraction peaks belonging to C 2 S and C 12 A 7 of aluminum-siliceous minerals are observed to increase with the rise of reduction roasting temperature, indicating that high temperature is favorable for the formation of cementing minerals in the system. The reason accounted for this phenomenon is that high temperature promotes the decomposition degree of C 3 (A,F) S 3 and improves the reactivity of the newly-released SiO 2 and Al 2 O 3 . Moreover, the high temperature also leads to an increased amount of C 2 AS. This is mainly due to insufficient CaO content in CRM. With the increase of the decomposition degree of C 3 (A,F)S 3 , CaO is gradually insufficient to combine with the newborn SiO 2 and Al 2 O 3 to form C 2 S and C 12 A 7 respectively, thus giving rise to C 2 AS which has no cementitious activity. Therefore, it is essential to adjust the chemical composition of raw meals in the reduction roasting process to ensure the formation of the cementitious mineral. Regarding iron-containing minerals, Fe and FeO are formed by the reduction of Fe 2 O 3 and C 3 (A,F)S 3 . With the increase of reduction roasting temperature, the contents of Fe and FeO show an upward trend and a downward trend respectively, illustrating the rising reduction roasting temperatures are favorable for iron reduction. Because the characteristic peaks of C 2 F and C 3 (A,F)S 3 are almost overlapped, the existence of C 2 F in CCRM is ambiguous, which hinders the illustration of the mineral transformation mechanism. Based on the aforementioned analytical findings, the phase transformation of CRM during the reduction roasting process is depicted below. (3) To further identify the iron-containing minerals in CCRM, the Mossbauer spectra were measured. As shown in Fig. 5, the test results of 800CCRM and 1100CCRM can be fitted by one sextet pattern and two paramagnetic doublet patterns, which indicates CCRM samples possess both magnetically ordered and paramagnetic characters whether with high and low reduction roasting temperature. The relevant hyperfine parameters including isomer shift (IS), quadrupole splitting (QS), line width (γ), and magnetic field (H) are presented in Table 2. For both CCRM samples, the sextet pattern parameters are following those of pure α-Fe, in agreement with the literature (Peters et al. 2020). In addition, the occupied area of the sextet pattern significantly increases Fig. 4 a, b Effect of roast temperature on products of CRM reduction roasting (M-C 12 A 7 , W-FeO, I-Fe, G-C 2 AS, L-C 2 S) with the increase of reduction roasting temperature, indicating that elevated temperature promotes iron reduction. This result is consistent with the XRD analysis. Two additional paramagnetic doublet spectra are associated with Fe (III) and FeO (Peters et al. 2020), respectively. Ferrites, such as C 3 (A,F)S 3 , C 2 F, and C 4 AF, are among the possible Fe (III)-containing minerals in the CCRM. The IS values of 800CCRM and 1100CCRM are 0.39 and 0.36mm/s respectively, which indicates the Fe 3+ ions predominantly exist in the octahedral form (Neupane et al. 2022;Hu et al. 2019). As shown in the crystal structure diagram (Fig. 6), the crystal structure of garnet contains octal-coordinated Fe (III), while C 2 F and C 4 AF have both octal-and tetra-coordinated Fe 3+ ions. Therefore, the Fe (III)-contained phase in CCRM is C 3 (A,F)S 3 . The QS and γ values of the two samples are higher than those in the reports which may be related to the partial substitution of Al 3+ ions in Fe 3+ sites (Labhasetwar et al. 1991). According to the analysis of Mossbauer spectra, the iron element in CCRM exists in free iron phases (Fe and FeO) and the residual C 3 (A,F)SH 4 , regardless of whether low or high reduction roasting temperature was employed. And a conclusion can be drawn that the iron element in hydrogarnet can be directly transformed into the free iron phases without the formation of intermediate products (such as ferrites C 2 F, C 4 AF, etc.). It lays a good theoretical foundation for defining the thermal transformation pathway of CRM, which is described in Fig. 7.
By analyzing the mineral composition and phase transformation of CCRM during the reduction roasting process, it can be found that CaO is preferentially combined with  Fig. 7 The path of thermal decomposition of CRM the active SiO 2 and Al 2 O 3 to form cementitious minerals, and iron element in hydrogarnet and hematite preferentially converts to free iron phases. The results confirm the feasibility of the calcification-reduction roasting strategy, in which the formation of cementitious material can be driven by the phase transformation of hydrogarnet in CRM. To further decrease the content of useless phases C 2 AS and FeO in CCRM and improve the relevant cementitious activity, the reduction roasting parameters are needed to be optimized.

Effects of reduction roasting parameters on CCRM
In order to inhibit the formation of C 2 AS at high temperatures, CaO was added in the form of CaCO 3 with dosages of 2 wt%, 5 wt%, and 10 wt% of CRM, to adjust the chemical composition of raw meals. The reduction roasting temperature range of 800~1200°C was investigated to study the phase composition of CCRM samples with different CaO contents. Based on the quantitative analysis of XRD data by the Rietveld method, the mineral phase distribution of CCRM samples is shown in Fig. 8. Minor phases such as CaTiO 3 , C 3 (A,F)S 3 , and CaO are represented as "other phases" in the figure. As shown in Fig. 8 (a), the mineral phases in 800CCRM are mainly C 12 A 7 , C 2 S, Fe, and FeO. And the mineral phase distribution is essentially unchanged with the increase of CaO addition, except for the increase of CaO (see Fig. A1 in Appendix A). This is due to the low activity of the solid phase reactions at this temperature, and the introduced CaO hardly reacts with CRM during the reduction roasting process. At a temperature of 900 °C, the introduced CaO actively participates in solid phase reactions, as evidenced by the decrease in the area of other phases in Fig. 8(b). This results in an increase of C 2 F content through the consumption of free iron phases. These findings suggest that CaO plays a significant role in regulating the mineral phase distribution of CRM through its ability to modify solid phase reactions. When the temperature rises to 1000 °C (Fig. 8(c)), the additional CaO has a significant contribution to the formation of C 2 S and C 12 A 7 , thereby the content of C 2 AS is gradually decreased with adding CaO. In the meantime, C 4 AF begins to appear, and the content is also increased by adding CaO dosage. The decrease of Fe and FeO contents is associated with the formation of C 4 AF. The amount of C 12 A 7 remains unchanged, which is mainly affected by the comprehensive variation of C 4 AF and C 2 AS. Under the reduction roasting temperature of 1100 °C (Fig. 8 (d)), the content of C 2 AS is significantly decreased as adding of CaO and almost disappears when the CaO addition reaches 5 wt.% (see Fig. A4 in Appendix A). This phenomenon illustrates the mineral phase distribution of CCRM can be effectively modulated by adjusting the chemical composition of raw meals. In addition, C 4 AF content is sharply increased with the increase of CaO dosage, resulting in the obvious decrease of Fe, FeO, and C 12 A 7 , respectively. It can be inferred that the reaction kinetics of ferrite formation is faster than that of iron reduction in this case. However, C 4 AF is absent when the CaO dosage is low (e.g., 0wt.%), implying that CaO is preferentially combined with SiO 2 and Al 2 O 3 rather than iron-containing compounds, even though the activity of ferrite formation is high. This result is consistent with the analysis of Mossbauer spectra described above. When the temperature further rises to 1200 °C (Fig. 8 (e)), a large amount of C 2 S and C 12 A 7 can be formed, which is benefited from the high rate of solid phase reactions. C 4 AF content stabilizes at a low level, suggesting the reaction kinetics of iron reduction is faster than that of ferrite formation in this condition. Unfortunately, owing to the lack of CaO content, C 2 AS is retained in all 1200CCRM samples.
According to the discussion about the influences of reduction roasting parameters on the phase composition of CCRM, the optimal reduction roasting temperature is 1100 °C, and a 5 wt % dosage of CaO is suitable for the formation of cementitious minerals. In that condition, the reduction roasting product (1100CCRM-5) is mainly composed of C 2 S, C 12 A 7 , C 4 AF, and Fe. Thus, the cementitious mineral including C 2 S, C 12 A 7 , and C 4 AF would be obtained after the separation of the magnetic Fe phase.

Morphology and elemental distribution of CCRM
The morphology and elemental distribution of the CCRM are evaluated by SEM-EDS analysis. Represented by 1100CCRM-5, the reduction roasting product appears to be irregular shape particles with a slight aggregation as shown in SEM micrographs ( Fig. 9 (a), (b), (c), on the left side). The particle size distribution is statistically estimated in the range of 1~15 μm, demonstrating a relative uniformity of crystal growth during the reduction roasting process. In addition, elemental mappings explicit the existence of Ca, Al, Si, Fe, and O in the CCRM sample ( Fig. 9 (a), (b), (c), on the right side). As presented, the regional concentrated distribution of Fe implies that the magnetic Fe phase is formed independently during the reduction roasting product and is less adhesion to the cementitious mineral particles. The desirable elemental distribution of the sample is beneficial for the subsequent magnetic separation process.

Cementitious performance of REIC
After magnetic separation, the REIC of 1100CCRM-5 can be considered cementitious material. The hydration heat analysis is implemented to study the cementitious properties. As shown in the hydration heat curve (Fig. 10 (a), (b)), the hydration process of the cementitious material can be divided into six stages: 1 3 1. Initial reaction: C 12 A 7 , C 4 AF, and C 2 S dissolve quickly after wetting and simultaneously release a large amount of heat. The sharp exothermic peak detected in the first few minutes is attributed to the dissolution and instantaneous reaction of the mineral phases (Sun et al. 2022). 2. Induction period: After the initial reaction period, the initial hydration products may cover the particles and prevent the hydration process. Thus, the exothermic rate decreases rapidly due to the low heat production of the cementitious material (Sun et al. 2022). 3. Acceleration period: As the hydrates coating layer is destroyed, the hydration reactions of C 12 A 7 and C 4 AF are accelerated. The exothermic peak is observed at approximately 0.6 h, which is earlier than the single C 12 A 7 or C 4 AF phase. This is mainly caused by the synergistic effect that promotes the hydration reactions of each phase (Raab and Poellmann, 2011;Zhang et al. 2021a). 4. Deceleration period: There is a steady decrease presented in the hydration heat curve, which is related to the hydration reactions of mineral phases controlled by a diffusion process at this age (Bullard et al. 2011). 5. Phase transformation: The second exothermic peak is located at 4 h. The enhanced heat flow is mainly caused by the conversion of C 12 A 7 hydration products from the layer phase to the lamellar phase (Raab and Poellmann, 2011). 6. Final period: With the end of phase transformation, the evolution of the hydration heat returns to a downtrend and the cumulative heat tends to be constant (230.6 J/g). In addition, the hydration of C 2 S is mainly taken place at later ages.
According to the hydration activity analysis, REIC possesses both early and late hydration activity that can be considered cementitious materials. Moreover, after 60 °C hydration with the w/c of 5 for 3 days, the relevant hydration products are crystalline C 3 AH 6 and C 3 (A,F)H 6 as depicted in XRD patterns (Fig. 10 (c)), and also the amorphous AH 3 and FH 3 gel. Owing to the short hydration time, CH and CSH gel formed by C 2 S hydration are not been detected. The results further indicate the cementitious material has both early and potential late strength.

Magnetic separation effect of CCRM
For determining magnetic property and the relevant application potential of the extracted Fe phase, the RIP of 1100CCRM-5 was characterized by VSM and ICP analyses. In the magnetic hysteresis loops (Fig. 11), the saturation magnetization intensity (Ms) of RIP is 53 emu/g, which is significantly higher than REIC and CCRM. Across the reduction roasting-magnetic separation process, the magnetic and non-magnetic minerals in the CCRM can be effectively separated. Moreover, the Fe recovery can reach 75% with a high iron content of 46.75% as determined by ICP analysis. Therefore, the obtained RIP has the potential for utilization in various applications.

Environmental and economic impact analysis
We propose a strategy for the utilization of red mud, whereby the excess alkali components present in the raw material are effectively removed via hydrothermal reaction, while simultaneously modifying its phase structure. Subsequently, a reduction roasting process is employed to facilitate the synchronous recovery of Fe and the formation of cementitious minerals. The research route of the calcification-reduction roasting strategy offers significant environmental and economic benefits.
One of the main advantages is that it could reduce the amount of greenhouse gas emissions associated with traditional cementitious materials production processes (e.g., Portland cement). By employment at such a low temperature (1100 °C) of the reduction roasting process, the proposed route can reduce CO 2 emissions compared to conventional methods. In addition, the proposed route can utilize RM, a waste materials residue. By repurposing these waste materials, the route reduces the amount of waste generated and provides a new source of revenue for alumina producers. Furthermore, the proposed approach is cost-effective as it enables the simultaneous recovery of valuable metals and preparation of cementitious materials through a single roasting step, thereby saving energy, equipment, and process steps. This approach can be scaled up for industrial applications, potentially reducing the environmental impact of red mud and contributing to the sustainable development of the industry. Overall, the strategy proposed in this work provides a sustainable and economically feasible new process for the production of cementitious materials. It has the potential to significantly reduce greenhouse gas emissions, promote waste utilization, and lower production costs, making it a promising solution for cementitious materials.

Conclusions
This study proposes a calcification-reduction roasting strategy to convert RM into a cementitious material with early and late hydration properties. The thermal decomposition of hydrogarnet and mineral phase formation in CCRM are elaborated upon to fabricate the cementitious material. The following conclusions are drawn: (1) The C 3 (A,F)SH 4 of CRM decomposes significantly at temperatures above 800°C to form aluminum and silicon compounds and the free iron phases. The decomposition degree increases with a reduction in roasting temperature.
(2) Optimized reduction roasting conditions are determined to be the reduction roasting temperature of 1100 °C, the reaction time of 120 min, and CaO dosing of 5%. Under optimal conditions, 90% of the RM can be directly applied. The cementitious material is composed of C 2 S, C 12 A 7 , and C 4 AF, with the rest being recoverable magnetic Fe. (3) According to the hydration activity analysis, the fabricated cementitious material possesses both early and late hydration activity. Besides, the iron recovery in RIP can reach 75%.
The proposed method is cost-effective, saving energy, related equipment, and process steps. It has the potential to significantly reduce greenhouse gas emissions, promote waste reuse, and reduce production costs, making it a promising solution in the cementitious material industry.