Effect of reductant on property and CO 2 sequestration for belite cement from phosphogypsum

Effect of reductant on property and CO 2 sequestration for belite cement from phosphogypsum | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effect of reductant on property and CO 2 sequestration for belite cement from phosphogypsum Kaiwen Li, Changrong Liu, Xiaoling Ma, Hongbin Tan, Yassine Taha, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4275746/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Gypsum can be decomposed at low-temperature by adding reductant, which is beneficial to obtain cement. The effects of different reductant on the phases of belite cement raw meals were studied by calcining at 1300℃ for 2h and using phosphogypsum as raw materials. Diffraction peaks of calcium sulfate were not observed in the samples when 10 wt% activated carbon was added and the molar ratio of CaS to CaSO 4 was 3, respectively. The Sulfur trioxide contents of in the clinkers were 0.80 and 0.64 wt%, respectively. The effect of carbonation curing on the cement properties was also studied. Carbonation curing can promote cement hydration and increase strength at 20 ℃, 75 % relative humidity and 20% CO 2 concentration. As the carbonation curing age increased, the compressive strengths of the samples gradually enhanced. When 10 wt% carbon was employed as the reducing agent, compressive strengths of the samples were 15.5, 15.6, and 15.6 MPa after carbonation curing at 3, 7 and 28 d, respectively. When CaS was employed as the reducing agent, the compressive strengths of the samples were 19.8, 27.2, and 34.1 MPa after carbonation curing for 3, 7 and 28 d, respectively. The carbon dioxide sequestration contents of the samples prepared with a 10 wt% carbon and CaS reductant, were 11.6% and 9.2% after carbonation curing for 28 d, respectively. These findings demonstrate the potential to use phosphogypsum with various reductants to enhance the quality of belite cement and at the same time consume more CO 2 in the atmosphere during curing process. phosphogypsum reductant belite cement CO2 sequestration Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction Phosphogypsum (PG) is a by-product of wet-process phosphoric acid industry, which gypsum (CaSO 4 ·2H 2 O) is the major component. About 4.5-5 kg of PG is generated for every kg of P 2 O 5 produced. Almost 75 million tons of PG is generated annually in China and its output is estimated to be around 415 million tons worldwide per year [ 1 , 2 ]. Currently, PG is applied in fields, as soil-stabilization amendments, agricultural fertilizers, cement retarders, building bricks/blocks and cementitious binders, etc. However, the reuse proportion of PG is lower than 10%, while the vast majority of PG is dumped in large stockpiles, which are exposed to weathering processes without any treatment [ 3 ]. PG contains metals, organic substances and other potentially toxic elements, which have potential environmental impacts. Therefore, the effective utilization of PG cannot only save the natural gypsum, but protect the environment. Portland cement, the glue of concrete, is the largest manufactured product by human society and the basic ingredient for the construction industry. The CaO in Portland cement is about 65%, which mainly comes from limestone and produces CO 2 in the cement manufacturing processes [ 4 ]. The co-production process of sulfuric acid and cement not only utilizes calcium resources from PG, does not discharge solid waste and cuts in carbon emissions, but also produces sulfuric acid for phosphoric acid industry [ 5 ]. The process research has gradually become a hot spot in recent years. Portland cement mainly contains alite and belite minerals. The formation enthalpy of alite mineral is 1810 kJ/kg, and the formation temperature is as high as 1450℃, while the formation enthalpy of belite mineral is only 1350 kJ/kg, which can be formed at 1300℃. Belite cement mainly contain belite mineral, which can reduce energy consumption than Portland cement [ 6 , 7 ]. However, belite mineral has slow hydration rate and low early strength. On the other hand, CO 2 can enhance the hydration activity of belite mineral and improve its early strength, which can sequestrate greenhouse gases CO 2 in cement hydration products. Gypsum initial decomposition temperature reaches 1662℃ by thermodynamic calculation, but the decomposition temperature decreases significantly in the presence of reducing agents, such as coke, sulfur, hydrogen or carbon monoxide, which the decomposition temperatures are 849, 1054, 909 and 925 ℃, respectively. Gypsum can be decomposed at low-temperature by adding reductant, which is beneficial to obtain cement. In this work, the effects of different reductant (CaS and carbon) on the phases of belite cement raw meals were studied by calcining at 1300℃ for 2h and using phosphogypsum as raw materials. Moreover, the effect of carbonation curing on the cement properties was also studied. 2. Materials and methods PG came from Lomon Co. Ltd. in Mianzhu, China. The chemical composition is shown in Table 1 . Alumina (analytical reagent) was provided by Tianjin Comio Chemical Co. Ltd., China. Iron oxide (analytical reagent) was provided by Tianjin Fengchuan Chemical Reagent Co. Ltd., China. Activated carbon powder (analytical reagent) was provided by Tianjin Beichen Chemical Reagent Co. Ltd., China. Silica (analytical reagent) and calcium carbonate (analytical reagent) were provided by Chengdu Kelong Chemical Reagent Co. Ltd., China. Table 1 Chemical Composition of PG (wt%) CaO SO 3 SiO 2 MgO Al 2 O 3 Fe 2 O 3 P 2 O 5 Others 42.56 52.67 2.79 0.02 0.33 0.03 1.17 0.43 The mineral composition of belite cement clinker was designed, which alite mineral did not presence in clinker, as in reference [ 7 ]. The limestone saturation factor (KH) was 0.67, which tricalcium silicate was absent in theory. The values of silica ratio (SM) and alumina ratio (IM) were 2.50 and 0.93, respectively. The theoretical mineral composition of the cement consisted of 77.40 wt% dicalcium silicate (C 2 S), 4.17 wt% tricalcium aluminate (C 3 A) and 16.33 wt% tetracalcium iron aluminate (C 4 AF). The chemical compositions of designed cement clinker were 60.5 wt% calcium oxide (CaO), 27.0 wt% silicon oxide (SiO 2 ), 5.4 wt% iron oxide (Fe 2 O 3 ), 5.0 wt% aluminum oxide (Al 2 O 3 ), and 2.1 wt % others impurities, which impurities came from PG. The granular clinker was not obtained only by using PG as calcium source, which the clinker sample was completely melted on ceramic tray after calcination. According to references [ 8 ], the belite cement can be prepared with 1/3 of the CaO from limestone (calcium carbonate) and 2/3 of the CaO from PG. According to designed chemical composition of belite cement, PG, iron oxide, silica, alumina, calcium carbonate and reductant were mixed by mill to obtain cement raw meal. Moreover, the CaS reductant was prepared from PG, according to references [ 1 ]. The prepared cement raw meal were mixed with a 2 wt% polyethylene glycol and pressed into Φ50mm×8 mm round cake molds. The pressed cakes were put into an oven at 110 ℃ for 2 h to drying treatment. After drying, the samples were put into a high-temperature furnace and held at 1300 ℃ for 2 h, with a heating rate of 5℃/min. After the calcined process was finished, the cement clinker was immediately removed from the high-temperature furnace and rapidly cooled by air blowing. The cement clinker was ground finely by mill to obtain belite cement. Water was added in the cement, whith a water-to-gypsum ratio was fixed at 0.30 for sample. Finally, the homogeneous slurry was poured into a mold (20×20×20 mm) and shaped through vibrations. After 24 h of hardening time, the molds were removed, and then, the samples were cured at a constant temperature of 20℃in relative humidity (RH) of 75% for different days, with/without CO 2 concentration of 20%. The chemical composition of the raw materials was measured by an X-ray fluorescence spectrometer (Axios-Poly, PANalytical, Netherlands). The morphology was observed by scanning electron microscopy (TM-1000, Hitachi, Japan). The phase analysis was observed by using an X-ray powder diffractometer (Smartlab, Rigaku, Japan), equipped with Cu-K α radiation (λ = 0.15406 nm). The mechanical properties of samples were tested by microcomputer-controlled electronic universal testing machine (TSE255D, Universal Testing Machine). 3. Results and discussion Figure 1 shows the XRD patterns of cement raw meal calcined at 1300 ℃ for 2 h, with different reducing agents. The diffraction peaks of CaSO 4 decreased with reductant increase. As the ratio of the added amount of carbon was 10 wt% (Fig. 1 (a)) and n(CaS)/n(CaSO 4 ) was 3 (Fig. 1 (b)), the diffraction peaks of CaSO 4 were not observed, respectively, which the main phases of samples were β-C 2 S and α' H -C 2 S. However, the sample contained a large amount of CaSO 4 with a 6.7 wt% carbon. According to theoretical calculation, the amount of reduction potence provided by carbon was similar by calcium sulfide, which it was possible that calcium sulfide was more suitable as reductant than carbon. In general, the decomposition temperature of gypsum can be effectively reduced and industrial production can be carried out in a more reasonable way. The CaS reductant was prepared at 900℃ for 1h by using activated carbon as a reductant. According to carbon amount of prepared CaS, the C/S ratio of cement prepared with CaS reductant was about 1.5. The C/S ratio of cement prepared with a 6.7 wt% carbon was about 1.6. The C/S ratio of cement prepared with a 10wt% carbon was about 3. More carbon was needed to prepare cement by directly using carbon as a reductant, probably because carbon easily reacted with air, which it increased carbon consumption. The primary phase of belite cement is dicalcium silicate. The main chemical reactions of produced belite (2CaO·SiO 2 ) can be simplified in the following equations: 2C + CaSO = CaS + 2CO(g) (1) CaS + 3CaSO 4 = 4CaO + 4SO 2 (g) (2) CaS + 3/2O 2 (g) = CaO + SO 2 (g) (3) 2CaO + SiO = 2CaO·SiO (4) The specific surface areas of cement prepared by adding, 6.7 and 10 wt% carbon, and CaS ( n (CaS)/ n (CaSO 4 ) = 3) were 0.580, 0.554, and 0.625 m 2 /g, respectively, which cement was obtained by grinding clinker at 720 rpm for 40 min using a vibration mill. The cement prepared with CaS had best grindability. According to Chinese standard GB/T176-2017 (Methods for chemical analysis of cement), free lime was not detected in the samples, which the content of free lime was below the detectable level. Figure 2 shows SO 3 content and Loss of cement clinker obtained with different reductant. Sulfur trioxide (SO 3 ) content of cement prepared by adding 6.7 and 10wt% carbon, and CaS ( n (CaS)/ n (CaSO 4 ) = 3), were 26.28, 0.80, and 0.64 wt%, respectively. And their Losses were 0.68, 1.34 and 1.10 wt%, respectively. SO 3 and Loss were all in accordance with Chinese standards GB/T175-2023 (Common portland cement, SO 3 ≤ 3.5 wt% and Loss ≤ 3.0 wt% ) for cement obtained by calcination with 10 wt% and carbon CaS ( n (CaS)/ n (CaSO 4 ) = 3). Figure 3 shows the compressive strength of hydrated cement obtained with different reductant by curing for different age. The compressive strength increased with curing age increase. The strength increased rapidly before 28 d and the growth slowed down after 28 d. When a 10 wt% carbon was employed as the reducing agent, compressive strengths of the samples were 15.5, 15.6, and 15.6 MPa after carbonation curing 3, 7 and 28 d, respectively. When CaS was employed as the reducing agent, compressive strengths of the samples were 19.8, 27.2, and 34.1 MPa after carbonation curing 3, 7 and 28 d, respectively. The strengths of samples cured by carbonized condition were much higher than samples cured by standard condition. The sample had high strength by using CaS as reductant, which strength were 2.4 and 34.1 MPa after curing 28 d without and with carbonation, respectively. And the strengths were 3.8 and 39.4 MPa after curing 90 d without and with carbonation, respectively. However, the samples had similar strength by adding 6.7 and 10 wt% carbon as reductant, which strength were 13.2 and 15.6 MPa after curing 28 d in carbonized condition, respectively. Figure 4 shows the XRD patterns of hydrated cement for different curing ages. Belite (β-C 2 S and α’ H -C 2 S) diffraction peaks were observed in all samples but the diffraction peaks intensity of carbonized samples (Fig. 5 (a 2 ), (b 2 ) and (c 2 )) were weaker than standard samples (Fig. 5 (a 1 ), (b 1 ) and (c 1 )), which CO 2 can enhance the hydration activity of belite mineral. The strong diffraction peaks of calcium carbonate were observed in carbonized samples and the peaks intensity increased with age increasing. On the other hand, the diffraction peaks of calcium carbonate were also observed in standard samples after curing 28 d, because of air containing CO 2 in standard curing room. The diffraction peaks of ettringite (AFt) were observed in standard samples (Fig. 5 (a 1 ), (b 1 ) and (c 1 )) but it were not observed in carbonized samples (Fig. 5 (a 2 ), (b 2 ) and (c 2 )), probably because ettringite did not generate in carbonation environment. The main chemical reactions of belite and CO 2 can be simplified in the following Eq. (5), though the actual reaction was complexity [ 8 ]: Ca 2 SiO 4 + 2CO 2 = 2CaCO 3 + SiO 2 (gel) (5) The early strength of belite cement was enhanced by the calcium carbonate generated during carbonation curing and the C-S-H gel, thus improving its micro-mechanical properties stability in cement hydration environment [ 9 ]. Moreover, the diffraction peaks of SiO 2 were observed in samples prepared with CaS reductant after curing 56 d (Fig. 5 (c 2 )). The strong diffraction peaks of gypsum (CaSO 4 ·2H 2 O) were observed in samples prepared a 6.7wt% carbon reductant (Fig. 5 (a 2 )) because the residual anhydrite had some hydration activity in carbonation curing condition. Figure 5 shows the microstructure of hydrated cement for 28 d under different curing conditions, respectively. The cement prepared with CaS reductant exhibited higher density because it had high hydration and carbonation activity (Fig. 5 (c 1 ) and (c 2 )). Belite cement produced a little of needle-shaped ettringite particles during hydration process (Fig. 5 (a 1 ), (b 1 ) and (c 1 )). The ettringites overlap and were intimately associated with the surrounding calcium silicate hydrate gel, thereby enhancing the strength of the hydrated cement [10] . A lot of CaCO 3 fibers were observed in carbonized curing samples (Fig. 5 (a 2 ), (b 2 ) and (c 2 )), which fibers and hydrated calcium silicate filled the loose porous structure and increased samples strength. Some short fibers were observed in the cement prepared by using a 6.7 wt% carbon as reductant (Fig. 5 (a 2 )), probably because residual CaSO 4 hindered the fibers growth. The strength of hydrated cement prepared with CaS reductant was the highest. The production of CaCO 3 fibers and hydrated calcium silicate resulted in a filling of the loose porous structure within the overall system, leading to a more compact structure and an increase in the strength of the samples [ 11 ]. Figure 6 shows the carbon sequestration amount of cements prepared with different reductant after curing 28d. The carbon sequestration amount of cement prepared by adding 6.7 and 10wt% carbon, CaS ( n (CaS)/ n (CaSO 4 ) = 3), were 7.5, 9.2, and 11.6 wt%, respectively. The cement prepared with CaS reductant had high hydration activity, as a result it had high carbonation activity. 4. Conclusion The effect of different reductant on the property and CO 2 sequestration of belite cement were studied by using PG as raw material. (1) The belite cement can be prepared by calcining at 1300 ℃ for 2 h, with CaS ( n (CaS)/ n (CaSO 4 )=3) and 10 wt% carbon reductant, respectively. The main phases were β-C 2 S and α' H -C 2 S and free lime was not detected in the samples. (2) The strength of belite cement can be significantly increased by carbonation curing, and the strength increased rapidly before 28 d and the growth slowed down after 28 d. The sample had high strength by using CaS as reductant, which strength were 2.4 and 34.1 MPa after curing 28 d without and with carbonation , respectively. (3) Some CaCO 3 fibers were observed in carbonized curing samples. The cement prepared with CaS reductant had high carbon sequestration ability (11.6 wt%) and compact microstructure. Declarations Author Contribution K.L. wrote the main manuscript text and prepared figures.All authors reviewed the manuscript. Acknowledgements This work was supported by the Research Foundation of the Sichuan Science and Technology Program of China (2024YFS0334) and State Key Laboratory of Solid Waste Reuse for Building Materials (SWR-2023-010). References Fang R, Tan H, Mao W, et al. Influence of carbon and additive on high-temperature decomposition behavior of phosphogypsum[J]. Materials and technology, 2020, 54(6) : 115-119 Taha Y, Elghali A, Hakkou R, et al. Towards Zero Solid Waste in the Sedimentary Phosphate Industry: Challenges and Opportunities [J]. Minerals, 2021, 11: 1250 Tan Hongbin, Zheng Aiguo, Kang Xiangmei, et al. Synthesis of a-hemihydrate gypsum from phosphogypsum and influence of different additives on mechanical performance[J]. Materials and technology, 2020, 54(5) : 697-703 Costa F N, Ribeiro D V.Reduction in CO 2 emissions during production of cement, with partial replacement of traditional raw materials by civil construction waste (CCW)[J]. Journal of Cleaner Production, 2020, 276: 123302 Ige O E, Olanrewaju O A, Duffy K J, et al. A review of the effectiveness of Life Cycle Assessment for gauging environmental impacts from cement production[J]. Journal of Cleaner Production, 2021, 324: 129213 Liu S, Wei L, Zhou S, et al. Research Development of high-strength low-calcium Portland cement[J]. Bulletin of the Chinese Ceramic Society, 2014, 33(3): 553-557 Chang J, Jiang T, Cui K. Influence on compressive strength and CO 2 capture after accelerated carbonation of combination β-C 2 S with γ-C 2 S[J]. Construction and Building Materials, 2021, 312: 125359 Guan X, Qiu M, Li H, et al. Preparation of self-pulverized low calcium cement and its carbonation-hardening properties[J]. Journal of Building Materials, 2018, 21(05): 775-779+785. Gong P, Zhang C, Wu Z, et al. Study on the effect of CaCO 3 whiskers on carbonized self-healing cracks of cement paste: Application in CCUS cementing[J]. Construction and Building Materials, 2022, 321: 126368-. Chen X, Zhang J, Lu M, et al. Study on the effect of calcium and sulfur content on the properties of fly ash based geopolymer[J]. Construction and Building Materials, 2022, 314(3): 125650. Lan Y, Gu Q, Peng Y, et al. Microstructure of early-age calcium sulphoaluminate and ordinary Portland cement paste cured under different CO 2 pressures[J]. Journal of Zhejiang University (Engineering Science), 2022, 56(12): 2454-2462. Additional Declarations No competing interests reported. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4275746","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":292105260,"identity":"8286fc79-1fad-4dfd-9817-1770b2fa7d6e","order_by":0,"name":"Kaiwen Li","email":"","orcid":"","institution":"Southwest University of Science and Technology，Mianyang","correspondingAuthor":false,"prefix":"","firstName":"Kaiwen","middleName":"","lastName":"Li","suffix":""},{"id":292105261,"identity":"6d2997b3-f6f7-456e-aca6-5ef19f2cda0b","order_by":1,"name":"Changrong Liu","email":"","orcid":"","institution":"Southwest University of Science and 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2","display":"","copyAsset":false,"role":"figure","size":17847,"visible":true,"origin":"","legend":"\u003cp\u003e(a) SO\u003csub\u003e3 \u003c/sub\u003econtent and (b) Loss of cement clinker obtained with different reductant\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4275746/v1/346f616cea6d015bff98f607.png"},{"id":55085360,"identity":"e5de57b2-980f-4198-b77d-bda989cd07ee","added_by":"auto","created_at":"2024-04-22 11:05:01","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":38636,"visible":true,"origin":"","legend":"\u003cp\u003eCompressive strength of hydrated cement obtained with (a) 6.7 and (b) 10 wt% carbon, (c) CaS reductant by curing for different age\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4275746/v1/e77087c67ac8bbdf43f8e6b6.png"},{"id":55085365,"identity":"ed785243-f906-44bd-9fcd-28a8a5f6c33d","added_by":"auto","created_at":"2024-04-22 11:05:01","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":64311,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of hydrated cements at different curing ages\u003c/p\u003e\n\u003cp\u003e(a) hydrated cement prepared with a 6.7 wt% carbon reductant in (a\u003csub\u003e1\u003c/sub\u003e) standard and (a\u003csub\u003e2\u003c/sub\u003e) carbonized curing condition; (b) hydrated cement prepared with a 10 wt% carbon reductant in (b\u003csub\u003e1\u003c/sub\u003e) standard and (b\u003csub\u003e2\u003c/sub\u003e) carbonized curing condition; (c) hydrated cement prepared with a CaS reductant in (c\u003csub\u003e1\u003c/sub\u003e) standard and (c\u003csub\u003e2\u003c/sub\u003e) carbonized curing condition\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4275746/v1/e8ce4bb28555f36c9740e198.png"},{"id":55085366,"identity":"0c0b8503-8785-46d0-8790-9e4e4fa9e6aa","added_by":"auto","created_at":"2024-04-22 11:05:01","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":994620,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructure of cements hydrated under different curing condition for 28 d\u003c/p\u003e\n\u003cp\u003e(a) hydrated cement prepared with a 6.7 wt% carbon reductant in (a\u003csub\u003e1\u003c/sub\u003e) standard and (a\u003csub\u003e2\u003c/sub\u003e) carbonized curing condition; (b) hydrated cement prepared with a 10 wt% carbon reductant in (b\u003csub\u003e1\u003c/sub\u003e) standard and (b\u003csub\u003e2\u003c/sub\u003e) carbonized curing condition; (c) hydrated cement prepared with a CaS reductant in (c\u003csub\u003e1\u003c/sub\u003e) standard and (c\u003csub\u003e2\u003c/sub\u003e) carbonized curing condition\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4275746/v1/03e65984c90eca03cb8c197d.png"},{"id":55085364,"identity":"e58b2098-5882-4aac-9c6a-66203ff153eb","added_by":"auto","created_at":"2024-04-22 11:05:01","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":12129,"visible":true,"origin":"","legend":"\u003cp\u003eCarbon sequestration rate of cement samples prepared with different reductant\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4275746/v1/73642727f1e57ba7e141a959.png"},{"id":55086158,"identity":"31a98d3f-6514-4166-a1b5-23d4c84e84ba","added_by":"auto","created_at":"2024-04-22 11:21:01","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1326331,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4275746/v1/8a57abc0-13f7-4e98-a812-337e70fb84e1.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effect of reductant on property and CO 2 sequestration for belite cement from phosphogypsum","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003ePhosphogypsum (PG) is a by-product of wet-process phosphoric acid industry, which gypsum (CaSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO) is the major component. About 4.5-5 kg of PG is generated for every kg of P\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e produced. Almost 75\u0026nbsp;million tons of PG is generated annually in China and its output is estimated to be around 415\u0026nbsp;million tons worldwide per year [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Currently, PG is applied in fields, as soil-stabilization amendments, agricultural fertilizers, cement retarders, building bricks/blocks and cementitious binders, etc. However, the reuse proportion of PG is lower than 10%, while the vast majority of PG is dumped in large stockpiles, which are exposed to weathering processes without any treatment [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. PG contains metals, organic substances and other potentially toxic elements, which have potential environmental impacts. Therefore, the effective utilization of PG cannot only save the natural gypsum, but protect the environment.\u003c/p\u003e \u003cp\u003ePortland cement, the glue of concrete, is the largest manufactured product by human society and the basic ingredient for the construction industry. The CaO in Portland cement is about 65%, which mainly comes from limestone and produces CO\u003csub\u003e2\u003c/sub\u003e in the cement manufacturing processes [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. The co-production process of sulfuric acid and cement not only utilizes calcium resources from PG, does not discharge solid waste and cuts in carbon emissions, but also produces sulfuric acid for phosphoric acid industry [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. The process research has gradually become a hot spot in recent years.\u003c/p\u003e \u003cp\u003ePortland cement mainly contains alite and belite minerals. The formation enthalpy of alite mineral is 1810 kJ/kg, and the formation temperature is as high as 1450℃, while the formation enthalpy of belite mineral is only 1350 kJ/kg, which can be formed at 1300℃. Belite cement mainly contain belite mineral, which can reduce energy consumption than Portland cement [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. However, belite mineral has slow hydration rate and low early strength. On the other hand, CO\u003csub\u003e2\u003c/sub\u003e can enhance the hydration activity of belite mineral and improve its early strength, which can sequestrate greenhouse gases CO\u003csub\u003e2\u003c/sub\u003e in cement hydration products.\u003c/p\u003e \u003cp\u003eGypsum initial decomposition temperature reaches 1662℃ by thermodynamic calculation, but the decomposition temperature decreases significantly in the presence of reducing agents, such as coke, sulfur, hydrogen or carbon monoxide, which the decomposition temperatures are 849, 1054, 909 and 925 ℃, respectively. Gypsum can be decomposed at low-temperature by adding reductant, which is beneficial to obtain cement.\u003c/p\u003e \u003cp\u003eIn this work, the effects of different reductant (CaS and carbon) on the phases of belite cement raw meals were studied by calcining at 1300℃ for 2h and using phosphogypsum as raw materials. Moreover, the effect of carbonation curing on the cement properties was also studied.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cp\u003ePG came from Lomon Co. Ltd. in Mianzhu, China. The chemical composition is shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Alumina (analytical reagent) was provided by Tianjin Comio Chemical Co. Ltd., China. Iron oxide (analytical reagent) was provided by Tianjin Fengchuan Chemical Reagent Co. Ltd., China. Activated carbon powder (analytical reagent) was provided by Tianjin Beichen Chemical Reagent Co. Ltd., China. Silica (analytical reagent) and calcium carbonate (analytical reagent) were provided by Chengdu Kelong Chemical Reagent Co. Ltd., China.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eChemical Composition of PG (wt%)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCaO\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMgO\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eP\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eOthers\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e42.56\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e52.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.43\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe mineral composition of belite cement clinker was designed, which alite mineral did not presence in clinker, as in reference [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. The limestone saturation factor (KH) was 0.67, which tricalcium silicate was absent in theory. The values of silica ratio (SM) and alumina ratio (IM) were 2.50 and 0.93, respectively. The theoretical mineral composition of the cement consisted of 77.40 wt% dicalcium silicate (C\u003csub\u003e2\u003c/sub\u003eS), 4.17 wt% tricalcium aluminate (C\u003csub\u003e3\u003c/sub\u003eA) and 16.33 wt% tetracalcium iron aluminate (C\u003csub\u003e4\u003c/sub\u003eAF). The chemical compositions of designed cement clinker were 60.5 wt% calcium oxide (CaO), 27.0 wt% silicon oxide (SiO\u003csub\u003e2\u003c/sub\u003e), 5.4 wt% iron oxide (Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e), 5.0 wt% aluminum oxide (Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e), and 2.1 wt % others impurities, which impurities came from PG. The granular clinker was not obtained only by using PG as calcium source, which the clinker sample was completely melted on ceramic tray after calcination. According to references [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e], the belite cement can be prepared with 1/3 of the CaO from limestone (calcium carbonate) and 2/3 of the CaO from PG.\u003c/p\u003e \u003cp\u003eAccording to designed chemical composition of belite cement, PG, iron oxide, silica, alumina, calcium carbonate and reductant were mixed by mill to obtain cement raw meal. Moreover, the CaS reductant was prepared from PG, according to references [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. The prepared cement raw meal were mixed with a 2 wt% polyethylene glycol and pressed into Φ50mm\u0026times;8 mm round cake molds. The pressed cakes were put into an oven at 110 ℃ for 2 h to drying treatment. After drying, the samples were put into a high-temperature furnace and held at 1300 ℃ for 2 h, with a heating rate of 5℃/min. After the calcined process was finished, the cement clinker was immediately removed from the high-temperature furnace and rapidly cooled by air blowing. The cement clinker was ground finely by mill to obtain belite cement. Water was added in the cement, whith a water-to-gypsum ratio was fixed at 0.30 for sample. Finally, the homogeneous slurry was poured into a mold (20\u0026times;20\u0026times;20 mm) and shaped through vibrations. After 24 h of hardening time, the molds were removed, and then, the samples were cured at a constant temperature of 20℃in relative humidity (RH) of 75% for different days, with/without CO\u003csub\u003e2\u003c/sub\u003e concentration of 20%.\u003c/p\u003e \u003cp\u003eThe chemical composition of the raw materials was measured by an X-ray fluorescence spectrometer (Axios-Poly, PANalytical, Netherlands). The morphology was observed by scanning electron microscopy (TM-1000, Hitachi, Japan). The phase analysis was observed by using an X-ray powder diffractometer (Smartlab, Rigaku, Japan), equipped with Cu-K\u003csub\u003eα\u003c/sub\u003e radiation (λ\u0026thinsp;=\u0026thinsp;0.15406 nm). The mechanical properties of samples were tested by microcomputer-controlled electronic universal testing machine (TSE255D, Universal Testing Machine).\u003c/p\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the XRD patterns of cement raw meal calcined at 1300 ℃ for 2 h, with different reducing agents. The diffraction peaks of CaSO\u003csub\u003e4\u003c/sub\u003e decreased with reductant increase. As the ratio of the added amount of carbon was 10 wt% (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (a)) and n(CaS)/n(CaSO\u003csub\u003e4\u003c/sub\u003e) was 3 (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (b)), the diffraction peaks of CaSO\u003csub\u003e4\u003c/sub\u003e were not observed, respectively, which the main phases of samples were β-C\u003csub\u003e2\u003c/sub\u003eS and α'\u003csub\u003eH\u003c/sub\u003e-C\u003csub\u003e2\u003c/sub\u003eS. However, the sample contained a large amount of CaSO\u003csub\u003e4\u003c/sub\u003e with a 6.7 wt% carbon. According to theoretical calculation, the amount of reduction potence provided by carbon was similar by calcium sulfide, which it was possible that calcium sulfide was more suitable as reductant than carbon.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn general, the decomposition temperature of gypsum can be effectively reduced and industrial production can be carried out in a more reasonable way. The CaS reductant was prepared at 900℃ for 1h by using activated carbon as a reductant. According to carbon amount of prepared CaS, the C/S ratio of cement prepared with CaS reductant was about 1.5. The C/S ratio of cement prepared with a 6.7 wt% carbon was about 1.6. The C/S ratio of cement prepared with a 10wt% carbon was about 3. More carbon was needed to prepare cement by directly using carbon as a reductant, probably because carbon easily reacted with air, which it increased carbon consumption.\u003c/p\u003e \u003cp\u003eThe primary phase of belite cement is dicalcium silicate. The main chemical reactions of produced belite (2CaO\u0026middot;SiO\u003csub\u003e2\u003c/sub\u003e) can be simplified in the following equations:\u003c/p\u003e\n\u003ch3\u003e2C + CaSO = CaS + 2CO(g) (1)\u003c/h3\u003e\n\u003cp\u003eCaS\u0026thinsp;+\u0026thinsp;3CaSO\u003csub\u003e4\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;4CaO\u0026thinsp;+\u0026thinsp;4SO\u003csub\u003e2\u003c/sub\u003e (g) (2)\u003c/p\u003e \u003cp\u003eCaS\u0026thinsp;+\u0026thinsp;3/2O\u003csub\u003e2\u003c/sub\u003e(g)\u0026thinsp;=\u0026thinsp;CaO\u0026thinsp;+\u0026thinsp;SO\u003csub\u003e2\u003c/sub\u003e(g) (3)\u003c/p\u003e\n\u003ch3\u003e2CaO + SiO = 2CaO·SiO (4)\u003c/h3\u003e\n\u003cp\u003eThe specific surface areas of cement prepared by adding, 6.7 and 10 wt% carbon, and CaS (\u003cem\u003en\u003c/em\u003e(CaS)/\u003cem\u003en\u003c/em\u003e(CaSO\u003csub\u003e4\u003c/sub\u003e)\u0026thinsp;=\u0026thinsp;3) were 0.580, 0.554, and 0.625 m\u003csup\u003e2\u003c/sup\u003e/g, respectively, which cement was obtained by grinding clinker at 720 rpm for 40 min using a vibration mill. The cement prepared with CaS had best grindability.\u003c/p\u003e \u003cp\u003eAccording to Chinese standard GB/T176-2017 (Methods for chemical analysis of cement), free lime was not detected in the samples, which the content of free lime was below the detectable level.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows SO\u003csub\u003e3\u003c/sub\u003e content and Loss of cement clinker obtained with different reductant. Sulfur trioxide (SO\u003csub\u003e3\u003c/sub\u003e) content of cement prepared by adding 6.7 and 10wt% carbon, and CaS (\u003cem\u003en\u003c/em\u003e(CaS)/\u003cem\u003en\u003c/em\u003e(CaSO\u003csub\u003e4\u003c/sub\u003e)\u0026thinsp;=\u0026thinsp;3), were 26.28, 0.80, and 0.64 wt%, respectively. And their Losses were 0.68, 1.34 and 1.10 wt%, respectively. SO\u003csub\u003e3\u003c/sub\u003e and Loss were all in accordance with Chinese standards GB/T175-2023 (Common portland cement, SO\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;\u0026le;\u0026thinsp;3.5 wt% and Loss\u0026thinsp;\u0026le;\u0026thinsp;3.0 wt% ) for cement obtained by calcination with 10 wt% and carbon CaS (\u003cem\u003en\u003c/em\u003e(CaS)/\u003cem\u003en\u003c/em\u003e(CaSO\u003csub\u003e4\u003c/sub\u003e)\u0026thinsp;=\u0026thinsp;3).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the compressive strength of hydrated cement obtained with different reductant by curing for different age. The compressive strength increased with curing age increase. The strength increased rapidly before 28 d and the growth slowed down after 28 d. When a 10 wt% carbon was employed as the reducing agent, compressive strengths of the samples were 15.5, 15.6, and 15.6 MPa after carbonation curing 3, 7 and 28 d, respectively. When CaS was employed as the reducing agent, compressive strengths of the samples were 19.8, 27.2, and 34.1 MPa after carbonation curing 3, 7 and 28 d, respectively.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe strengths of samples cured by carbonized condition were much higher than samples cured by standard condition. The sample had high strength by using CaS as reductant, which strength were 2.4 and 34.1 MPa after curing 28 d without and with carbonation, respectively. And the strengths were 3.8 and 39.4 MPa after curing 90 d without and with carbonation, respectively. However, the samples had similar strength by adding 6.7 and 10 wt% carbon as reductant, which strength were 13.2 and 15.6 MPa after curing 28 d in carbonized condition, respectively.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the XRD patterns of hydrated cement for different curing ages. Belite (β-C\u003csub\u003e2\u003c/sub\u003eS and α\u0026rsquo;\u003csub\u003eH\u003c/sub\u003e-C\u003csub\u003e2\u003c/sub\u003eS) diffraction peaks were observed in all samples but the diffraction peaks intensity of carbonized samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (a\u003csub\u003e2\u003c/sub\u003e), (b\u003csub\u003e2\u003c/sub\u003e) and (c\u003csub\u003e2\u003c/sub\u003e)) were weaker than standard samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (a\u003csub\u003e1\u003c/sub\u003e), (b\u003csub\u003e1\u003c/sub\u003e) and (c\u003csub\u003e1\u003c/sub\u003e)), which CO\u003csub\u003e2\u003c/sub\u003e can enhance the hydration activity of belite mineral.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe strong diffraction peaks of calcium carbonate were observed in carbonized samples and the peaks intensity increased with age increasing. On the other hand, the diffraction peaks of calcium carbonate were also observed in standard samples after curing 28 d, because of air containing CO\u003csub\u003e2\u003c/sub\u003e in standard curing room. The diffraction peaks of ettringite (AFt) were observed in standard samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (a\u003csub\u003e1\u003c/sub\u003e), (b\u003csub\u003e1\u003c/sub\u003e) and (c\u003csub\u003e1\u003c/sub\u003e)) but it were not observed in carbonized samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (a\u003csub\u003e2\u003c/sub\u003e), (b\u003csub\u003e2\u003c/sub\u003e) and (c\u003csub\u003e2\u003c/sub\u003e)), probably because ettringite did not generate in carbonation environment.\u003c/p\u003e \u003cp\u003eThe main chemical reactions of belite and CO\u003csub\u003e2\u003c/sub\u003e can be simplified in the following Eq.\u0026nbsp;(5), though the actual reaction was complexity [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]:\u003c/p\u003e \u003cp\u003eCa\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e4\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;2CaCO\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;SiO\u003csub\u003e2\u003c/sub\u003e (gel) (5)\u003c/p\u003e \u003cp\u003eThe early strength of belite cement was enhanced by the calcium carbonate generated during carbonation curing and the C-S-H gel, thus improving its micro-mechanical properties stability in cement hydration environment [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Moreover, the diffraction peaks of SiO\u003csub\u003e2\u003c/sub\u003e were observed in samples prepared with CaS reductant after curing 56 d (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (c\u003csub\u003e2\u003c/sub\u003e)). The strong diffraction peaks of gypsum (CaSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO) were observed in samples prepared a 6.7wt% carbon reductant (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (a\u003csub\u003e2\u003c/sub\u003e)) because the residual anhydrite had some hydration activity in carbonation curing condition.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the microstructure of hydrated cement for 28 d under different curing conditions, respectively. The cement prepared with CaS reductant exhibited higher density because it had high hydration and carbonation activity (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (c\u003csub\u003e1\u003c/sub\u003e) and (c\u003csub\u003e2\u003c/sub\u003e)). Belite cement produced a little of needle-shaped ettringite particles during hydration process (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (a\u003csub\u003e1\u003c/sub\u003e), (b\u003csub\u003e1\u003c/sub\u003e) and (c\u003csub\u003e1\u003c/sub\u003e)). The ettringites overlap and were intimately associated with the surrounding calcium silicate hydrate gel, thereby enhancing the strength of the hydrated cement \u003csup\u003e[10]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eA lot of CaCO\u003csub\u003e3\u003c/sub\u003e fibers were observed in carbonized curing samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (a\u003csub\u003e2\u003c/sub\u003e), (b\u003csub\u003e2\u003c/sub\u003e) and (c\u003csub\u003e2\u003c/sub\u003e)), which fibers and hydrated calcium silicate filled the loose porous structure and increased samples strength. Some short fibers were observed in the cement prepared by using a 6.7 wt% carbon as reductant (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (a\u003csub\u003e2\u003c/sub\u003e)), probably because residual CaSO\u003csub\u003e4\u003c/sub\u003e hindered the fibers growth.\u003c/p\u003e \u003cp\u003eThe strength of hydrated cement prepared with CaS reductant was the highest. The production of CaCO\u003csub\u003e3\u003c/sub\u003e fibers and hydrated calcium silicate resulted in a filling of the loose porous structure within the overall system, leading to a more compact structure and an increase in the strength of the samples [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the carbon sequestration amount of cements prepared with different reductant after curing 28d. The carbon sequestration amount of cement prepared by adding 6.7 and 10wt% carbon, CaS (\u003cem\u003en\u003c/em\u003e(CaS)/\u003cem\u003en\u003c/em\u003e(CaSO\u003csub\u003e4\u003c/sub\u003e)\u0026thinsp;=\u0026thinsp;3), were 7.5, 9.2, and 11.6 wt%, respectively. The cement prepared with CaS reductant had high hydration activity, as a result it had high carbonation activity.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThe effect of different reductant on the property and CO\u003csub\u003e2\u003c/sub\u003e sequestration of belite cement were studied by using PG as raw material.\u003c/p\u003e\n\u003cp\u003e(1) The\u0026nbsp;belite\u0026nbsp;cement can be prepared by calcining at 1300 ℃ for 2 h, with CaS (\u003cem\u003en\u003c/em\u003e(CaS)/\u003cem\u003en\u003c/em\u003e(CaSO\u003csub\u003e4\u003c/sub\u003e)=3) and 10 wt% carbon reductant, respectively. The main phases were β-C\u003csub\u003e2\u003c/sub\u003eS and α'\u003csub\u003eH\u003c/sub\u003e-C\u003csub\u003e2\u003c/sub\u003eS and\u0026nbsp;free lime was not detected\u0026nbsp;in the samples.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e(2) The strength of belite cement can be significantly increased by carbonation curing, and the strength increased rapidly before 28 d and the growth slowed down after 28 d. The sample had high strength by using CaS as reductant, which strength were 2.4 and 34.1 MPa after curing 28 d without and with carbonation , respectively.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e(3) Some CaCO\u003csub\u003e3\u003c/sub\u003e fibers were observed in carbonized curing samples. The cement prepared with CaS reductant had high carbon sequestration ability (11.6 wt%) and compact microstructure.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eK.L. wrote the main manuscript text and prepared figures.All authors reviewed the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the Research Foundation of the Sichuan Science and Technology Program of China (2024YFS0334) and State Key Laboratory of Solid Waste Reuse for Building Materials (SWR-2023-010).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eFang R, Tan H, Mao W, et al. Influence of carbon and additive on high-temperature decomposition behavior of phosphogypsum[J]. Materials and technology, 2020, 54(6) : 115-119\u003c/li\u003e\n \u003cli\u003eTaha Y, Elghali A, Hakkou R, et al. Towards Zero Solid Waste in the Sedimentary Phosphate Industry: Challenges and Opportunities [J]. Minerals, 2021, 11: 1250\u003c/li\u003e\n \u003cli\u003eTan Hongbin, Zheng Aiguo, Kang Xiangmei, et al. Synthesis of a-hemihydrate gypsum from phosphogypsum and influence of different additives on mechanical performance[J]. Materials and technology, 2020, 54(5) : 697-703\u003c/li\u003e\n \u003cli\u003eCosta F N, Ribeiro D V.Reduction in CO\u003csub\u003e2\u003c/sub\u003e emissions during production of cement, with partial replacement of traditional raw materials by civil construction waste (CCW)[J]. Journal of Cleaner Production, 2020, 276: 123302\u003c/li\u003e\n \u003cli\u003eIge O E, Olanrewaju O A, Duffy K J, et al. A review of the effectiveness of Life Cycle Assessment for gauging environmental impacts from cement production[J]. Journal of Cleaner Production, 2021, 324: 129213\u003c/li\u003e\n \u003cli\u003eLiu S, Wei L, Zhou S, et al. Research Development of high-strength low-calcium Portland cement[J]. Bulletin of the Chinese Ceramic Society, 2014, 33(3): 553-557\u003c/li\u003e\n \u003cli\u003eChang J, Jiang T, Cui K. Influence on compressive strength and CO\u003csub\u003e2\u003c/sub\u003e capture after accelerated carbonation of combination \u0026beta;-C\u003csub\u003e2\u003c/sub\u003eS with \u0026gamma;-C\u003csub\u003e2\u003c/sub\u003eS[J]. Construction and Building Materials, 2021, 312: 125359\u003c/li\u003e\n \u003cli\u003eGuan X, Qiu M, Li H, et al. Preparation of self-pulverized low calcium cement and its carbonation-hardening properties[J]. Journal of Building Materials, 2018, 21(05): 775-779+785.\u003c/li\u003e\n \u003cli\u003eGong P, Zhang C, Wu Z, et al. Study on the effect of CaCO\u003csub\u003e3\u003c/sub\u003e whiskers on carbonized self-healing cracks of cement paste: Application in CCUS cementing[J]. Construction and Building Materials, 2022, 321: 126368-.\u003c/li\u003e\n \u003cli\u003eChen X, Zhang J, Lu M, et al. Study on the effect of calcium and sulfur content on the properties of fly ash based geopolymer[J]. Construction and Building Materials, 2022, 314(3): 125650.\u003c/li\u003e\n \u003cli\u003eLan Y, Gu Q, Peng Y, et al. Microstructure of early-age calcium sulphoaluminate and ordinary Portland cement paste cured under different CO\u003csub\u003e2\u003c/sub\u003e pressures[J]. Journal of Zhejiang University (Engineering Science), 2022, 56(12): 2454-2462.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"phosphogypsum, reductant, belite cement, CO2 sequestration","lastPublishedDoi":"10.21203/rs.3.rs-4275746/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4275746/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eGypsum can be decomposed at low-temperature by adding reductant, which is beneficial to obtain cement. The effects of different reductant on the phases of belite cement raw meals were studied by calcining at 1300℃ for 2h and using phosphogypsum as raw materials. Diffraction peaks of calcium sulfate were not observed in the samples when 10 wt% activated carbon was added and the molar ratio of CaS to CaSO\u003csub\u003e4\u003c/sub\u003e was 3, respectively. The Sulfur trioxide contents of in the clinkers were 0.80 and 0.64 wt%, respectively.\u003c/p\u003e\n\u003cp\u003eThe effect of carbonation curing on the cement properties was also studied. Carbonation\u0026nbsp; curing can promote cement hydration and increase strength at 20 ℃, 75 % relative humidity and 20% CO\u003csub\u003e2\u003c/sub\u003e concentration. As the carbonation curing age increased, the compressive strengths of the samples gradually enhanced. When 10 wt% carbon was employed as the reducing agent, compressive strengths of the samples were 15.5, 15.6, and 15.6 MPa after carbonation curing at 3, 7 and 28 d, respectively. When CaS was employed as the reducing agent, the compressive strengths of the samples were 19.8, 27.2, and 34.1 MPa after carbonation curing for 3, 7 and 28 d, respectively. The carbon dioxide sequestration contents of the samples prepared with a 10 wt% carbon and CaS reductant, were 11.6% and 9.2% after carbonation curing for 28 d, respectively. These findings demonstrate the potential to use phosphogypsum with various reductants to enhance the quality of belite cement and at the same time consume more CO\u003csub\u003e2\u003c/sub\u003e in the atmosphere during curing process. \u0026nbsp;\u003c/p\u003e","manuscriptTitle":"Effect of reductant on property and CO 2 sequestration for belite cement from phosphogypsum","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-04-22 11:04:56","doi":"10.21203/rs.3.rs-4275746/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"c632663a-917a-41b8-b0fa-539e29949eb3","owner":[],"postedDate":"April 22nd, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-05-09T18:41:01+00:00","versionOfRecord":[],"versionCreatedAt":"2024-04-22 11:04:56","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4275746","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4275746","identity":"rs-4275746","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}