Study On Mineral Composition, Structure And Properties Of Alite Modified Calcium Sulphoaluminate Cement Clinker

The main line is the direct calcination of calcium sulphoaluminate cement clinker containing alite in this thesis.Focusing on how to break through the stable coexistence of C4A3$and C3S minerals under high SO₃ conditions,the regulation of high temperature molten liquid phase and its combination with ion doping are carried out.The main purpose is to prepare alite modified calcium sulphoaluminate cement?hereinafter referred to as“AMCSA”cement?to improve the strength and bonding properties of calcium sulphoaluminate cement?“CSA”cement?in the middle and later period,thereby prolonging the service life of engineering structures in severe environments.X-ray diffraction,thermal analysis,optical microscope and fluorescence spectrum analysis were employed to investigate systematically the mineral composition,structures,properties and hydration of AMCSA cement clinker,and industrial trial production and application.The following main parts are studied in this paper:?1?Study on composition design and mineral structure of AMCSA clinkerThe effect of ferrite phase on the formation and coexistence of C₃S and C₄A₃$minerals was studied.It is clear that the reduction of the molar ratio of Al₂O₃/Fe₂O₃ in ferrite phase composition could significantly reduce the formation temperature of melting liquid phase,improve the burnability of raw meal and promote the formation of C₃S minerals,which is beneficial to the formation and coexistence of C₃S and C₄A₃$minerals.The structure and bonding properties of C₃S mineral were studied.The result indicates that the C₃S is T₁ type structure in the triclinic system The C₃S hydration hardened paste has good bonding properties,and its flexural bond strengths at 3 d,7 d and 28 d are 1.89 MPa,2.45MPa and 3.21 MPa,respectively.The mechanical interlocking force formed between hydration products C-S-H gel and matrix is the main source of bond strength,which provides a theoretical basis for further research on the bonding properties of AMCSA cement.The structure of C₂S and the identification of?-C₂S phase were investigated via Eu³⁺ion fluorescence probe.The existence of?-C₂S with no hydration activity in clinker mineral has been clarified,and it is the first time that the Eu³⁺ions occupying the positions of four non-equivalent Ca²⁺and their emission wavelengths within the ⁵D₀?⁷F₀ transition range have been determined.They are Ca?1?O₆?578.9 nm?and Ca?2?O₆?577.6 nm?of?-C₂S and Ca?1?O₇?573.8nm?and Ca?2?O₈?571.8 nm?of?-C₂S.This method has prominent advantages in identifying a small amount of?-C₂S phase,and provides a new characterization method for the phase analysis of AMCSA cement clinker minerals.?2?Study on preparation,properties and hydration of AMCSA cement clinkerThe preparation and properties of AMCSA cement clinker were studied.The results indicate that barium ion doping promotes the coexistence of alite and C₄A₃$minerals.The diffraction peaks of alite and C₄A₃$in AMCSA cement clinker are obvious.Mineral compositions of the clinker quantitatively analyzed by the Rietveld method:bellite 55.5 wt.%,C₄A₃$25.6 wt.%,alite 9.6 wt.%and iron phase 9.3 wt.%,respectively.The compressive strength of AMCSA cement increases by 20.4 MPa during the curing time from 3 d to 28 d,which significantly improves the mechanical properties of belite calcium sulphoaluminate cement.The preparation of AMCSA cement clinker with BaSO₄ as an alternative part of CaSO₄ is investigated.The results show that the diffraction peak and the optical microstructure of alite are obvious with the increase of BaSO₄ content.The effect of SO₃ in CaSO₄ on the crystal growth and development of silicate minerals is clarified.It is found that the more SO₃ content,the more serious silicate crystal is corroded,the shape is irregular and the size is reduced.AMCSA cement has good mechanical properties.The compressive strength of the mortar increased by 12.2 MPa from 3 d to 28 d,and the flexural bond strength is 3.39 MPa at 28 d curing time,which is 24.6%higher than that of CSA.It is clear that alite minerals can play an important role in improving the bonding properties of the cement.The accumulated heat of the AMCSA cement after hydration for 3 d is lower than that of the CSA cement,which explains the reason why the early strength of AMCSA cement is lower than that of CSA cement.AMCSA cement hardened paste is composed of ettringite,C-S-H gel,AH₃ and BaSO₄,other hydration products and unhydrated cement particles.AFt crystal and C-S-H gel mutually overlap,forming the skeleton of hardened cement paste?HCP?,and improving the microstructure of HCP.Consequently,the macroscopic properties are the increase of compressive strength and bond strength.?3?The industrial pilot manufacturing and engineering application of AMCSA cementBased on the existing cement production equipment and industrial raw fuel,the key technology of AMCSA cement industrial trial production was carried out for the first time in the production line of the NSP.The proportioning scheme of AMCSA cement was determined,which is 56.81 wt.%limestone,23.43 wt.%bauxite,3.09 wt.%gypsum,8.83 wt.%barite and7.85 wt.%tailings sand.The appearance of AMCSA cement clinker is green-grey and slightly black,with uniform granulation and no pulverization.The measured vertical lifting weight is more than 1060 g/L,and the optical microscopic characteristics of the clinker are clear.The quantitative results of the clinker compositions are:the sum of C₄A₃$and C₃BA₃$48.5 wt.%,belite 32.6 wt.%,alite 11.7 wt.%and ferrite phase 7.2 wt.%.The key technological parameters of AMCSA cement industrial preparation were also determined,which lays a foundation for the industrial production of the cement.The setting time of the trial production of cement is comparable to that of CSA cement.The compressive strengths of AMCSA cement at 3 d and28 d are 46.0 MPa and 56.9 MPa,respectively,which increased by 10.9 MPa from 3 d to 28 d.The bond strength of AMCSA cement 28 d mortar is 3.51 MPa,which is higher than that of CSA cement 2.72 MPa,and the bond strength is increased by 29%,indicating that the properties of the industrial trial production of cement has reached the expected target,that is,the introduction alite minerals can improve the bonding properties of CSA cement.The adaptability of water reducing agent and retarder to AMCSA cement was studied.The results indicate that the polycarboxylate superplasticizer has good adaptability to the cement.Boric acid has the best retarding effect on the cement and has little effect on the strength.The concrete mix ratio adjustment scheme was proposed.According to the workability and strength of the concrete,the final mix ratio?per m³?was determined as cement,limestone powder,sand,stone,admixture and water quality?kg?:400.30,835,975,5.2,and 160,respectively.The compressive strength of concrete samples at l d,3 d,7 d and 28 d are 47.7 MPa,54.1 MPa,60.1 MPa and 64.8 MPa,respectively,meeting the C40 strength grade requirements of concrete design,and providing a basis for the application of AMCSA cement in severe environmental engineering such as offshore and offshore.