Study On The Activation And Hydration Mechanism Of Belite Calcium Sulfoaluminate Cement

High belite calcium sulfoaluminate cement has a wide application potential due to its low energy consumption,low CO₂ emission and excellent durability performance.However,due to the low hydration rate of C₂S in the early hydration stage,the application of high-bellite calcium sulfoaluminate cement is limited to a certain extent.In this study,C₂S phase is activated by dissolving ions into the crystal lattice to improve the hydration rate.On the other hand,a new cement system with a hydration rate gradient is created by adjusting the mineral composition of cement clinker,to be specific,by adjusting the ratio of calcium sulfoaluminate phase and activated C₂S phase.Unlike the traditional analysis methods of thermodynamics and dynamics theroy of solid phase reaction,the first principle and density functional theory were applied to study the effect of different ions on the activation of C₂S,especially on the crystal structure and lattice parameters,bond energy and electronic structure change.The crystal sturcture of C₂S can be activated by doping Ba and Al atoms and the crystal formation energy increases and the band gap betweenVBM and CBM become more narrow in the activated C₂S crystal structure.The original conapical or conangular ligands of[CaO₆]hexagon or[CaO₈]octagonal ligands can exist in the form of conangular or coplanar structures,thus greatly reducing the stability of crystal structure.Further comparision on the activation effect between Ba and Al,it can be concluded that the activation effect of Ba on the crystal structure is more obvious.The first principle and density functional theory which explains the change of the electronic structure of the activated crystal structure,provides a theoretical basis for the purposeful design of material structure.Since the anion SO₄^2-can also activate the lattice structure,the SO₄^2-in the calcium sulfoaluminate phase can also activate the bellite phase during the synthesis of bellite sulfoaluminate cement system.Therefore,in this study,the hydration process and hydration products of activated belite calcium sulfoaluminate cement system were further studied by several hydration testing methods,such as XRD,NMR,FT-IR and ICP-MS.The results show that there are two main hydration stages in the bellite calcium sulfoaluminate cement system.The first stage is the calcium sulphoaluminate and gypsum co-control stage.In this stage,the hydration reaction rate is fast and provides early strength development for the whole system.The second stage is the secondary reaction between C₂S phase and the hydration products in the first stage.Hydration reaction rate is mainly controlled by C₂S phase and the SO₄^2-concentration in the system.Due to the interaction of the unsaturated Ca²⁺and Si⁴⁺ions,the formaiton of C-S-H gel is inhibited and delayed.In this stage,hydrated calcium aluminate such as C₂ASH₈,and C-A-S-H are gradually generated.When the concentration of Al³⁺ions is reached to a certain limitaion,C-S-H gel is nucleated.In addition,the influence of gypsum on the hydration process of bellite calcium sulfoaluminate cement system was studied,and it was found that the SO₄^2-concentration inhibited the hydration reaction of C₂S in the second hydration stage.To sum up,bellite calcium sulfoaluminate cement system has the advantage of rapid hydration rate of calcium sulfoaluminate phase in the early stage and also promoting the hydration of C₂S phase in the later stage.In the bellite calclium sulfoaluminate cement system,hydrated products are mainly comprised by calcium aluminate hydrate(AFt,AFm,C₄AH₁₃,C₃AH₆,etc.)and gel phase?AH₃,C-A-S-H,C-S-H?.For cement-based systems with large gel phase content,there are some limitations for the traditional hydration kinetics test method such as calorimetry test,chemical shrinkage test and chemical bound water content test.By applying the electrochemical impedance spectroscopy?EIS?test method,the limitations of traditional hydration testing methods can be overcomed.Cement hydration process can be characterized by the curve of impedance with five classical hydration age.Point T_IIIII is regarded as the boundary of dissolution control stage and diffusion control stage,which are characterized by the equation Z_cp=R_s+l?dissolution control stage?and Z_cp=R_s+l+R_int?diffusion control stage?.Therefore,the mathematical equation between EIS method and traditional hydration test method can be established to improve the evaluation the hydration degree model of cementitous mateirals.It provides theoretical and practical support for evaluation of hydration process of cementitious materials and non-destructive testing technology,also provides strong guarantee for the development of application system of new cementitious material system.There are multi-layered structures in the hydration products such as AFm,AFm-CO₂,AH₃,C₄AH₁₃ and C₂ASH₈,bellite calcium sulphoaluminate cement system has the chloride resistance capacity and heavy metal immobilization properties.The hydrated calcium alumiante can absorb Cl^-ions and then generate the Friedel salt,so they can effectively absorb chloride ions intruded in the concrete and prevent reinforcement corrosion.In addition,the layered double hydroxide with AFm-Cl can adsorb heavy metal ions in waste water,especially cobalt ions.In this study,the adsorption behavior of hydrated calcium alumiante with Co?II?ions?[Co]=8 mM?is investigated.The adsorption process follows the pseudo-second-order model and can be described by the Langmuir adsorption isotherm.The experimental results demonstrate that the hydrated calcium alumiante has a high Co?II?adsorption?up to capacity 3.682mmol Co?II?/g?and a strong adsorption efficiency?>99.98%Co?II?adsorption at[Co]<16.00 mM with 4.0 g/L of adsorbent?.Due to isomorphic substitution behavior,less than 0.2%Co?II?will be leached from the newly formed Co-adsorbed hydrocalumite during the ions stability tests.The present study reveals that the hydration products of high belite calcium sulfoaluminate cement can be applied into heavy metal ions immobilization.