Study On Hydration Hardening And Durability Of Sulfoaluminate Cement-Slag-Gypsum Cementing System

With the implementation of China's low-carbon economy and sustainable development strategy,the cement-based materials with a quality of green high performance has gradually been the main direction of development.As a special cement independently developed by China,calcium sulfoaluminate cement?CSAC?has the advantages of high early strength,short setting time,impermeability,frost resistance and corrosion resistance,as well as energy-saving,environment-protecting production process.All of the advantages satisfy the need of modern engineering,and conform to national sustainable development strategy.However,due to the limitation of the price of raw materials,the production cost of CSAC is high,which hinders its further development and application.Therefore,based on the hydration hardening mechanism of CSAC and the influence mechanism of mineral admixtures,this article used ground granulated blast furnace slag?GGBFS?and flue gas desulfurized gypsum?FGDG?to compound with cement to reduce the cost of CSAC and improve industrial by-products utilization rate,as well as simultaneously ensuring the performance of composite cement to meet the needs of the engineering.Combining traditional test methods with modern analytical testing technique,this subject systematically researches the effects of GGBFS and FGDG content on the qualities of hydration properties,mechanical strength,freeze resistance,and sulfate resistance of CSAC.The main contents are as follows:?1?The effects of GGBFS on the standard consistency water consumption,setting time,hydration rate,compressive and flexural strength,hydrate phase composition,hydration product morphology and pore structure changes were investigated through combining GGBFS with CSAC.The results showed that the introduction of GGBFS reduced the standard consistency water consumption and early hydration rate,which makes the setting time of the cement slurry increased.When the GGBFS content is less than or equal 10%,the pores in the hardened cement pastes are mainly harmless or less harmful,and the compressive strength of the composite system has no significant change comparing with the pure cement.However,when the GGBFS content is more than 10%,the number of harmful pores in the hardened cement pastes increased,resulting in a significant decrease in the compressive strength of the system.Meanwhile,the addition of GGBFS has no obvious effect on the hydrate phase of CSAC,and the main hydration products are AFt,AFm,C-S-H gel and AH₃ gel.However,with the increase of GGBFS,the generation of AFt decreases,and the generation of AFm and C-S-H gel increases,while the overall hydration degree of the composite system decreases.?2?The effects of FGDG on the macroscopic properties,hydrate phase composition and microstructure of CSAC-GGBFS system were investigated by using isothermal calorimeter,MIP,SEM,XRD and thermal analysis.Results showed that the addition of FGDG reduces the hydration rate in the early stage of the system?before5h?,but has a certain promotion effect on the later hydration.As the content of FGDG increases from 0%to 15%,the standard consistency water consumption of the composite system gradually increases,and the initial setting time is slightly shortened,but the final setting time is extended.Meanwhile,the later strength of the system showed the change rule of increasing first and decreasing later,the optimal content of FGDG was 6%.Comparing with the samples without FGDG,the increase rate of flexural strength and compressive strength was 11.6%and 15.1%respectively after hydration for 120d.FGDG has no significant influence on the mineral composition of hydration products of the composite system,but promotes the hydration generation of AFt and C-S-H gel,and inhibits the transformation of AFt to AFm.The total generation of hydration products increases,resulting in the decrease of porosity of hardened cement pastes and the denser internal structure.?3?The freezing-thawing cycle test was designed to evaluate the criteria of mass loss rate,strength loss rate,internal damage degree and defect change,examine freezing-thawing cycle capability of the composite system,and research the effect of GGBFS and FGDG on the freezing-thawing performance of CSAC.The results showed that the frost resistance of the CSAC-GGBFS system decreases significantly with the content of GGBFS increases from 0%to 30%.Under the same freezing-thawing conditions,the more the content of GGBFS is contained,the more serious the damage degree of the specimen get.While on the basis of the binary system,the introduction of FGDG can effectively improve the antifreeze performance of the system.The antifreeze performance of the ternary composite system can be equal to that of pure cement,when the mass ratio of each material is 75.2%CSAC+18.8%GGBFS+6%FGDG.?4?The sulfate erosion test was designed to test the sulfate corrosion resistance of the composite system,and the influence of the content of GGBFS and FGDG on the sulfate corrosion resistance of CSAC was further investigated.Results showed that the proper addition of GGBFS is beneficial to improve the corrosion resistance of CSAC.When the GGBFS content is 20%,the corrosion resistance coefficient of the specimen reaches 1.18?erosion for 12 months?,which is slightly higher than that of the pure cement specimen?1.14?.In the ternary system,the corrosion resistance of the mortar decreases with the increase of FGDG,but after 360 days of erosion,the mortar has not been damaged in spite of the content of FGDG is ranged from 0%to 6%,and its flexural and compressive strength has increased steadily,and the corrosion resistance coefficient is always maintained above 1.05,which shows that the ternary composite system still has excellent resistance to sulfate attack.