Study On Reaction Mechanism And Early Shrinkage Properties Of Modified Supersulfate Cement

The 14th Five-Year Plan clearly sets out the goal of achieving carbon peak by 2030 and carbon neutral by 2060.However,the carbon emission of China’s cement industry is as high as1.5 billion tons per year,ranking the top three in the country.Cement as an indispensable material in the construction industry,the annual demand is growing,so the development of low-carbon environmental protection of the new cement material has become one of the major issues in the world today.Alkali slag/gypsum cement（also known as super sulfate cement）is a kind of cement with little clinker or no clinker,which takes granulated blast furnace slag as the main raw material,gypsum as the sulfate activator and clinker or lime as the alkaline activator.It has the characteristics of extremely low hydration heat release,high late strength and good resistance to sulfate attack.Supersulfate cement can replace most of cement by industrial by-products such as blast furnace slag and gypsum,and it has low CO₂ emission and has become one of the new green cementing materials with the most development potential.The sulfate cement early strength is low,construction maintenance for a long time,strength development is slow,and the related reaction mechanism and contract performance study is less,this topic by adding alkaline excitation agent on modification,the sulfate cement with blast furnace slag,cement and industrial by-product phosphogypsum as raw materials,the preparation of surplus sulfate cement.Different influencing factors are explored:The effects of different alkali activator content,cement content（5%,10%,15%）,blast furnace slag content（80%,90%,100%）,phosphogypsum content（5%,10%,15%）on the mechanical properties and early reaction process of supersulfate cement were studied,and the reaction mechanism and early shrinkage characteristics were also studied.The results show that:（1）Alkaline activator can significantly improve the early strength of supersulfate cement,and the 3-day compressive strength increases from 3.85 MPa to 38.62 MPa,and has a higher late strength.The increase of cement content has a certain inhibitory effect on the strength development of supersulfate cement,while both slag and phosphogypsum have an enhanced effect on the strength of supersulfate cement.（2）The hydration heat release of super sulfate cement is very low,only half of the heat release of ordinary cement,which is a kind of low heat release cement.Alkaline activator and slag can accelerate the hydration exothermic rate of supersulfate cement and the emergence time of the second exothermic peak.The increase of phosphogypsum content delayed the early reaction rate,the time delay of the emergence of the second heat release peak,and the total heat release increased.（3）The reaction products of supersulfate cement are mainly calcium-vanadium stone and C-S-H gel.Calcium-vanadium stone will crystallize to form a hard skeleton structure and c-S-H gel overlap each other to fill pores and improve strength.The addition of alkaline activator can significantly accelerate the formation time of ca-vanadium stone,and the morphology of ca-vanadium stone changes from long needle shape to long rod shape.（4）The increase of cement content will reduce the reaction rate and delay the formation time of caovanadite.The higher the slag content,the faster the reaction rate and the more C-S-H gel.The reaction rate decreases with the increase of phosphogypsum content,but the yield of calcium vanadite increases obviously.（5）The autogenous shrinkage and drying shrinkage of super sulfate cement are small,and the super sulfate cement without alkaline activator will appear micro expansion phenomenon.Alkaline activator will increase the autoshrinkage of supersulfate cement.The self-shrinkage and drying shrinkage of supersulfate cement with 10%cement content are the smallest,38%and 28%smaller than that of ordinary Portland cement.The increase of slag content has little effect on the self-shrinkage and drying shrinkage.（6）Alkaline activator can reduce the early porosity of super sulfate cement and improve the density.With the increase of cement content,the porosity increases and the number of capillary pores increases obviously.The porosity decreases with the increase of slag and phosphogypsum content.