Study On The Rebar-Protecting Property Of Sulphoaluminate Cement-Portland Cement-Gypsum Ternary Composites

In recent years,more and more scholars have begun to study the compounding of different series of cements,with findings showing that different series of cements have complementary advantages.Among those compounds,sulphoaluminate cement-Portland cement-gypsum ternary composite system(abbr: ternary system)has attracted much attention due to its characteristics of quick setting,early strength,shrinkagecompensating,etc.In addition,in repairing mortar or concrete,non-shrinkage grouting materials and other aspects of the ternary system has application prospects.Although there have been many reports on the study of ternary systems,most of them focus on mix design,hydration products,microstructure and basic technical properties,while there has been no reported research on the rebar protecting property of the ternary system.In this paper,by measuring the alkalinity and porosity of cement paste,the formation of passivation film and the strength of mortar,the chloride diffusion coefficient of concrete and the cracking time of reinforced concrete electrode after electrochemical accelerated corrosion,the rebar-protecting property of ternary system is studied from different aspects.The main research results and conclusions of this paper are as follows:(1)When the ratio of the two cements is about 1:1,the compressive and flexural strength of the mortar decreases as the amount of gypsum increases.While the gypsum content is 3.3%,the compressive and flexural strength is the highest,and it grows fastest with age.(2)By measuring the pH of the artificial hole solution of cement paste,it is found that the alkalinity of the ternary system was lower than that of Portland cement and sulphoaluminate cement.The pH of Portland cement paste is about 13,so that the passivation film can be formed completely on the surface of the steel bar.The pH of sulphoaluminate cement paste is between 11 and 11.5,so that the passivation film is incomplete,between passivation and activation.The pH in the ternary system is between 10 and 11.5,inhibiting the passivation film from being formed.(3)With the porosity and the chloride diffusion coefficient of the ternary system being obviously higher than those of Portland cement,the ternary system's density is poorer,allowing water and oxygen to permeate the concrete cover easily and thereby corrode the steel bar.(4)In the electrification accelerated corrosion test,the relative potential of the ternary system at the beginning is very low,about-600 mV,so the steel is very likely to rust.As the energization time goes by,the relative potential increases,but remain less than-200 mV.The ternary system with high gypsum content is too low in strength to crack when the electricity is energized for 3 days.The ternary system with low gypsum content has high porosity,and the corrosion product is rapidly extravasated,and does not accumulate on the surface of the steel bar to crack the concrete cover.(5)From the above,it can be known that compared with Portland cement and sulphoaluminate cement,the ternary system's alkalinity is too low to form a passivation film,the structure is loose and the density is poor;thus,the rebar-protecting property of ternary system is poor.Owing to the rebar-protecting property of the ternary system having hidden dangers,it needs to be demonstrated when the ternary system is applied specifically to engineering.It is possible to improve the density of the ternary system by selecting the well-graded aggregate and a suitable gypsum type,adjusting the proportion of each cementitious materials in the ternary system,incorporating the polymer,and avoiding high curing temperature,so that the rebar-protecting property of ternary system can be improved.