The Effect Of Nano-SiO₂ On The Properties Of Cementitious Material With Large Dosage Of Slag

The environmental load of cement can be effectively reduced by replacing the cement with slag,so the systems of cement with large dosage of slag（20-80%of Portland cement replaced by slag）and supersulfated cement（75-85%of slag）have been developed.However,the existing research results show that the slow development of early performance of the system with large slag content is the key problem restricting the performance development of the two kinds of cement.Related studies have shown that nano Si O₂（NS）can improve the performance of slag cement system,but the understanding of its action law is not consistent,even contradictory.This paper starts from the simple cement system of slag to study the influence of nano Si O₂ on the hydration and hardening of the system,and further explore their influence on the supersulfated cement hydration,with clear nano Si O₂ modified regulation with large dosage of slag system.In this paper,qualitative and quantitative analysis methods were used to study the influence of NS on hydration and phase composition evolution of the cement system when the substitution fraction of slag was from 0 to 70 wt.%,so as to reveal the source of performance improvement.The results show that the mechanical strength of the sample with slag is lower than that of the control sample at the early ages.When the addition amount is less than 50%,the strength is improved at the later ages.However,when the replacement rate is 70%,the strength is always lower than that of the control sample.The addition of NS improves the mechanical properties of the samples,especially at the early stage.The hydration heat release rate and cumulative heat release rate of the samples decreased with the increase of slag content,while NS promoted the hydration heat release of each group of samples.TG results showed that NS consumed CH to generate additional C-S-H gel,which promoted the hydration of the whole sample.XRD and selective dissolution results showed that this was due to the simultaneous promotion of hydration of clinker and slag.The effect of NS on promoting hydration is most significant at the early ages,which makes up for the disadvantage of poor early performance of the cementitive material with large dosage of slag,and also provides ideas for the improvement of other aluminosilicate materials with poor early performance.Then this paper studied the influence of NS on the macroscopic and microscopic properties of supersulfated cement with different gypsum content,the results show that the 10%gypsum content of each performance is better than 4%gypsum sample.The strength of supersulfated cement samples with different gypsum content at each age is low,and the compressive strength at 90 d is only about 22 MPa.When NS is introduced,the strength of supersulfated cement samples at different ages increases to varying degrees,and the strengthening effect of NS against the compressive strength gradually increases with the increase of curing age within 90 days,the increment reaches about 100%at 90 days,and the compressive strength value reaches 44 MPa.The improvement of NS on the strength of supersulfated cement is also due to its promoting effect on the hydration of the system.Within90 days,the promoting effect of NS on hydration increases with the increase of age.However,the introduction of NS hinders the hydration of slag in the early stage,and the content of silicon atom in C-（A）-S-H gel is less,the average chain length is shorter,and the degree of polymerization is lower.After curing for 7 d,NS had a promoting effect on the hydration of slag,and after curing for 90 d,the promoting effect gradually increased.Compared with the control sample,the content of silicon atoms in C-（A）-S-H gel was higher,the average chain length was longer,and the degree of polymerization was higher.By studying the microstructure of supersulfated cement,it was found that in the sample without NS,there were more capillaries with larger size.Through SEM observation,there were more pits on the sample section,the structure was loose,and there were more unreacted raw material particles attached.In the sample mixed with NS,there were more small gel pores,and the introduction of NS compacted the pore structure of the supersulfated cement,and the critical pore size decreased from 0.1μm to 0.01μm.Through SEM observation,the structure of the sample section was also very dense,without obvious holes and cracks,and there was no adhesion of unreacted raw materials.In the supersulfated cement system,the addition of NS reduces the concentration of OH^-ions in the liquid phase,and hinders the precipitation of S and Al ions in the liquid phase.The hindrant effect of NS on gypsum consumption may make more gypsum promote the hydration of silicon phase in the system at the later ages,thus accelerating the hydration of the whole system,and thus greatly increasing the strength at the later ages.