Effect Of Sodium Hexametaphosphate On Properties Of MEA-cement-PCE Paste System

As a new type of expansive agent,magnesium oxide expansive agent(MEA)is widely used in engineering because of its advantages such as less hydration water demand,stable hydration products,adjustable expansion performance and compensation for concrete shrinkage.However,adding MEA will make the fluidity of the paste worse and affect the workability of concrete.In order to improve the bad effect of MEA on the fluidity of cement paste,sodium hexametaphosphate was used as fluidity control agent.The influence factors of different water binder ratio(0.3,0.4,0.5),different types of MEA(R type and M type)and different MEA dosage(6%,9%,12%)were taken into account The improvement effect of sodium hexametaphosphate(SHMP)on the fluidity of MPA-cement-PCE paste,as well as the influence on the mechanical properties and expansion properties of hardened paste were studied through the combination of experimental test and theoretical analysis.Research showed:(1)MEA can reduce the fluidity of plasticizing paste.With the increase of the content of sodium hexametaphosphate,the fluidity of slurry is improved.The Ultraviolet visible spectrophotometry test(UV)results show that due to the strong adsorption capacity of MEA on polycarboxylate superplasticizer,some cement has little or no adsorption of superplasticizer,which affects the dispersion of polycarboxylate superplasticizer and reduces the fluidity.By comparing the fluidity of the three water binder ratios,MEA had the least influence on the fluidity of the 0.4 water binder ratio test group,the most influence on the 0.5 water binder ratio test group,and the 0.3 water binder ratio was in the middle.(2)The addition of sodium hexametaphosphate significantly improves the fluidity of MEA-cement-PCE paste.With the increase of the content of sodium hexametaphosphate,the fluidity increased.The reason is that the structure of "inner phosphate(multilayer)-outer superplasticizer(monolayer)" is formed on the surface of cement with the addition of sodium hexametaphosphate,which increases the thickness of adsorption layer,enhances the steric hindrance effect and enhances the dispersion of PCE.However,the effect of sodium hexametaphosphate on the fluidity of slurry does not increase with the increase of its dosage.In general,the effect of sodium hexametaphosphate on the fluidity of 0.3 water binder ratio test group was the worst,and the effect of sodium hexametaphosphate on the fluidity of 0.4 water binder ratio and 0.5 water binder ratio was similar.(3)The addition of sodium hexametaphosphate has a certain effect on the strength of the mixed MEA mortar.The results showed that the strength of the experimental group was better than that of the experimental group with high dosage of MEA,and the strength of sodium hexametaphosphate was better than that of the experimental group.XRD and MIP experiments show that calcium magnesium phosphate is formed by the addition of sodium hexametaphosphate,which fills the pores and reduces the number of harmful and harmful pores.(4)Under each dosage,the curve of restrained expansion rate basically presented threestage growth,namely "more rapid growth in the early stage,slower growth in the middle stage and more stable growth in the later stage".The effect of different restrained ratio of sodium hexametaphosphate on the expansion in water is more complex.When the content of sodium hexametaphosphate is 0.3%,although it can improve the expansion of some test blocks with low water binder ratio and low MEA content(9%),it can weaken the expansion of test blocks with high MEA content(12%),and the larger the water binder ratio is,the greater the expansion weakening is.When the content of sodium hexametaphosphate is 0.15%,the effect on the expansion test block is small,and some test blocks have an increase in the expansion rate.Although a small part of the test blocks have a decrease in the expansion rate,they are all within10%.TG results showed that the addition of sodium hexametaphosphate reduced the formation of magnesium hydroxide,resulting in the reduction of restrained expansion rate of some experimental groups.MIP shows that the addition of sodium hexametaphosphate makes the test block more dense,which provides favorable conditions for MEA to push away the cement stone and produce macro expansion,resulting in the increase of restrained expansion rate of some test groups.