Hardening,Corrosion Properties Of Seawater Sea Sand Cementitious Based Materials With Ultra Low Water To Binder Ratio

Seawater and sea sand,generally,contain lots of soluble chloride,the utilization of seawater and sea sand in conventional reinforced concrete will accelerate the corrosion of re-bar and result in the degradation of concrete.Thus,the use of seawater and sea sand in the production of conventional reinforced concrete is prohibited by various of international standards due to the reported high risk of corrosion.Unlike the conventional concrete,the w/b of UHPC is very low,and abundant of cementitious materials are used in UHPC,which make the use of seawater and sea sand in UHPC become possible.On one hand,the introduced chloride by seawater will be much lower than conventional concrete because of the very low w/b in UHPC;On the other hand,the free chloride can be bound by the amount of cementitious materials,and further decrease the content of introduced chloride.What’s more,the dense microstructure of the UHPC can immobilize the corrosive compounds available in seawater and sea sand to reach the steel reinforcements.Therefore,in this study,the effect of seawater and sea sand on property of low w/b paste or UHPC has been studied.Compared with the fresh water specimens,the mixing of seawater accelerated the hydration process of ternary system by 2 to 10h.With the mixing of seawater,the role of slag on hydration evolution was more significant that of silica fume and cement.This is mainly due to that the seawater increased the reactivity of slag and enhanced its interaction with cement due to triggered reactions between seawater components and rich Al-phases in cement and slag.The microstructure and early compressive strength of ternary cementitious system was improved by the mixing of seawater,this effect was more obvious in slag containing specimens.Moreover,the autogenous shrinkage of cement-silica fume-slag paste can be restrained by the mixing of seawater.The replacement of cement with Meta Kaolin（MK）,fly ash or slag improved the chloride binding capacity,while the MK containing specimens exhibited the highest chloride capacity,followed by slag and fly ash containing specimens.The increase of w/b from 0.16 to 0.24,the chloride binding capacity decreased.This shows that the w/b had an inverse effect on the total chloride binding process when the chloride was introduced internally by seawater.The increase in the enhanced the formation of chemical chloride binding,which indicated the chemical chloride binding capacity highly related the content of Al₂O₃.A direct relationship between the amount of CaO and the zeta potential of hydrated cement paste was observed,which implied that the physical chloride binding capacity of hydrated cement paste highly rely on the content of CaO.It can be concluded that the chemical chloride binding is more effective than physical chloride adsorption capacity in the paste with ultra-low w/b with the mixing of seawater.The influential role of Al₂O₃and CaO on chloride binding capacity was highlighted,the effect of SiO₂content was not significant on chloride binding.The addition of slag can both improved the early properties of fresh and salt UHPC,this effect was more effective in salt UHPC than fresh UHPC.The optimum addition of slag was determined as 20%.The thickness of ITZ between sand-cement paste was less than 20 um in UHPC,the property of ITZ can be enhanced by the mixing of seawater and sea sand.This can be caused by two reasons,on one hand,the mixing of seawater and sea sand result in the acceleration of hydration,which promote the formation of hydration products,and in turn enhance the microstructure and compressive strength;On the other hand,the precipitation of Friedel’s salt and Brucite in ITZ improved the bond strength of ITZ.The high corrosion rate was observed in seawater mixed conventional reinforced concrete,the corrosion rate of sea sand mixed conventional reinforced concrete exhibited the trend of increase after the decrease.Technically,the mixing of seawater specimens showed higher corrosion rate than the sea sand mixed specimens.The corrosion was not detected in reinforced UHPC while mixed with seawater and sea sand by the age of 1080d.Which is mainly because of the dense matrix and very low porosity in UHPC that insufficient oxygen and water can be used to initiate the corrosion.What’s more,the content of introduced chloride by seawater and sea sand may not reach the threshold chloride concentration to motivate the electrochemical reaction of corrosion.