Study On Chloride Binding In Seawater Mixed Cement Paste With Low Water-binder Ratio

Seawater and sea sand,as raw material for concrete construction,have a broad application prospect in areas where fresh water and river sand resources are scarce（such as deserts,islands and natural disaster prone areas,etc.）.The introduction of chloride ions into concrete by sea sand and seawater will increase the risk of steel corrosion embedded in ordinary concrete,which has become a consensus at present.Compared with ordinary concrete,UHPC has an ultra-low water to binder（w/b）ratio,extremely dense microstructure and better durability.The effect of seawater and sea sand on the corrosion of steel reinforcement in ultra-high-performance concrete（UHPC）remains controversial.The chloride binding in UHPC is an important factor affecting the risk of corrosion of steel reinforcement,and it is also a key indicator directly related to the durability design of seawater sea sand UHPC.The purpose of this paper is to investigate the chloride binding behavior in cement-based materials with low w/b ratio and to establish a theoretical basis for the future preparation of UHPC with seawater and sea sand.In this paper,a water extraction method suitable for measurement of free chloride in cement-based materials with low w/b ratio is proposed and the testing accuracy was determined.Then,the chloride binding capacity,chloride binding mechanism and relationship between chloride binding and compressive strength and pore structure of seawater cement paste with low w/b ratio are studied by this method.At the same time,the effect of chloride binding on the risk of steel rebar in seawater cement paste with the low w/b ratio is discussed.Finally,the stability of bound chloride in seawater cement paste in different corrosive environments and its corrosion mechanism are analyzed and discussed.As the first part of the study,the effects of w/b ratio,curing age and extraction time on the testing results of water extraction method are studied.The optimal extraction time and test accuracy are determined while the testing results of pore pressing method are considered as the control group.The influence of water extraction on the chemical composition of seawater cement paste is clarified through the quantitative analysis of the chemical composition of seawater cement paste before and after water extraction.The specific effects of water extraction on the chemically binding and physically adsorption of chloride in seawater cement paste are calculated and analyzed.The results show that the influence of water extraction on the chemical composition of seawater cement paste is limited when the extraction time is set at 3min and the testing results are comparable to that of the pore pressing method.Based on the water extraction method,the effects of four supplementary cementitious materials（SCMs）–MK,GGBS,FA and SF on the chloride binding capacity,pore structure and compressive strength of seawater cement paste with low w/b ratio are studied.The chloride binding capacity is measured by the chloride binding ratio,which can magnify the difference of the chloride binding capacity of seawater cement paste with different mix proportions.The results of the study show that the chloride binding ratio of seawater cement paste with low w/b ratio is higher than that of ordinary seawater cement paste.Among the four SCMs,the influence of metakaolin on the chloride binding ratio is the greatest,the effects of granulated blast-furnace slag（GGBS）,fly ash（FA）and silica ash（SF）are similar.The effects of MK and SF on the chloride binding capacity of seawater cement paste with low w/b ratio are better than those on the chloride binding capacity of ordinary seawater cement paste.The compressive strength of the seawater cement paste with low w/b ratio increases with chloride binding ratio,which shows a power function relationship.The total porosity presents a downward trend with the increase of chloride binding ratio.By XRD quantification method,the contents of chemically bound chloride and physical adsorption chloride in the seawater cement paste with low w/b ratio are calculated.It is found that the content of chemically bound chloride in the seawater cement paste with low w/b ratio is only about 50%of that of the ordinary seawater cement paste,while the content of physical adsorption chloride is comparable to that of the ordinary seawater cement paste.After freshwater corrosion,more than 30%of the bound chloride ions in the seawater cement paste with low w/b ratio releases,which is 50%less than the release of the bound chloride in the ordinary seawater cement paste.The most released chloride is mainly the physically adsorbed chloride.The content of chemically bound chloride does not even decrease but increases after freshwater attack.After freshwater attack,the morphology of C-S-H gel in the seawater cement paste changes from the original flocculent structure to a fiber network structure,and a new hexagonal plate hydration product appeared,mainly Friedel’s salt crystals.After seawater attack,the content of bound chloride in seawater cement paste with low w/b ratio increases by more than 2-3 times compared with the seawater cement paste.The content of chemically bound chloride and physically adsorbed chloride is greatly improved by seawater attack.After seawater corrosion,the increment of bound chloride in seawater cement paste with low w/b ratio is slightly smaller than that of the ordinary seawater cement paste.Ettringite crystals appear after seawater attack,mainly longer needle-like crystals.After the sulfate attack,the content of bound chloride in the seawater cement paste with low w/b ratio is significantly reduced by more than 60%,which is much higher than the loss of the bound chloride under freshwater attack,but it is far less than the loss of bound chloride in the ordinary seawater cement paste.The influence of Mg SO₄on the bound chloride is the strongest among the three types of sulfates,while the effects of Na₂SO₄ and K₂SO₄ on bound chloride are similar.After the sulphate attack,the content of chemically bound chloride in the seawater cement paste with low w/b ratio is basically reduced to 0,while the physically adsorbed chloride is still retained.After the sulfate attack,a large number of ettringite crystals appears in the seawater cement paste,mainly smaller short rod crystals.It is worth noting that a large-sized long plate-like gypsum crystal and a loose porous amorphous form of M-S-H appears in the seawater cement paste after the Mg SO₄ corrosion.In the corrosion environment,the SCMs with high-Al phase（GGBS and MK）can enhance the stability of the bound chloride efficiently.The corrosion rate of steel rebars（Icorr）and[Cl^-]/[OH^-]in the seawater cement paste with low water-to-binder ratio were lower than those of ordinary seawater cement paste in the later curing stage,and the steel rebars were in a passivation state.The use of MK and SF can both improve the corrosion resistance of seawater cement paste with low water-to-binder ratio,and the effect of MK is better than that of SF.The impedance of the passive film on the steel formed in the cement paste with low water-to-binder ratio is higher than that of ordinary cement paste.