Effect of Chloride Ion on Free Nitrite Ion in Cement

This was an experiment in which chloride was externally permeated into cement paste. The influence of Cl on the content and distribution of free-form NO2 in the cement paste was researched using the chemical quantitative analysis method. The action mechanism was investigated by the micro-means of X-ray diffraction (XRD) and scanning electron microscopy (SEM).The results show that the physical competitive adsorption of Cl with NO2 on C-S-H and the chemical substitution of Cl to NO2-AFm caused more free-form NO2 in the cement paste. In the cement paste with chloride salt erosion, the NO2 concentration in the erosion surface was the lowest, and the NO2 concentration reached the highest value at 10mm from the erosion surface.The concentration decreased gradually with the depth from the erosion surface.


Introduction
Corrosion of rebar is a very important factor that affects the durability of reinforced concrete structures.This has great impact on the durability of concrete structures.There are varieties of methods of repair for steel corrosion used in China and in other parts of the world [1][2][3], such as concrete coating, concrete realkalization, cathodic protection, and electrochemical chlorine removal methods.Through a comprehensive comparison of the commonly used methods, findings showed that the most simple and effective way to improve durability is to mix steel rust inhibitor into the concrete.Not only does this effectively slow the corrosion rate of steel and delay the onset of rebar corrosion, but also it solves the problem of the repassivation of the steel.
Nitrite is the best corrosion inhibitor.It is the one used most widely and in the largest quantities [4][5][6].Concrete mixed with nitrite rust inhibitor is used to protect the steel in the reinforced concrete.There are many reports of use of this method at home and in other countries [7][8][9][10][11][12].However, at present, nitrite rust inhibitors are mainly used to evaluate corrosion inhibition mechanisms and the corrosion inhibition effects of steel bars in concrete structures [13][14][15][16].
There is a small amount of literature on binding, decomposition of nitrite, and the influencing factors in cement paste.After mixing nitrite rust inhibitor into the concrete structure, NO 2 -in the structure generally exists in two forms: one is free-form NO2-that is free to move in pore fluids and the other is a hardened NO 2 -bound by cement hydration products [10,11].The binding NO 2 can be divided into two types, according to curing mechanism: the chemical reaction of the solid state ions and the hydration products by physical adsorption of the binding state of the solid state ions.The internal content of free-form NO 2 -in concrete will directly affect the final rust inhibition effect.
On the other hand, two approaches are generally used to study steel corrosion in concrete: the use of simulated pore solution and the use of actual concrete or mortar.The use of electrolytes that mimic pore solutions facilitates control of the many parameters that influence rebar corrosion.This was the approach chosen for the present investigation.By contrast, the use of cement paste is more intuitive and more accurately reveals the role of the free-form NO 2 -reaction mechanism.In summary, the characteristics and influencing factors on the concentration of NO 2 -in cement paste need to be further explored.In particular, the influence and action principle of Cl -on the free-form NO 2 -concentration and its distribution are not yet clear [17][18][19].
In this paper, the introduction of Cl -by external infiltration of chloride salt and the effects of Cl -erosion on the free-form NO 2 -concentration in cement paste and its microscopic mechanism were studied by means of chemical quantitative analysis combined with X-ray diffraction (XRD) and scanning electron microscopy (SEM), in order to improve the durability and service life of China's reinforced concrete structures and to provide a theoretical basis for the approaches.1.

Experimental Program
Distilled water was used in the fabrication of concrete mixtures; Cl -and NO 2 -were obtained from NaCl analytical reagent (AR) and NaNO 2 analytical reagent (AR), respectively.The tested concrete types were 40mm × 40mm × 160mm rectangular specimens, having cement pastes of three water-to-cement ratios of 0.30, 0.40, and 0.50.
After demolding, the specimens were cured in a curing room for 28 days.Prior to environmental conditioning, only one surface of concrete was allowed to be penetrated by nitrite solution.The other surfaces were coated with epoxy resin.This was to realize one-dimensional diffusion.The cement paste specimens were separated into two groups representing different exposure conditions: test group (including A-C group, sodium chloride solution containing 2.0% chloride attack) and control group (including Ac-Cc group, soaked in aqueous solution).More details of the experimental scenarios for these two groups of specimens are given in Table 2.These were removed after 7 days of curing, dried naturally in the shade and sliced at 1cm intervals.

Specimen Sampling Method
(1) Determination of Free-Form  2 -in Cement Paste.The sliced cement paste specimen was ground into powder and put into standard square hole stone sieves with apertures of 0.6mm, 0.3 mm, 0.15 mm, and 0.075 mm.The sieve was vibrated on a vibrating screen machine.The 0.075 mm filter powder was then put into a dry box for drying.1g dry powder was put into a conical flask containing 100ml of water and then placed in a water bath pot.It was heated at a constant temperature of 60 ∘ C, while being agitated with a glass rod for 10min.The flask was shaken in an air shaker and centrifuged for 10 min.1ml of the soak solution diluted 500 times was used to obtain spare extract.Finally, the "water qualitydetermination of nitrogen (nitrite) -spectrophotometric method" (GBT-7493-1987VYD) was used to determine the concentration of free-form NO 2 -in the samples.
(2) X-Ray Diffraction Analysis.The specimens after curing, soaking, and drying were ground with a mortar and passed through a hole of a 0.3 mm sieve to achieve a particle size of less than 0.3mm powder.They were then dried for 24h in an oven at 60 ± 5 ∘ C.After cooling, the powder was placed in a small plastic sample bag and sealed.The concrete paste specimens were then completed for sampling.The prepared samples were prepared for phase analysis and quantitative analysis by using a D8 ADVANCE X-ray diffractometer manufactured by Bruker AXS.
(3) Scanning Electron Microscopy Analysis.After a time the curing cement mortar samples were removed from the curing room.Then a small hammer was used to knock the samples gently into 10mm square pieces.An amount of the fragments was dried for 24h in the oven at 60 ± 5 ∘ C.After cooling these were placed and sealed in plastic sample bags.Scanning for structure took place using a SU-70 field emission scanning electron microscope.

Effect of Erosion Cl -Concentration on Free-Form NO 2 -
Concentration.The combination of ions in cement generally has two forms: one is the result of a chemical reaction between C 3 A and other hydration products in the cement.When Cl -and NO 2 -exist in the cement paste, Cl -will react with AFm to form Friedel's salt, and NO 2 -will react with AFm to form NO 2 -AFm.Another bound form is when the ions are physically adsorbed into the surface of a hydration product, such as calcium silicate hydrate gel C-S-H [20][21][22][23].
Figure 1 is a contrast graph: It shows the free-form NO 2 -concentration of cement paste specimens with 0.5%, 1.0%, 1.5%, and 2.0% NO 2, -in water-to-cement ratio of 0.4, immersed in 2.0% NaCl solution and aqueous solution, respectively.The cement paste specimens with a watercement ratio of 0.4 and with 0.5%, 1.0%, 1.5%, and 2.0% NO 2 were immersed in 2% NaCl solution and aqueous solution.Figure 1 shows the internal free-form of the NO 2 -concentration contrast curve.It can be seen from Figure 1 that when the same cement paste test piece is immersed in the chloride salt solution the internal free-form NO 2 -concentration is higher than that in the aqueous solution.
Figures 2 and 3 are X-ray diffraction contrast charts for cement paste specimens with 1.0% and 2.0% NO 2 -mixed in 2.0% NaCl solution and aqueous solution for 7 days, respectively.Figure 2 shows that NO 2 -AFm was produced under the condition of natural curing with cement paste containing 1.0% and 2.0% NO 2 -, respectively.After soaking International Journal of Corrosion 3  in an aqueous solution, the value of the NO 2 -AFm diffraction peak of hydration products basically disappeared.It is possible that water molecules had infiltrated to lower the pH, causing them to break down or change to other substances.Friedel's salt diffraction peaks then appeared, which indicated that the NO 2 -in NO 2 -AFm had partially been replaced by Cl -chemistry.At the same time, diffraction peaks of C-S-H gel, in the cement hydration products, soaked in two kinds of solution, had obviously increased, and no NO 2 -AFm had been generated.Figure 1 shows that that there was a clear competitive adsorption between Cl -and NO 2 -on the surface of C-S-H gel when Cl -entered the cement paste and competed with NO 2 -for adsorption on the surface of C-S-H gel.Due to the limited amount of C-S-H gel formation, the  infiltration of Cl -lowered the physical adsorption of NO 2 -, resulting in a relative increase of NO 2 -in the pore solution.

Effect of Chloride Erosion Depth on
Free NO 2 -Concentration.Figures 4-6 show the curves of the free-form NO 2 concentration with erosion depth in cement paste specimens, with water-to-cement ratios of 0.3, 0.4, and 0.5, respectively, in NaCl solution.
As can be seen from Figures 4-6, the free NO 2 -concentration decreased with increasing erosion depth.cement paste specimens (depth 0 mm from the erosion surface) was the lowest, far lower than the NO 2 -concentration inside the cement paste specimens, reached highest value at a distance of 10 mm from the erosion surface, and achieved a steady state of 80mm.It was considered that there was a large amount of dissolution of NO 2 -at the erosion surface.However, in the interior of the cement paste, the content of NO 2 -dissolution was much lower, so the concentration of free-form NO 2 -ions on the surface of the test piece was the lowest.As erosion time increased, Cl -diffused into the cement paste specimens, which effected the chloride ion concentration.On the one hand, Cl -displaced NO 2 in the hydration product of NO 2 -AFm to form Friedel's salt, which, relatively, reduced the formation of NO 2 -AFm.On the other hand, it competed with NO 2 -to be adsorbed into the surface of calcium silicate hydrate gel (C-S-H).The greater the Cl -concentration, the greater the concentration of free-form NO 2 -.At a distance of 10 mm from the erosion surface, a large amount of Cl -accumulated due to diffusion, so the corresponding free-form NO 2 -concentration also reached the maximum at this position.At the same time, it can be seen from Figures 4-6 that as the water-to-cement ratio increased, the free-form NO 2 -concentration inside the cement specimens decreased with the erosion depth.This is because the greater the water-to-cement ratio is, the more the complete hydration within the paste will result in more C-S-H.However, at the same time, the greater the porosity of the paste, the faster the migration of Cl -within the paste.The substitution effect of Cl -and the physical adsorption capacity of C-S-H also increased, as did the free-form NO 2 concentration in the paste.
Figure 7 shows the SEM image of cement paste specimens without NO 2 -, and Figure 8 shows the SEM image of cement paste specimens with 2% NO 2 -. Figure 8 shows that the internal structure of cement paste without NO 2 -was more compact.The surface of cement stone was surrounded by hydration products.These were intertwined and filled the holes in the cement slurry.At the same time, a large amount of granular material was attached to the surface of  the cement slurry, and large amounts of acicular ettringite crystals and irregular plate crystals were found.The cement paste specimens with NO 2 -were looser specimens: there were more pores between the crystals and a large number of petal-shaped crystals at the surface.With pure cement paste, however, there were none of these petal-like crystals, from which it can be inferred that this material may be a form of NO 2 -AFm crystals.At the same time, under the action of aqueous solution and chloride salt solution, hydration of cement matrix was promoted, and the internal structure became more compact.The diffusion of Cl -slowed down with the increase of soaking time and produced a weakening of the competition with the NO 2 -relationship.

Conclusions
In this work, the effect of chloride ion on nitrite was investigated using chemical quantitative analysis and microanalysis method.The following conclusions were reached: (1) The larger amount of free NO 2 -in the cement paste under chloride salt attack than that when it is soaked in aqueous solution was mainly due to the competitive adsorption of NO 2 -at the surface of C-S-H and the chemical substitution of Cl -for NO 2 -AFm.
(2) Both the aqueous solution and the chloride salt solution promoted the hydration of the cement matrix and made the internal structure more compact.The diffusion of Cl -slowed down with the increase of soaking time and weakened the competition with NO 2 -.
(3) The cement paste affected by chloride salt had the lowest NO 2 -concentration on the surface of erosion, and the NO 2 -concentration reached the highest value at 10mm from the erosion surface.As the depth from the erosion surface deepened, the concentration of NO 2 -trended downwards.

5 :
The freeform NO 2 -concentration on the erosion surface of the International Journal of Corrosion N／ 2 -AFm

Table 1 :
Chemical composition and physical characteristics of ordinary Portland cement.

Table 2 :
Mix proportions of cement paste specimens.