The Effect of Mg 2+ on the Solidification of Chloride Ions in Portland Cement Paste

: In order to investigate the mechanism of the effect of Mg 2+ on the solidification of chloride ions in hardened Portland cement slurry, a NaCl-MgCl 2 composite solution was used to soak and corrode the hardened Portland cement slurry. Under the condition of fixed total chloride ion concentration of 0.6mol/L, the effects of Mg 2+ concentration (0.1mol/L, 0.2mol/L, 0.3mol/L) and soaking time in environmental solutions on Friedel salt production, leaching Ca 2+ ion concentration, and leaching of hardened Portland cement were studied The influence law of pH value and other properties in erosive environments. The results showed that in the early stage of immersion erosion, Mg 2+ had little effect on the rate of pH increase of the erosion solution; After the pH value of the solution stabilizes, the final pH value of the solution decreases with the increase of Mg 2+ concentration; As the concentration of Mg 2+ in the erosion solution increases, Mg 2+ promotes the dissolution of hydration product Ca(OH) 2 and the dissolution of Ca 2+ , while generating Mg(OH) 2 precipitates, resulting in a decrease in solution alkalinity and an increase in the amount of chloride ions solidified; When the pH of the solution exceeds 12.1, a lower pH value will promote the formation of Friedel salts; When the pH of the solution is less than 12.1, higher concentrations of Mg 2+ cause Friedel salt to decompose.


Introduction
Concrete has become the largest and most important engineering material in the 21st century, and it is widely used in various marine infrastructure construction. At present, marine engineering construction is in a new stage of development, and a large number of marine buildings are needed for national key strategies and maritime powers. However, in the marine environment, due to tides and other complex factors, the high content of inorganic salts leads to severe structural corrosion, especially the damage caused by chloride ion corrosion, which has a serious impact on the service life of building structures [1]. In order to gain a deeper understanding of the mechanism by which chloride ions penetrate concrete and cause deterioration of reinforced concrete, we divided the total chloride ions in the solution into chloride ions that combine with free chloride ions [2]. Free chloride ions refer to chloride ions that exist in pore solutions and can migrate within cement slurry. Combined chloride ions refer to chloride ions adsorbed on the surface of hydration products or chemically combined with hydration products [3]. Free chloride ions can move freely in cement slurry, and the combined chloride ions can be regarded as fixed chloride ions that can be released when the surrounding environment changes. Under normal circumstances, concrete buildings in marine environments are subject to erosion and destruction by various ions in seawater, including common corrosive ions such as Cland Mg 2+ . Concrete will be subject to the erosion of multiple ions, which will accelerate the entry of chloride ions into the concrete and lead to the corrosion of steel bars. The cement paste in the concrete can solidify chloride ions to generate Friedel salts, which can reduce the degree of erosion damage of reinforced concrete to a certain extent. Therefore, it is very meaningful to study the chloride ions solidified in concrete. At present, some studies on the solidification of chloride ions in cement slurry both domestically and internationally have found that the types of chloride cations have a significant impact on the solidification of chloride ions in cement slurry. Zhu [5] studied the chloride ion content in concrete with different water binder ratios and found that in different types of chloride ion solutions, the ability to solidify chloride ions was in the order of C b (CaCl 2 )>C b (MgCl 2 )>C b (KCl) ≈C b (NaCl). Tritthart's [6] study showed that the decrease in OHconcentration in the pore solution of cement slurry immersed in CaCl 2 and MgCl 2 erosive solutions is due to the formation of Ca(OH) 2 and Mg (OH) 2 precipitates. In addition, it is found that the OHconcentration of pore solution is inversely proportional to the chloride ion binding capacity of cement slurry, which indicates that the competitive adsorption between Cland OHaffects the chloride ion binding. B. Reddy [7] studied the effect of pH value on solidified chloride ions and believed that as pH value decreased, some of the bound chloride ions would be released. Cheng [8] found that Mg 2+ in Portland cement mortar has a certain blocking effect on the chloride diffusion rate. The presence of Mg 2+ reduces the pH value, and the decalcification dissolution of C-S-H gel greatly increases the chloride diffusion rate. The above related studies mainly considered the effects of some ions and pH changes on the solidification of chloride ions in cement slurry. Given that the concentration of Mg 2+ significantly increases in tidal environments, the mechanism by which its concentration affects the solidification behavior of chloride ions in cement slurry is still unclear. Therefore, based on the above issues, this study conducted a study on the regularity of Mg 2+ on cement solidification chloride ions in tidal environments.

Raw materials
The experimental materials were fly ash (FA) produced by Gongyi Longze Water Purification Materials Co., Ltd. and ground granulated blast furnace slag (GGBS) produced by Gongyi Longze Water Purification Materials Co, Ltd. The Portland cement used in the experiment was produced by Jiaozuo Qianye Cement Co, Ltd. Its chemical composition and physical and mechanical properties are shown in Tables 1 and 2. The mineral composition and content of Portland cement are shown in Table 3. The chemical composition of fly ash and mineral powder is shown in Figure 4. Fly ash is industrial solid waste discharged from fossil-fuel power station, which has micro aggregate effect and chemical activity effect. Mineral powder is a powder material obtained by grinding the slag discharged from an ironmaking blast furnace after quenching and quenching with water. The design of cement paste mix proportion is shown in Table 5.

Specimen Forming and Curing
In order to eliminate the impact of aggregate interface on erosion, this article uses the GB/T1346-2011 "Cement Standard Viscosity Water Consumption, Setting Time, and Stability Testing Method" standard to prepare clean slurry test blocks with a water cement ratio of 0.4 as the research object. The formed slurry is demolded and cured in saturated lime water for 28 days. The hardened cement slurry cured for 28 days is broken into cement powder with a particle size of 200 mesh, and the obtained samples are placed in a vacuum drying oven at 40℃ for 3 days. Finally, immerse the dried sample in an erosion solution in a constant temperature chamber, with a solid-liquid mass ratio of 1:20. In order to simulate the ion concentration in the tidal zone environment and accelerate the testing process, the Clconcentration in the erosion solution was fixed at 0.6mol/L. Choose three concentration gradients for Mg 2+ concentration: 0.1mol/L, 0.2mol/L, and 0.3mol/L. A detailed combination of erosion solutions is shown in Tables 5. A total of four sets of erosion solution environments were designed to study the erosion damage mechanism of cement slurry under the coupling effect of different concentrations of Clsuch as Mg 2+ .

Test methods
Use a pH meter to measure the pH value of each erosion solution at different immersion times. Before each use, the pH electrode is washed with deionized water and placed in the solution to be measured. At the same time, the solution is stirred gently with a glass rod. After the value is stable, the observed value is recorded. 0.22 passed before the test μ After filtering with the m filter membrane, 1mL of erosion solution and 9mL of dilute nitric acid solution with a mass fraction of 2% were added and mixed evenly. The solution was analyzed using spectral Arcos ICP-OES in semi quantitative analysis mode, and the concentration of Ca2+ was detected within the calibration range. Thermogravimetric analysis: Place the soaked sample in a 40℃ vacuum drying oven for 3 days, and use the HCT-3 microcomputer differential thermal balance produced by Beijing Science Instrument Factory to test and analyze the prepared sample. The temperature range for testing is 25~500℃, and the heating rate is 10℃/min. X-ray diffraction analysis: The X-ray diffractometer produced by Dandong Haoyuan Co., Ltd., manufactured in China, was used for testing and analysis, and the obtained samples were characterized for phase composition. Cu target and K were used in the experiment α Radiation, tube voltage 40KV, tube current 150mA, scanning area 5-70°, scanning speed 5°/min.

The effect of Mg 2+ on the solidification of
Cl-in cement slurry Figure 1 shows the partial phase composition of cement slurry in four different Mg 2+ concentration erosion solutions. Under pure NaCl corrosion environment, the composition of cement hydrate phase mainly includes Ca(OH) 2 and Friedel salt crystals, with characteristic peaks of Ca(OH) 2 at 18.0°,34.1°, and 45.5°, respectively; The characteristic peak of Friedel is 11.2°. In the NaCl MgCl 2 erosive environment, in addition to Friedel salt and Ca(OH) 2 crystals, Mg(OH) 2 was also found, with characteristic peaks at 18.5° and 37.9°. At the same time, it was also found that with the appearance of the diffraction peak of magnesium hydroxide crystal, Ca(OH) 2 disappeared from the 18.0° diffraction peak, indicating that the generation of Mg(OH) 2 consumed a portion of Ca(OH) 2 in the solution.

Thermogravimetric analysis
According to Figure 2, thermal analysis shows the phase composition of cement hydration products after erosion by Cl -Mg 2+ solution. Under the erosion environment of pure NaCl solution, the phase composition of cement hydration products mainly includes Friedel salt and Ca(OH) 2 . Among them, the Friedel salt endothermic peak range is 250-410℃ [9] , and according to the TG curve calculation, its weight loss is 7.67%. The endothermic peak range of Ca(OH) 2 is 410-520℃ [9] , and calculated by TG curve, its weight loss is 20.9%. The phase composition of cement hydration products in NaCl MgCl 2 erosion solution not only includes Friedel salt and Ca(OH) 2 , but also generates Mg(OH) 2 . The endothermic peak range of Mg(OH) 2 is 350-420℃ [10] : Due to the partial overlap of the endothermic peaks of Mg (OH) 2 and Friedel salt, the weight within the range of 250-350℃ is divided into Friedel salt, and the weight within the range of 350-420℃ is divided into Mg(OH) 2 . Therefore, the contents of Friedel salt, Mg(OH) 2 , and Ca(OH) 2 calculated based on the TG curve are shown in Figure 2. The content of Ca(OH) 2 decreased from 20.9wt.% to 11.68wt.%, and the content of Mg(OH) 2 increased from 0% to 8.77wt.%. This indicates that as the concentration of Mg 2+ in the erosion solution increases, Mg 2+ promotes the dissolution of Ca(OH) 2 and the dissolution of Ca 2+ , while generating precipitation of Mg(OH) 2 and promoting the formation of Friedel salts. In the corrosive environment of 0.6mol/L Cland 0.2mol/L Mg 2+ to 0.6mol/L Cland 0.3mol/L Mg 2+ , the Friedel salt content in the samples decreased from 8.98wt.% to 8.75wt.%, the Ca(OH) 2 content decreased from 11.68wt.% to 10.77wt.%, and the Mg(OH) 2 content increased from 8.77% to 9.50% by weight. Therefore, it indicates that as the concentration of Mg 2+ increases in the eroded environment, the Friedel salt content first increases and then decreases, the Ca(OH) 2 content gradually decreases, and the Mg(OH) 2 content continues to increase, indicating that the following reactions have occurred:

Curve of pH Change in Friedel Salt and Erosion Solution under Different Mg 2+ Concentrations
From Figure 6,it can be seen that the pH value of the pure cement slurry in the corrosive environment of 0.6mol/L Clto 0.6mol/L Cland 0.2mol/L Mg 2+ decreased from 13.03 to 12.10,and the Friedel salt content in the corresponding environment samples increased from 7.67wt.% to 8.98wt.%, while the Ca(OH) 2 content decreased from 20.9wt.% to 11.68wt.%. The content of Mg(OH) 2 increased from 0% to 8.77wt.%.With the increase of Mg 2+ concentration in the erosion solution,Mg 2+ promotes the dissolution of Ca(OH) 2 and the dissolution of Ca 2+ ,forming Mg(OH) 2 precipitation, and promoting the formation of Friedel salt, which indicates that the chloride ion binding capacity of cement paste decreases with the increase of pH value [11] because of the competition between hydroxyl ion and chloride ion [11] . Under the erosion environment of 0.6mol/L Cland 0.2mol/L Mg 2+ to 0.6mol/L Cland 0.3mol/L Mg 2+ in pure cement slurry, the pH value of the erosion solution decreased from 12.10 to 11.65, and the Friedel salt content in the corresponding environment decreased from 8.98wt% to 8.75%. The Ca(OH) 2 content decreased from 11.68wt% to 10.77wt%. The content of Mg(OH) 2 increased from 8.77Wwt.% to 9.5wt.%. This indicates that as the concentration of Mg 2+ in the erosion solution increases, Mg 2+ promotes the dissolution of Ca(OH) 2 and the dissolution of Ca 2+ , resulting in the formation of Mg(OH) 2 precipitates and partial decomposition of Friedel salts. When the pH is too low, the solubility of Friedel salts increases [12]. The reason can be explained by the following formula: The above reaction consumes a large amount of OH -, and the chemical reaction shifts to the right, increasing the solubility of Friedel salts, which is not conducive to the solidification of chloride ions [13,14].  Figure 7(a), it can be seen that there is Friedel salt present in the control group sample, and its chemical composition can be demonstrated by energy dispersive elements (Figure 7(b)). It is not difficult to find from Figure 7(a) that the shape of Friedel salt is similar to a hexagonal plate shape [15]. Figure 7(c) shows the microstructure of the sample surface in a 0.4mol/L NaCl-0.1mol/LMg 2+ etching solution. It is found that there are a large number of pores located on the sample surface, and Mg(OH) 2 and Ca(OH) 2 are filled in the pores. When the sample is placed in an erosion solution with a high concentration (≥0.2mol/L), a large amount of Mg(OH) 2 phase is found in the erosion product (Figure 7(e)(g)).At the same time,it was found that when the concentration of Mg 2+ in the erosion solution was 0.2mol/L, the samples in the erosion solution exhibited more cracks compared to the samples in the 0.6mol/L pure NaCl erosion solution (Figure 7(a)), and there were Mg(OH) 2 with better crystalline morphology around the cracks. This is because the transformation of Ca(OH) 2 into the product Mg(OH) 2 led to the appearance of cracks, and the average width of the cracks in the erosion product of the sample under 0.2mol/L Mg 2+ erosion solution was 116nm, Under 0.3mol/L Mg 2+ corrosion solution, the crack width increases to 265nm, so an increase in Mg 2+ concentration in the corrosion solution will lead to an increase in crack width.

Conclusion
(1) In the early stage of immersion erosion, Mg 2+ has little effect on the rate of pH rise of the erosion solution; After the pH value of the solution stabilizes, the final pH value of the solution decreases with the increase of Mg 2+ concentration.
(2) As the concentration of Mg 2+ in the erosion solution increases, Mg 2+ promotes the dissolution of hydration product Ca(OH) 2 and the dissolution of Ca 2+ , while generating Mg(OH) 2 precipitates, resulting in a decrease in solution alkalinity and an increase in chloride ion solidification.
(3) When the pH of the solution exceeds 12.1, a lower pH value will promote the formation of Friedel salts; When the pH of the solution is less than 12.1, higher concentrations of Mg 2+ cause Friedel salt to decompose.