The Study's Chemical Interaction of the Sodium Silicate Solution with Extender Pigments to Investigate High Heat Resistance Silicate Coating

Silicate coating is water-based paint with many advantages and wide applications in many different industries. However, there are still some problems with silicate coating: how to increase its resistance to heat at high temperatures and prolong the life of the coating. Silicate paints have high durability and longevity dependent mainly on the chemical interaction of the silicate binder with extender pigments. Therefore, our groups have studied the geopolymerization process of the sodium silicate solution with extender pigments to investigate high heat resistance silicate coating. The effect of curing time on the chemical interaction between sodium silicate solution and extender pigments (ZnO, TiO2, Fe2O3, CaCO3, and Na2SiF6) was investigated by Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA), and X-ray diffraction (XRD). The shift of antisymmetric stretching vibration of the Si-O-Si bond (1060 cm−1) to low frequency and increase of the intensity of the Si-O-Si stretching as curing time increases from 1 to 20 days are due to the increased chemical interaction between extender pigments (ZnO, TiO2, Fe2O3, CaCO3, and Na2SiF6) and sodium silicate solution. Moreover, TG results of ZnO-silicate, TiO2-silicate, CaCO3-silicate, Na2SiF6-silicate, and Fe2O3-silicate coating at 1 and 20 days of curing show high residual geopolymer about 69–90% at 800°C. Thus, we proposed that the geopolymerization process between sodium silicate solution and extender pigments (ZnO, TiO2, Fe2O3, CaCO3, and Na2SiF6) increases when the curing time from 1 to 20 days leads to forming geopolymer silicate with high thermal stability. In addition, the optimal mixing ratio between sodium silicate solution and extender pigments (ZnO, TiO2, Fe2O3, CaCO3, and Na2SiF6) is as follows: 25% binder (sodium silicate solution), 8% ZnO; 5% TiO2, 5% Fe2O3, 1% Na2SiF6, 21% CaCO3, 34% H2O, and 1% additives to make high heat resistance silicate coating with temperature resistance at 1000°C.


Introduction
Vietnam is a country that has a tropical climate with high humidity and high temperature.Moreover, there are many impurities in the air such as dust, microorganisms, and toxic chemicals.With the above climate, many types of building materials are damaged, mossy, and rusty.Coating technology is used as a film to protect buildings from chemical attack and corrosion of the environment.
ere are two commonly used paints, organic and inorganic coating [1].
e organic coating such as epoxy resin shows good mechanical performance, chemical resistance, and anticorrosive effect [2].However, it has been shown that organic coating has many disadvantages such as high aging and especially high price [3].Besides, organic-inorganic hybrid materials are materials in which organic (epoxy resin) and inorganic (rice husk silica [4,5], nano-glass fiber, and silanized coal fly ash [6]) components are mixed to enhance the mechanical and fracture toughness of epoxy resin.e inorganic coating is the main component of cross-linked inorganic binders such as silicon resin [7], water-and solvent-based silicates [8], silanes [9], and a mixture of the organic binder with silicate [10].In comparison with organic coating, inorganic coating (silicate coating) shows more advantage because of its corrosion and heat resistance [11].However, there are still some problems with silicate coating such as long-term durability, low resistant water, and high price, which have limited the wide commercial applications of organic coating in the construction and building industry [12].e need to address issues to develop and deploy in the practice of new silicate paint materials required the development of new approaches to developing silicate coating.Herein, recent achievements in the modified properties and working mechanism of silicate coating by adding organic and inorganic compounds make the silicate coating even more promising for the application.Yuan et al. reported a study on the properties and working mechanism of the waterborne polyurethane-modified silicate-based coating.
e bending, compressive strengths, and flexibility of the coating modified by adding polyurethane were greatly improved compared with the unmodified coating.e temperature resistance of the waterborne polyurethanemodified silicate-based coating is low at 250 °C [13].Parashar et al. reported about alkali silicate-based zinc-rich coating with excellent abrasion resistance, prevention of undercutting of coatings by corrosion, unaffected by temperatures up to 528 °C, and unaffected by the organic solvents such as ketones and gasoline [14].Bahreini's group also synthesized silica supported ZnO nanoparticle as the new pigment for thermal control coating [15].Besides, Irfan Khan et al. have used TiO 2 pigment to improve the photocatalytic activity of silicate paint [16].However, the silicate paint which is the suspension of extender pigments (Zn, ZnO nanoparticle, and TiO 2 ) in liquid potassium glass has some disadvantages, for example, low flexibility and high cost.e liquid sodium glass (sodium silicate solution) is of interest for use as a binder for silicate coating, which ensures more high-performance properties and low cost [12].e use of liquid sodium glass does not provide the water-resistance of silicate coatings [12].Extender pigments are of interest for use as filler for silicate paints that ensure more high-performance properties.Moreover, silicate coating has high heat resistance, durability, and longevity dependent mainly on the curing process of the silicate binder with extender pigments.erefore, understanding the interaction of liquid glass with extender pigments and colorants will help us to create paint products that are heat resistant and of high durability and longevity.Silicate coating with sodium silicate film is a new direction due to its high heat resistance, long life, and low price and completely suitable for the climatic conditions of Vietnam.To investigate the effects of the interaction of liquid glass with extender pigments on the properties of silicate coating, our groups have studied the curing process of sodium silicate solution with each of the extender pigments such as ZnO, TiO 2 , CaCO 3 , Na 2 SiF 6 , and Fe 2 O 3 .e chemical interaction between sodium silicate solution and extender pigments according to curing time was investigated by FT-IR, TGA, and XRD.In addition, the novel material based on extender pigments (ZnO, TiO 2 , CaCO 3 , Na 2 SiF 6 , and Fe 2 O 3 ) was prepared to investigate the high heat resistance silicate coating.e thermal stabilization of silicate coating was investigated by TGA and SEM. ) was added to the 5.5-6.5 g sodium silicate solution and stirred at room temperature until getting powder (note that the amount of the extender pigments was added to sodium silicate solution so that the drying time of the mixture to get dry powder is less than 20 minutes).e coating powders were crushed by the ball milling method for 1 hour; after that the coating powder was cured from 1 to 20 days in the air under room temperature.

Materials and Methods
erefore, the formula of the mixture of liquid sodium glass with each of the extender pigments shows in Table 1.
e effect of curing time on the chemical interaction between sodium silicate solution and extender pigments (ZnO, TiO 2 , Fe 2 O 3 , CaCO 3 , and Na 2 SiF 6 ) was investigated by Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA), and X-ray diffraction (XRD).Fourier transform infrared spectroscopy (FT-IR) measurements were conducted on a Nicolet 380 spectrometer (Waltham, MA, USA) operated in the mid-IR range of 4000-400 cm −1 , with spectra obtained at a spectral resolution of 8 cm −1 in transmittance mode.ermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) were performed using a METTLER TOLEDO SDTA851e.
e coating powder at 1 and 20 days of curing was placed in aluminum oxide (AlO x ) TGA pan.e sample was heated at rate of 10 °C min −1 from 20 to 900 °C under a nitrogen gas flow of 50 mL.min−1 .X-ray diffraction (XRD) spectra of absorption materials were obtained using D8 Advance (Bruker-Germany) and D5005 (Siemens-Germany).

Preparation of the Silicate Coating.
e silicate coating was prepared (Scheme 1) by mixing sodium silicate solution with extender pigments (ZnO, TiO 2 , Fe 2 O 3 , CaCO 3 , Na 2 SiF 6 , and additives) and stirred at room temperature for 20 minutes until getting dry powder by following the formula in Table 2. e dry silicate coating powders were crushed by ball milling for 1 hour; after that the coating powder was cured from 1 to 20 days.e silicate coating sample at 20 days of curing was mixed with water and coated on the steel plate.
e silicate coating was prepared (Scheme 1) by following the formula in Table 2. e role of ZnO, TiO 2 , and Fe 2 O 3 in silicate paints is to enhance the mechanical properties of the paint such as acid strength, heat resistance, and leaching resistance of the silicate coating due to chemical interaction with liquid glass to form the geopolymer.Na 2 SiF 6 is added to the paint to increase the leaching resistance of the paint film due to the chemical interaction between Na 2 SiF 6 and liquid sodium silicate by the following reaction: (1) CaCO 3 is the primary filler of most silicate coatings, because a part of CaCO 3 interacts with the liquid sodium glass to form calcium silicate that is both mechanically stable and water resistant.e thermal stabilization of the silicate coating was investigated by TGA and scanning electron microscopy (SEM).e SEM was performed with S-4800 (SEM, Hitachi) at an accelerating voltage of 30 kV. e heat resistance of silicate coating was investigated by calcination of the paint film at 1000 °C for 10 h.

Results and Discussion
e chemical interaction between sodium silicate solution and extender pigments (ZnO, TiO 2 , Fe 2 O 3 , CaCO 3 , and Na 2 SiF 6 ) results are as follows.
e FT-IR spectra of extender pigments-silicate coating (Figures 1(a)-1(e)) show the variation in the chemical structure according to the curing time from 1 day to 20 days.
e ZnO-silicate, TiO 2 -silicate, CaCO 3 -silicate, Na 2 SiF 6 -silicate, and Fe 2 O 3 -silicate coating at 1 day of curing's sample show the peaks at 3450 and 1650 cm −1 , indicating the presence of the asymmetric stretching and bending of hydroxyl (-OH) groups from silanol (Si-O-H) groups or residual water on the surface of the sample [16].When the curing time increased to 20 days, the intensity peak at 3450 cm −1 decreased due to the dehydration and dehydroxylation to form silicate geopolymer.e FT-IR results of SiO 2 /TiO 2 composite show vibration at 925 cm −1 , which is assigned to the formation of Ti-O-Si bonds [21,22].erefore, we suggest that the shift of the antisymmetric stretching vibration (1060 cm −1 ) of the Si-O-Si bond to the low frequency (955 and 960 cm −1 ) and the decrease in the intensity of Si-OH bending vibration of sample of 20 days of curing compared with 1-day sample were because of the complete geopolymerization process between sodium silicate solution and ZnO and TiO 2 to form the chemical bonds Si-O-Zn and Ti-O-Si [16,17,20].
e chemical bonds between the Fe 2 O 3 and sodium silicate solution at curing time of 1 and 20 days (Figure 1(c)) were further investigated using FT-IR spectra.In Figure 1(c), the absorption band near 500 and 1398 cm −1 of Fe 2 O 3 -silicate coating samples at 1 day and 20 days of curing are assigned to Fe-O stretching mode, indicating the presence of the Fe 2 O 3 in the coating samples [23].e peak at 1060 cm −1 corresponding to Si-O-Si stretching vibration of Na 2 O.nSiO 2 .xH 2 O shifts to the low frequency at 990 cm −1 with increasing curing time from 1 to 20 days, which is assigned to the formation of the Fe-O-Si bond [24].
erefore, we suggest that the shift of the antisymmetric stretching vibration of the Si-O-Si bond (1060 cm −1 ) to low frequency at 990 cm −1 and the decrease in the intensity of Si-OH bending vibration of sample of 20 days of curing compared with 1-day sample are because of the complete geopolymerization process between sodium silicate solution and Fe 2 O 3 to form of new chemical bonds such as Fe-O-Si.
e FT-IR spectra of CaCO 3 -silicate coating (Figure 1(d)) at 1 day and 20 days of curing compared with FT-IR spectra of CaCO 3 and Na 2 O.nSiO 2 .xH 2 O show the variation in the chemical structure according to curing time from 1 to 20 days.e FT-IR results of samples of 1 and 20 days reveal that the absorption peak at 1060 cm −1 confirms the formation of silicate (SiO 2 ) due to the chemical interaction between CaCO 3 and liquid sodium silicate.e absorption peaks at 712, 871, and 1460 cm −1 are attributed to the C-O bond out-of-plane bending, in-plane bending, and stretching vibrations of CO 3  2-, respectively [25,26].In addition, the intensity peak at 1060 cm −1 which is related to the constitutional SiO 2 increases as the curing time increases from 1 to 20 days [16,17].erefore, when the curing time increases from 1 to 20 days, the geopolymerization process between CaCO 3 and liquid sodium silicate is more complete due to the chemical interaction between CaCO 3 and liquid sodium silicate according to the following reaction: In Figure 1(e), FT-IR spectra of Na 2 SiF 6 -silicate coating at 1 day and 20 days of curing compared with Na 2 SiF 6 and Na 2 O.nSiO 2 .xH 2 O show the variation in the chemical structure according to curing time from 1 to 20 days.e FT-IR results of samples at 1 day and 20 days of curing reveal that the absorption peak at 1060 cm −1 of SiO 2 is due to the chemical interaction between Na 2 SiF 6 and liquid sodium silicate [16,17].In addition, the intensity peak at 1060 cm −1 which is related to the constitutional SiO 2 increases as the curing time increases from 1 to 20 days.erefore, when the curing time increases from 1 to 20 days, the geopolymerization process between Na 2 SiF 6 and liquid sodium silicate is more complete due to the chemical interaction between Na 2 SiF 6 and liquid sodium silicate according to the following reaction: e FT-IR spectrum of extender pigments-silicate coating (Figure 1) shows the peak at 1060 cm −1 corresponding to Si-O-Si stretching vibration of Na 2 O.nSiO 2 .xH 2 O shift to low frequency about 900-960 cm −1 when sodium silicate solution is mixed with extender pigments (ZnO, TiO 2 , and Fe 2 O 3 ).Besides, the intensity peak at 1060 cm −1 of silicate (SiO 2 ) due to the chemical interaction between extender pigments (CaCO 3 and Na 2 SiF 6 ) and sodium silicate solution increases as the curing time increases from 1 to 20 days.erefore, the geopolymerization process between extender pigments (ZnO, TiO 2 , Fe 2 O 3 , CaCO 3 , and Na 2 SiF 6 ) and sodium silicate solution to form new bonds suggested provoking the significant changes observed in the shift to the low frequency of antisymmetric stretching vibration of the Si-O-Si bond (1060 cm −1 ) and increase of the intensity of the Si-O-Si stretching as curing time increases from 1 to 20 days.

ermal Analysis.
ermal stability of extender pigments-silicate coating was investigated by thermal gravimetric analysis.TG and DTG curves at 10 °C/min heating rate for 1 and 20 days of curing were given in Figure 3.As shown in Figure 3, the total residual geopolymer of 1 and 20 days of curing's samples is 89 and 90% for ZnO-silicate coating, 84 and 89% for TiO 2 -silicate coating, 64 and 69% for CaCO 3 -silicate coating, 71 and 79,5% for Na 2 SiF 6 -silicate coating, and 85 and 88% for Fe 2 O 3 -silicate coating at 800 °C, respectively.Besides, the DTG plots of ZnO-silicate, TiO 2 -silicate, Fe 2 O 3 -silicate, CaCO 3 -silicate, and Na 2 SiF 6silicate coating show the increase in dehydration temperature of surface water (physically and chemically absorbed water) when curing time increases from 1 to 20 days.Drying or dehydration of surface water occurred below 100 °C, whereas fixed water was released about 95-118 °C for 1 day of curing and 95-128 °C for 20 days of curing's samples. is increase in dehydration temperature resulted from the increase of interaction of water molecules and geopolymer.
e DTG plots of ZnO-silicate coating at 1 and 20 days of curing (Figures 3(a e ZnO-silicate coating at 1 and 20 days of curing shows that the small peak at 500 °C that accounts for 2-5% of the mass loss (1 day of curing) and the one at 650 °C that accounts for 1-2% of the mass loss (20 days of curing) are considered due to the melting of silica catalyzed by metal impurities (Na + ) and formation of bulk silicate [32].
e DTG plots of CaCO 3 -silicate coating at 1 and 20 days of curing (Figures 3(g) and 3(h)) show that the peak at 694 °C of CaCO 3 -silicate coating at 1 and 20 days of curing's samples represents the thermal decomposition of CaCO 3 process according to the following reaction: e CaCO 3 -silicate coating at 1 and 20 days of curing shows the small peak at 800 °C that accounts for 2-5% of the mass loss due to the melting of silica catalyzed by metal impurities (Na + and Ca 2+ ) and formation of bulk silicate [32].
e DTG plots of Na 2 SiF 6 -silicate coating at 1 and 20 days of curing (Figures 3(i) and 3(j)) show that the peak at 600 °C of Na 2 SiF 6 -silicate coating at 1 day and the one at 615 °C of Na 2 SiF 6 -silicate coating at 20 days of curing's samples represent the decomposition of Na 2 SiF 6 according to the following reaction: Na 2 SiF 6 ⟶ NaF + SiF 4 (5) We proposed that the increased decomposition temperature of Na 2 SiF 6 as curing time increases from 1 to 20 days is due to the increased chemical reaction between Na 2 SiF 6 and sodium silicate solution binder.
TG results of ZnO-silicate, TiO 2 -silicate, CaCO 3 -silicate, Na 2 SiF 6 -silicate, and Fe 2 O 3 -silicate coating at 20 days of curing show high residual geopolymer about 69-90% at 800 °C.e DTG plots show the increase in dehydration and thermal decomposition temperature resulted as the curing time increases from 1 to 20 days.As the discussion in FT-IR results, the shift of antisymmetric stretching vibration of the Si-O-Si bond (1060 cm −1 ) to low frequency and increase of the intensity of the Si-O-Si stretching as curing time increases from 1 to 20 days are due to the chemical interaction between extender pigments (ZnO, TiO 2 , Fe 2 O 3 , CaCO 3 , and Na 2 SiF 6 ) and sodium silicate solution to form new bonds.erefore, we proposed that the geopolymerization process between sodium silicate solution and extender pigments (ZnO, TiO 2 , Fe 2 O 3 , CaCO 3 , and Na 2 SiF 6 ) increases when the curing time increases from 1 to 20 days leading to forming geopolymer silicate with high thermal stability.And the newly introduced material based on extender pigments was expected to give a high heat resistance silicate coating, which is very useful for applications in devices working at high temperatures.

Silicate Coating.
e thermal stability of the silicate coating was investigated by thermal gravimetric analysis.Journal of Analytical Methods in Chemistry showed that the surface morphology after calcination of the silicate coating on the steel plate at 1000 °C did not have much difference compared to the silicate coating on the steel plate before calcination.e needle shape morphology was found in the microstructural analysis of geopolymers in this study.In particular, the surface and color of the film that can be seen with the naked eye do not change after calcination at 1000 °C for 10 h.Hence, the silicate paint is of high heat resistance and completely suitable for the climatic conditions of Vietnam.

Conclusion
In this report, we have investigated the effect of curing time on the chemical interaction between sodium silicate solution and each of the extender pigments such as ZnO, TiO us, the newly introduced material based on extender pigments was expected to give a high heat resistance silicate coating, which is very useful for applications in devices working at high temperatures.
) and 3(b)) show that the peak at 315 °C of ZnO-silicate coating at 1 and 20 days of curing represents the dehydration of bonded water (Si-O-H) process.

Figure 4 :
Figure 4: TGA and DTG curves of the stable mixing of silicate coating sample at 1 day (a); TGA and DTG curves of the stable mixing of silicate coating sample at 20 days (b).

Figure 5 :
Figure 5: SEM image results of the silicate coating film (curing at 20 days) on the steel plate before (a) and after heating at 1000 °C (b).

Table 1 :
e components of the mixture of liquid sodium glass with each of the extender pigments (ZnO, TiO 2 , Fe 2 O 3 , CaCO 3 , and Na 2 SiF 6 ).

Table 2 :
e components of the silicate coating. 2 SiF 6 , and additives Stirring 3. Mixing with H 2 O and coating on steel plate Silicate coating Scheme 1: Preparation of the silicate coating.
2 , CaCO 3 , Na 2 SiF 6 , and Fe 2 O 3 by FT-IR, XRD, and TGA. e geopolymerization process between extender pigments (ZnO, TiO 2 , Fe 2 O 3 , CaCO 3 , and Na 2 SiF 6 ) and sodium silicate solution to form new bonds suggested provoking the significant changes observed in the shift of antisymmetric stretching vibration of the Si-O-Si bond (1060 cm −1 ) to low frequency and increase of the intensity of the Si-O-Si stretching as curing time increases from 1 to 20 days.TGA results of mixture between extender pigments (ZnO, TiO 2 , Fe 2 O 3 , CaCO 3 , and Na 2 SiF 6 ) and sodium silicate solution at 1 and 20 days of curing show high residual geopolymer about 69-90% at 800 °C.Also, the optimal mixing ratio between sodium silicate solution and extender pigments such as ZnO, TiO 2 , Fe 2 O 3 , CaCO 3 , and Na 2 SiF 6 is as follows: 25% binder (sodium silicate solution), 8% ZnO, 5% TiO 2 , 5% Fe 2 O 3 , 1% Na 2 SiF 6 , 21% CaCO 3 , 34% H 2 O, and 1% additives to make high heat resistance silicate coating with temperature resistance at 1000 °C.