LIU Jun ,YANG Qile ,GUO Yuchen ,YANG Yuanquan

(1.School of Architecture and Civil Engineering, Liuzhou Institute of Technology, Liu Zhou 545026, China; 2.School of Materials Science and Engineering, Shenyang Ligong University, Shenyang 110159, China; 3.School of Political Science and International Relations, Tongji University, Shanghai 200092, China; 4.School of Economics and Management, Tongji University, Shanghai 200092, China)

Abstract: K-struvite was prepared by precipitation method,and the stability of K-struvite in high temperature and acid-base environment were investigated by X-ray diffraction (XRD),thermogravimetric analysis (TG/DSC),and infrared spectroscopy (FT-IR).The results show that K-struvite decomposes from 50 to 110 ℃,and the mass loss begins at 50 ℃ before being completely destroyed at 110 ℃,then further heating at temperature above 500 ℃ leading to complete loss of the binding water in K-struvite.Moreover,K-struvite is more stable in alkaline environments than acidic environment.

Key words: K-struvite;temperature;magnesium potassium phosphate cement;acid-base environment

1 Introduction

Magnesium potassium phosphate cement(MKPC) is composed of dead burnt magnesium oxide and potassium dihydrogen phosphate accompanied by rapid setting and hardening through acid alkali reaction,which presents good properties of fast setting and hardening,high strength and volume stability[1,2].In the past five years,MKPC has attracted domestic and international attention.Hydration mechanism,hydration products and macro performances have been widely investigated[3,4].As a kind of repairing materials for construction,MKPCs face a variety of environments,such as high temperature environments and complex environments like acid and alkaline salt solutions.

Recent studies have found that MKPC can be used in disposal of waste resources such as curing of hazardous materials.In repair or reinforcement work,the service environment is different,especially in areas near the equator which may be in a higher temperature environment.In addition,MKPC was used in solidification of nuclear waste[5].The solidification process of nuclear waste requires an extremely high temperature environment due to thermal radiation.K-struvite is the main factor affecting the mechanical properties of MKPC.Study on thermal stability,acid and alkali resistance of K-struvite is helpful for the application of MKPC in complex environments.Therefore,it is particularly important to clarify the effect of different temperatures on the stability of K-struvite,a hydration product of MKPC.

Struvite,a hydration product of magnesium ammonium phosphate and similar to K-struvite,dehydrates and decomposes with increasing temperature[6,7].Previous studies[8,9]show the process of struvite decomposition: the struvite starts to decompose at 55 ℃ under nitrogen atmosphere,loses 6 water molecules and produces ammonia gas,no significant mass change with temperature is found after 250 ℃,and the decomposition is complete.While,another study found that the struvite starts to decompose at 65℃ under nitrogen atmosphere,loses 5 crystal water to MgNH4PO4·H2O firstly,the remaining crystal water is lost after 235 ℃ and becomes MgNH4PO4.As the temperature rises to 300 ℃,it will lose an NH3and becomes MgHPO4.When the temperature rises to 575℃,it becomes Mg2P2O7,and Mg3(PO4)2at 850 ℃.Lai[10]investigated the mechanical properties of MKPC at different temperatures and found that high temperature causes decomposition of hydration products and reduction of mechanical strength.Most of the current research concerned about improving stability of the binders by admixtures or modifiers[11-17],while less attention on the development of pure hydration product from binders.How was the mechanical properties of MKPC affected by temperature and to what extent does the temperature affect the mechanical properties of MKPC? The questions were not answered.A study on the properties of K-struvite from hydration products of MKPC may probably answer the above mentioned questions,since the K-struvite is a main binding phase in MKPC,which dominates the mechanical performance of MKPC.For understanding the mechanical development of MKPC in high temperature and acid-base environment,it is necessary to clarify the stability of K-struvite in variable environments.

In this work,K-struvite was prepared by precipitation method,and then treated by heating at different temperatures,soaking in acid or alkaline solutions.The influence of temperature and pH environment on K-struvite were investigated by X-ray diffraction (XRD),thermogravimetric analysis (TG/DSC),and infrared spectroscopy (FT-IR).

2 Experimental

2.1 Materials

The analytical grade KH2PO4(KDP,net content＞99.5%,impurities of Cl,Na and Fe are less than 0.2%),KOH with net content＞85.0% (impurities of Na and K2CO3are less than 2% and other impurities include Cl,Ca,Fe and Ni are less than 0.02%),and MgCl2·6H2O with net content＞98.0% (impurities include N,Ca,Fe,and each content is less than 0.05%)were used in this experiment.All the above mentioned materials are provided by Tianjin Damao Chemical Reagent Factory.To avoid excessive impurities affecting the sample purity and data accuracy,deionized water is used to prepare the solution in this experiment.

2.2 Preparation and test methods

2.2.1 Synthesis of K-struvite

The K-struvite (MgKPO4·6H2O) was synthesized under weak alkaline environment.Firstly,250 mL of 1 mol/L KOH solution,100 mL of 1 mol/L KH2PO4solution and 100 mL of 1 mol/L MgCl2solution were prepared.Secondly,the as-prepared KH2PO4solution was poured into a beaker,which is in a water bath at 22℃ with magnetic stirring.1 mol/L KOH solution was gradually added to make its pH value stable at 7.9-8.1.Approximately 120 mL of KOH solution is used in this process.The instruments used in this process include DF-101 thermostatic heating magnetic stirrer and pH/conductivity tester.Thirdly,keep constant temperature and stirring state,adding 1 mol/L MgCl2solution in the beaker gradually,while adding 1 mol/L KOH solution to adjust the pH value to make it stable at 7.9-8.1.In this process KOH solution was used about 90 mL,and the reaction last for 30 min.Fourthly,turn off the magnetic stirring,let it stand for a few minutes,then filter it to get the white powder which is K-struvite,wash it with deionized water and filter it again,and dry it in the natural state to get the final sample.Finally,we examined the sample by X-ray diffraction with a scanning range of 5-65° and a scanning speed of 10 °/min,and detected the wave number range of 4 000-400 cm-1by Nicolet iS50 FTIR spectrometer.

2.2.2 Heating treatment

Xl-1 high temperature muffle furnace and 1200 vacuum atmosphere tube furnace were used to heat K-struvite at different temperatures,namely,low heating temperature of 50,70,90,110 ℃ for 2 hours and high temperature of 500,600,700,800,900 ℃ for 1 hour with a heating rate of 10 ℃/min.

2.2.3 Acid-base environment curing

100 mL acid solutions of pH=2,pH=4 and pH=6 were prepared with 36% concentrated hydrochloric acid (HCl),and 100 mL alkali solutions of pH=10 and pH=12 were prepared with potassium hydroxide (KOH).Soak high purity K-struvite in the above mentioned solution for 4 hours,extract and dry in natural state.

Environment conditions and standing time for the K-struvite corresponding to each group are shown in Table 1,and K1 was set as a blank control group without any treatment.

Table 1 Environment conditions and standing time for K-struvite

2.2.4 Test methods

The samples were examined by Rigaku Ultima IV X-ray diffraction analyzer with a scanning range of 10-55° and scanning speed 10 °/min.Nicolet iS50 FTIR spectrometer with the wave number range of 4 000-400 cm-1was also used to examine the K-struvite.The NetzschSTA449 analyzer was used to study the mass and heat flow of K-struvite with increasing temperature.All samples were powdered and passed through 80 μm pore size,and 5-15 mg samples were weighed in alumina crucible and heated at 10 ℃/min from 35 to 900 ℃ using nitrogen atmosphere.

3 Results and discussion

3.1 Characterization of K-struvite

High purity K-struvite is synthesized by chemical reaction in weak alkaline environment,the pH of the solution is stabilized at 8.0 by KOH solution,and KH2PO4solution and MgCl2solution are fully reacted in constant pH condition.By stirring,standing,filtering,washing,drying and other processes,the K-struvite (white powder) was obtained.

Fig.1 shows the XRD diffraction pattern of K-struvite (MgKPO4·6H2O).Matching with the K-struvite of PDF,the sample has high purity and no other crystal impurities were found.

Fig.1 XRD patterns of K-struvite

Fig.2 shows the FTIR spectrum of K-struvite with the detection wave number range of 4 000-400 cm-1.The absorption band of K-struvite spectrum consists of three main parts[18],the absorption band of bound water H-O-H(3 550-1 590 cm-1),the absorption peaks generated by PO4,v1(995-930 cm-1),v2(470-404 cm-1),v3(1 163-1 017cm-1),v4(554-509 cm-1),and the absorption band of metal-oxygen bond (880-400 cm-1).The peak H-O-H vibrations at 2 870,2 330 and 1 590 cm-1indicate the presence of crystalline water.The absorption peak at 990 cm-1is caused by the symmetric stretching vibration ofv1(PO4).The absorption peak at 433 cm-1is caused by the symmetric bending vibration ofv2(PO4).The absorption peak at 1 220 cm-1is caused by the asymmetric stretching vibration ofv3(PO4).The absorption peak at 561 cm-1is caused by the asymmetric stretching vibration ofv4(PO)4.The absorption peak of metal-oxygen bond at 730 cm-1indicates the presence of Mg-O or K-O.The results showed no other functional groups,indicating the synthesis of K-struvite with high purity.

Fig.2 FTIR spectra of K-struvite

Fig.3 shows the TG/DTG curves of high-purity K-struvite.The results show that the sample started to lose mass at 80.01 ℃,and the highest decomposition rate was at 117.77 ℃.There is no significant mass change with increasing temperature after 400 ℃,and the total mass loss is 40.95%.The theoretical mass loss of K-struvite decomposed into MgKPO4is 40.5%.The total mass loss matches with the theoretical mass loss of K-struvite,which further proves the synthesis of K-struvite with high purity.

Fig.3 TG-DTG curves of K-struvite

3.2 Effect of temperature on K-struvite

The content and crystalline quality of K-struvite have important effects on the strength,water resistance,and adhesion of MKPC[19].MKPC is commonly used for concrete structure repair or curing nuclear waste,etc,and may be exposed to high ambient temperatures or thermal radiation effects.

Fig.4 shows the XRD patterns and FTIR spectra of K-struvite after low temperatures heating.K2,K3,K4 and K5 are the samples obtained by low temperature heating at 50,70,90 and 110 ℃,holding for 2 hours.K1 is the blank control without any process.

Fig.4 XRD patterns and FTIR spectra of K-struvite after low temperature heating

The decomposition of K-struvite is accelerated by heating at 50 ℃.The XRD diffraction peaks of K-struvite after heating at 50 ℃ are slightly weakened compared to the untreated sample K1.The content of the material phase is reduced,indicating that a small portion of K-struvite decomposes after heating at 50 ℃ for 2 hours.Combined with FTIR spectra,the absorption peaks in the detection wave number range 4 000-400 cm-1were at the same position with slightly reduced intensity.

The XRD diffraction peaks of K-struvite after heating at 70 ℃ were significantly weaker compared with those of the sample heated at 50 ℃.Combined with FTIR spectra,there is no change in the position of the vibration peak,but the peak is obviously weakened,indicating that the content is less and the crystallinity of K-struvite is reduced.

After heating at 90 ℃,there was only one significant diffuse scattering peak in the XRD patterns at the range of 25-35°,indicating that the sample was amorphous at this time.XRD patterns of K-struvite after heating at 110 ℃ were basically the same as that at 90 ℃,with only one significant diffuse scattering peak at the range of 25-35°.Each peak was greatly reduced,indicating a large amount of K-struvite has been decomposed.

Fig.5 shows TG/DTG and DTG/DSC curves of K-struvite after low temperature heating.The weight loss of K-struvite was mainly concentrated from 50 to 200 ℃.The mass loss started after 75 ℃for each sample.The solid phase mass loss of each group of K-struvite at 50,70,90 and 110 ℃ were 47.41%,32.30%,13.96% and 8.48% respectively.The maximum decomposition rates of each sample were different,the highest decomposition rates of K2 and K3 were 117.8 and 99.8 ℃ respectively,while no significant weight loss peaks appeared for K4 and K5.K-struvite showed a slight weakening of the weight loss peak after heating at 50 ℃.After heating at 70 ℃,the weight loss peak was weakened more and reached the maximum decomposition rate at 99.8 ℃,and the decomposition process was basically completed at 130 ℃.Then the weight loss curve tended to be flat.There is no obvious weight loss peak of K-struvite after heating at 90 and 110 ℃,which proves that the physical phase of K-struvite at this temperature has been completely destroyed.

It has been shown that MKPC has good frost resistance[20].However,MKPC is less resistant to high temperatures.The water will be evaporated at high temperatures,resulting in volume shrinkage and tensile stresses inside the substrate.The tensile stress cracks the material.

Fig.6 shows XRD patterns and FTIR spectra of the samples obtained from K-struvite after high temperature heating.K6,K7,K8,K9 and K10 are the samples obtained by high temperature heating at 500,600,700,800 and 900 ℃ and keeping for 1 hour.K1 is the blank control without any process.

Fig.6 XRD patterns and FTIR spectra of K-struvite after high temperature heating

No diffraction peak of MgKPO4·6H2O appeared.The crystal structure of K-struvite was completely destroyed after high temperature heating.Due to the high heating temperature,potassium and phosphorus were detected in the form of oxides Mg2P2O7,and some phosphates appeared in Mg3(PO4)2and KMgPO4.The infrared spectra deviated greatly from the original samples,and the H-O-H vibrational peaks of crystalline water all disappeared,and the original phases were completely destroyed.Due to the complex impurities in the air,the peaks in the detection wavenumber range 1 500-400 cm-1are disordered.

After high temperature treatment,TG/DTG and DTG/DSC curves show a nearly horizontal straight line as shown in Fig.7.There is almost no mass loss during the heating process from room temperature to 900 ℃,which proves that the crystal structure of K-struvite is completely destroyed after high temperature heating.

Fig.7 TG/DTG and DTG/DSC curves of K-struvite after high temperature heating

3.3 Effect of pH on K-struvite

Research has been shown that the pH of the solution has an effect on the hydration of MKPC,the more strongly acidic environment pH of the solution,the more dissolution of magnesium oxide[21].The pH have a strong impact on strength,sustainability,durability and defect-free long service life of MKPC[22-24],due to the dissolution of phosphate in the solution which changes the pH[25].However,how K-struvite evolved and was affected by the pH was not clear,since the stability of K-struvite is not the same in the form of an independent component with that in MKPC.Fig.8 gives the XRD patterns and FTIR spectra of K-struvite obtained at different pH values.

Fig.8 XRD patterns and FTIR spectra of K-struvite in different pH environment

As shown in Fig.8,the XRD patterns and FTIR spectra of K-struvite obtained for K11,K12,K13,K14,and K15 were obtained by soaking the solutions at pH=2,pH=4,pH=6,pH=10,and pH=12 for four hours.K1 is the blank control without any treatment.But it should be noted that the sample of K1 itself was made from the solution environment pH=8.

The diffraction peaks of K-struvite were significantly enhanced for the samples soaked in different pH solutions compared to the untreated samples.XRD patterns and FTIR spectra weakened after soaking in acidic solution but the waveform did not change.The stability of K-struvite was poor in pH=2,pH=4 environment.XRD patterns and FTIR spectra for K22,K23 are almost consistent with K1.pH=6 and pH=10 solutions have less effect on K-struvite.XRD patterns and FTIR spectra were significantly enhanced after immersion in strongly basic environment at pH=12.In summary,the stability of k-struvite is poor in pH=2-4,and there is no big difference in the stability in pH=6-10,and the stability is the optimum in the alkaline environment of pH=12.K-struvite crystal growth is more suitable for high alkaline environment.

Fig.9 shows the TG/DTG and DTG/DSC curves of the samples obtained from K-struvite soaking in different pH solutions,which showed significant differences in the mass loss process during the increase from 50 to 200 ℃.Mass loss occur after 50 ℃.The mass losses of K11,K12,K13,K14 and K15 were 39.64%,41.16%,41.86%,42.81% and 43.01%.The final mass loss of K1 sample was 40.95%,and the mass losses of all samples increased except K11.

Fig.9 TG/DTG and DTG/DSC curves of K-struvite in different pH environment

All samples had a significant weight loss process near 70 ℃ except K11.The DTG/DSC curves in the ranges of 100-150 ℃ were enlarged for specific analysis.The heat absorption peaks of K14 samples were significantly steeper,with faster weight loss and higher decomposition rates.

4 Conclusions

a) K-struvite begins to decompose at 50 ℃,and changes rapidly at 70 ℃.Most of the original phases are destroyed after heating at 90 ℃.After heating at 110 ℃,K-struvite is completely destroyed.

b) The crystal structure of K-struvite was completely destroyed after high temperature heating.Due to the high heating temperature,potassium and phosphorus were detected in the form of oxides Mg2P2O7,and some phosphates appeared in Mg3(PO4)2and KMgPO4.

c) The K-struvite is of low stability in pH=2-4,and there are small differences in the stability in pH=6-10,and the stability is the optimum in the alkaline environment of pH=12.K-struvite crystal growth is more suitable for high alkaline environment.

Conflict of interest

All authors declare that there are no competing interests.