Wenlong Wang*,Guolin Li,Lizhi Zhou,Changzai Ren

National Engineering Laboratory of Coal-fired Pollutants Emission Reduction,Shandong Provincial Key Lab of Energy Carbon Reduction and Resource Utilization,Shandong University,Jinan 250061,China

Keywords:Sulfoaluminate cement Iron phase Thermodynamic data Gibbs free energy Solid waste utilization

ABSTRACT The preparation of sulfoaluminate cementitious materials(SCM)is a promising way to massively utilize solid wastes.Iron phases are significant in SCM system but the thermodynamic data of some key minerals,such as 6CaO·Al2O3·2Fe2O3(C6AF2)and 6CaO·2Al2O3·Fe2O3(C6A2F),are missing,which greatly hinders the SCM optimization in a theoretical way.This work,for the first time,calculated the standard formation enthalpy,Gibbs free energy of formation,entropy and molar heat capacity for C6AF2and C6A2F and lowered the errors to the least with the reference of C4AF data in the literature.By building the function diagram of Gibbs free energy changes with temperature for the basic iron phase formation reactions with the obtained thermodynamic data,it is proved that the formation likeliness of C6AF2is higher than that of C6A2F,as is accordant to the literatures and verifies the correctness of obtained data.This work provides a good theoretical foundation to optimize SCM mineral system and to study relevant mechanism deeply.

1.Introduction

It is a promising way to realize a large-scale utilization of massive solid wastes through the preparation of sulfoaluminate cementitious materials(SCM)[1,2].The sulfoaluminate cement,known as the third type cement after silicate and aluminate ones,is regarded as“green”because of its low calcination temperature,low calcium demand and therefore low cost and low CO2emission[3-5].It has a great potential as an environmentally-benign alternative to current cementitious materials[6-8].Another significant feature of SCM is its wide coverage of chemical compositions,e.g.SiO23wt%-25 wt%,Al2O315wt%-32 wt%,Fe2O34wt%-15 wt%,CaO 38wt%-50 wt%,and SO33wt%-15 wt%.This makes SCM highly adaptive to different solid wastes as raw materials,especially suitable for high-Fe wastes,like red mud,steel slag,furnace slag and other tailings[9].And,the SCM product with higher Fe content usually shows better performances in anti-corrosion,being particularly suitable for marine engineering or underground geotechnical applications as a high-value material[10].Therefore,SCM preparation using solid wastes is a research hotspot in building materials field[11,12].

While using different wastes to prepare SCM,the optimization of final clinker mineral system is critical.The main minerals in SCM are calcium sulfoaluminate(3CaO·3Al2O3·CaSO4),dicalcium silicate(2CaO·SiO2)and iron phases.Thereof,the iron phases may include 2CaO·Fe2O3(C2F),4CaO·Al2O3·Fe2O3(C4AF),6CaO·Al2O3·2Fe2O3(C6AF2)and 6CaO·2Al2O3·Fe2O3(C6A2F).The raw material batching and calcination temperature decide the specific forms of iron phases in clinker.The traditional experimental way has to rely on a large number of batching tests.So far,there is no effective way to optimize the mineral compositions of SCM through calculation,for the biggest barrier is the lack of thermodynamic data of some key minerals in the system.For instance,the thermodynamic data of C6AF2and C6A2F cannot be found in any previous literatures and a theoretical prediction of what iron phases can be formed is entirely limited.And the systematic data report for 3CaO·3Al2O3·CaSO4also only exists in our previous publication[13].Since the data of other chief minerals,i.e.2CaO·SiO2,C2F and C4AF are known,the mineral optimization of SCM will be viable once obtaining those of C6AF2and C6A2F.

To directly measure the thermodynamic data of C6AF2and C6A2F,such as standard formation enthalpy ΔH,Gibbs free energy of formation ΔG,entropy S,and molar heat capacity cp,is difficult because it is hard to synthesize their pure minerals.No previous research is especially focused on this due to its particularity.

This work,for the first time,managed to obtain the thermodynamic parameters of C6AF2and C6A2F through calculation.The calculation approaches referred to the theoretical system of 3CaO·3Al2O3·CaSO4[13].The verification of the obtained data was carried out by comparing them to those of C4AF,which is similar to the two 6-Ca minerals and has available thermodynamic data in literatures[13].Finally,the Gibbs free energy changes with temperature were built up based on the obtained data,which was also further verified in the process.This work completes the thermodynamic data system of SCM and provides a good foundation to unveil the reaction mechanisms within SCM system or in other cases involving the related minerals theoretically.

2.Research Methods

2.1.Data obtainment method

The key thermodynamic data(standard formation enthalpy ΔH,Gibbs free energy of formation ΔG,entropy S,and molar heat capacity cp)of iron phases were obtained through empirical calculation methods for complicated oxyacid salt minerals.These methods were proved effective and accurate in the study of 3CaO·3Al2O3·CaSO4.It does not need to know the microscopic structure of a certain compound.The crystal structure and parameters are necessary if toolbox like Gauss is adopted to calculate any mineral's thermodynamic data but the related information of C6AF2and C6A2F is devoid in literatures.

In order to ensure the reliability of empirical calculation,it was first adopted to calculate the data of C4AF,which are available in the textbook of Silicate Physical Chemistry(see Table 1)[14].By comparing the data from calculation and reference,the possible errors can be known and a modification can be given to the calculation results.Then,the methods could be extrapolated to the two 6-Ca minerals.

2.2.Extended application of calculation method

The empirical calculation method,which generally breaks complicated oxyacid minerals into simple oxide units and obtains the thermodynamic data through synthesizing the values of each known unit,was applied to C6AF2and C6A2F by referring to the calculation of C4AF.Because the three iron phase minerals are very close in chemical composition and containing oxide units,the modification steps used in improving the data of C4AF can be effectively used to modify the other two minerals.The reliability of the finally obtained data can be ensured in this approach.

2.3.Data application and verification

The completion of thermodynamic parameters of key iron phases enables the study of related reaction mechanisms of SCM system.To apply these data and verify their application value,the Gibbs free energy changes of the formation reactions of C4AF,C6AF2and C6A2F were calculated by using the obtained thermodynamic data below 1800 K.The correctness and effectiveness of the data were verified through the comparison to their experimental formation rules reported in literatures[15].

3.Results and Discussion

3.1.The thermodynamic data for C4AF

3.1.1.Standard formation enthalpyand Gibbs free energy of formationof C4AF

The data was obtained as in Table 2.The data of oxides and double oxides are from the thermochemical software package,FactSage.Finally,the standard formation enthalpy and Gibbs free energy of formation for C4AF can be obtained.is-5064.888 kJ·mol-1;is-4780.005 J·mol-1.

Fig.1.The combination process of C4AF.

Table 2 The thermodynamic data of oxides and relevant reactions①

3.1.2.Molar heat capacity cpof C4AF

The heat capacity of C4AF was calculated following Kubaschewski's[19]method,which considered an individual ion or atom has basically equal heat capacity at the melting point of a solid and expressed the molar heat capacity cpas:

where Tmrepresents the melting point of inorganic substance(K),n is the total number of atoms in the chemical formula of inorganic substance(18 for C4AF),and θ is the contribution of anion and cation to molar heat capacity at 298 K[15].Table 3 shows the contribution of each anion and cation to molar heat capacity at 298 K,which is from the work of Spencer[20].According to this approach,4CaO·Al2O3·Fe2O3can be viewed as 4(Ca2+)+Then the molar heat capacity of C4AF at 298 K is

So the sum of group contributions of heat capacity is 360.15 J·mol-1·K-1.The melting point of C4AF is 1688 K[15].Based on the above data,we can obtain the coefficients in Eqs.(5)-(8)a=424.383,b=74.327,and c=-75.6.

Table 3 Group contributions of heat capacity cpat 298 K

3.1.3.Standard entropy S of C4AF

The method of estimating the standard entropy was referred to the work of Latimer,Mills,Richter and so on[21-23].It calculates the summation of the contribution of anion and cation functional groups of a specific mineral[24].Table 4 lists the corresponding functional groupsof C4AF and their contribution data[20].Therefore,the standard entropy of C4AF at 298 K can be figured out as:

Table 4 Standard entropy contributions of functional groups of C4AF

3.1.4.Comparison

Table 5 shows the calculated values and Silicate Physical Chemistry's values for the thermodynamic data of C4AF in standard formation enthalpy,Gibbs free energy of formation,molar heat capacity and standard entropy.When calculating standard formation enthalpy an Gibbs free energy of formation,the combination ability between 3CaO·Al2O3and CaO·Fe2O3is ignored and therefore some errors may be caused,but it can be found that the results are very close to Silicate Physical Chemistry's values.For the molar heat capacity,the error is 8.195%,which should be attributed to the complicated data acquisition and formula citation.Latimer's method to calculate standard entropy indicated tolerant error,about 3%.In summary,compared with Silicate Physical Chemistry's values,the calculation data for C4AF are relatively accurate.

Table 5 Comparisons between calculation values and Silicate Physical Chemistry's values for C4AF

3.2.The thermodynamic data of C6AF2and C6A2F

3.2.1.The standard formation enthalpy and Gibbs free energy

The C6AF2and C6A2F are assumed to be formed on the basis of C4AF[25-27].The process is that the in priority generated C4AF phase intakes excessive Fe2O3or Al2O3and causes the phase transformation to C6AF2or C6A2F.To calculate the standard formation enthalpy and Gibbs free energy of the two 6-Ca phases,Eqs.(11)and(12)can be used to express their formation following Wen's method.The related data of oxides and binary oxides are available in literatures and listed in Table 6.The thermodynamic data are obtained from software package,FactSage.

Table 6 The ΔH and ΔG data of related oxides of C6AF2and C6A2F

3.2.2.The heat capacity

In Kubaschewski's method[19],firstly,we calculated the contributions of heat capacity(the value of ∑θ).Then,the coefficients a,b,c and molar heat capacity can be obtained after modification in Eqs.(6)-(8).

The error(8.195%)of cpfor C4AF is too big,meaning an adjustment needs to be done.Accordingly,the calculated cpof C6AF2and C6A2F also needs to be adjusted.However,because the cpvalues of the two 6-Ca minerals are bigger than that of C4AF,the rectification would be excessive if the same error was assumed.So we take this error to multiply a coefficient to guarantee the accuracy.For C6AF2,it is 0.8.For C6A2F it is 0.4.The rectification errors of cpfor C6AF2and C6A2F become 6.556%and 3.278% individually.Thus,the cpof C6AF2can be obtained as 591.785 J·mol-1·K-1and that of C6A2F as 543.312 J·mol-1·K-1.Based on the cpvalues at 298 K,the two sets of coefficients,a,b,and c,can obtained again by Eqs.(6)-(8)as shown in Table 7.

Table 7 The coefficients a,b,c for C6AF2and C6A2F

3.2.3.Standard entropy

According to Latimer's method[21],C6AF2can be viewed asC6A2F can be viewed asHence,the standard entropy of C6AF2and C6A2F at 298 K is calculated by using the data in Table 4 respectively as:

Referring to the error of S298K(C4AF),3.145%,the S298Kdata can be adjusted as 510.144 J·mol-1·K-1for C6AF2and 469.568 J·mol-1·K-1for C6A2F.

3.3.Data application and verification

The usefulness of the above obtained thermodynamic data is to evaluate relevant chemical reactions.The basic formation reactions of C4AF,C6AF2and C6A2F are as formulae(17),(18)and(19).In order to ensure uniformity and comparability for these iron phases,they all are based on 6CaO.So the molar quantity of C4AF actually is 1.5 in Eq.(17).According to the Gibbs function(ΔG=ΔH-TΔS),theof these reactions at different temperatures can be figured out[14].To make the calculation,the thermodynamic data of relevant oxides and iron phases is collected in Table 8.According to literatures[14],formulae(20)-(22)can be used to calculate the Gibbs free energy of a reaction at a specific temperature.The involved values in the calculation are listed in Table 9.

Table 8 The thermodynamic data of iron phases and some oxides

Fig.2 shows the final calculated Gibbs free energy changes,with the temperature,T.It can be seen that the trends of ΔG changes with temperature for the three iron phases are the same and all drop steadily with the temperature rising.The larger negative values of ΔG mean the reactions can take place easier,so higher temperature is apt to promote the reactions working.Another important finding is that the line for C6AF2is always at the bottom,that for C4AF is in the middle position and that for C6AF2is on the top.The Gibbs free energy for C6AF2is smallest,meaning that it is the most stable mineral or under the same calcination temperature it is the most likely to form.The formation tendency of C6AF2is definitely higher than that of C6A2F.The literature[27-29]indicates that C6A2F is a highly hydraulic mineral and has very good mechanical strength performances,with 3-day compressive strength being over 80 MPa and much higher than that of C4AF.Because C6AF2contains higher proportion of Fe2O3than C4AF and C6A2F,it is a better mineral phase to use high-Fe content raw materials like solid waste.Therefore,in SCM mineral design,C6AF2can be an ideal phase.The derived information indicates the importance of thethermodynamic data of iron phases here.The obtained data enrich the database of sulfoaluminate cementitious material and may also promote the use of software tools like FactSage.

Table 9 The involved data in the thermodynamics calculation

Fig.2.Gibbs free energy changes with temperature for reactions(17)to(19).

4.Conclusions

Through theoretical calculation,the thermodynamic data of some key iron phases in SCM,such as C4AF,C6AF2and C6A2F,were obtained.The standard formation enthalpy,Gibbs free energy of formation,entropy and molar heat capacity were calculated following empirical methods and the errors were lowered to the least with the reference of C4AF data in the literature.

With the obtained thermodynamic data,the function diagram of Gibbs free energy changes with temperature for the basic iron phase formation reactions was built.It is proved that the formation tendency of C6AF2is higher than that of C6A2F,as is accordant to the literatures and verifies the correctness of obtained data.

This work focuses on the calculation of thermodynamic parameters for iron phases and provides a good theoretical foundation to study SCM system deeply.The database expansion can play a great role on SCM mineral optimization and promote the preparation of SCM with massive solid waste.Also,deeper mechanism for SCM preparation can be explored and the methods presented here for calculating thermodynamic parameters can be used to study other inorganic minerals.