Article ID: 1003-6326(2005)05-1145-05

Behavior of calcium silicate hydrate in aluminate solution

LI Xiao-bin(李小斌), ZHAO Zhuo(赵 卓), LIU Gui-hua(刘桂华),

ZHOU Qiu-sheng(周秋生), PENG Zhi-hong(彭志宏)

(School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China)

Abstract: Using calcium hydroxide and sodium silicate as starting materials, two kinds of calcium silicate hydrates, CaO·SiO₂·H₂O and 2CaO·SiO₂·1.17H₂O, were hydro-thermally synthesized at 120℃.The reaction rule of calcium silicate hydrate in aluminate solution was investigated. The result shows that CaO·SiO₂·H₂O is more stable than 2CaO·SiO₂·1.17H₂O in aluminate solution and its stability increases with the increase of reaction temperature but decreases with the increase of caustic concentration. The reaction between calcium silicate hydrate and aluminate solution is mainly through two routes. In the first case, Al replaces partial Si in calcium silicate hydrate, meanwhile 3CaO·Al₂O₃·xSiO₂·(6-2x) H₂O (hydro-garnet) is formed and some SiO₂ enters the solution. In the second case, calcium silicate hydrate can react directly with aluminate solution, forming hydro-garnet and Na₂O·Al₂O₃·2SiO₂·nH₂O (DSP). The desilication reaction of aluminate solution containing silicate could contribute partially to forming DSP.

Key words: calcium silicate hydrate; aluminate; caustic; reaction behavior CLC number: TF802

Document code: A

1 INTRODUCTION

Calcium silicate hydrate (mCaO·nSiO₂·xH₂O, C-S-H) is an important hydrate product of Portland cement. Because of its key rules in setting and hardening performance of cement^[1], it has attracted a lot of interest of many researchers^[2-4]. Calcium silicate hydrate is also a very important substance in alumina production, but its reaction behavior with aluminate solution has not been paid adequate emphases upon.

The key project in alumina production is the separation of aluminum and silicon. Calcium silicate hydrate is an important compound containing silicon in the process of alumina production. In order to separate aluminum and silicon, the silicate compounds without or with less alumina, such as C-S-H, are formed in high-pressure hydro-chemical process under very rigorous condition of high temperature and high caustic concentration^{[5, 6]}. A method to obtain C-S-H from sodium alumino-silicate hydrate (DSP) under mild condition was put forward^{[7, 8]}. Moreover, in sintering process of alumina production, the secondary loss of alumina and soda is actually caused by the reactions between calcium silicate hydrate and aluminate solution in the operation of leaching sinter^[9]. Therefore it is of great importance for alumina production to know the reaction law between C-S-H and aluminate solution.

The reactions between C-S-H and aluminate solution are generally considered as follows^[9]:

mCaO·nSiO₂·xH₂O+mNa₂CO₃=

  nNa₂SiO₃+mCaCO₃+2(m-n)NaOH+

  (n-m)H₂O(1)

mCaO·nSiO₂·xH₂O+2nNaOH=

  mCa(OH)₂+nNa₂SiO₃+

  (x+n-m)H₂O(2)

3Ca(OH)₂+2NaAl(OH)₄+xNa₂SiO₃=

  3CaO·Al₂O₃·xSiO₂·(6-2x)H₂O+

  2(1+x)NaOH+xH₂O(3)

2Na₂SiO₃+2NaAl(OH)₄=

  Na₂O·Al₂O₃·2SiO₂·xH₂O+

  4NaOH+(2-x)H₂O(4)

Conclusions can be drawn from the above reactions, i.e., the decomposition reactions of C-S-H with Na₂CO₃ and NaOH in aluminate solution (reactions (1) and (2)) are the basis of forming hydro-garnet and DSP (reactions (3) and (4)). The concentration of SiO₂ in the solution from leaching sinter can reach 4-6g/L, which is generally considered the decomposition of C-S-H by Na₂CO₃ and especially by NaOH^{[9, 10]}. However, Liu et al^[11] experimentally studied the reaction behavior of C-S-H with Na₂CO₃ and NaOH and the results showed that the SiO₂ concentration in the obtained solution is only 0.1-0.5g/L in both NaOH and Na₂CO₃ system with concentration of 100g/L Na₂O. The SiO₂ concentration is much less than that in sinter leaching, indicating the limited reaction degree of C-S-H with Na₂CO₃ and NaOH. Therefore the reaction mechanism between C-S-H and aluminate solution is not clear and further investigation is necessary.

2 METHODOLOGY

Firstly, the possibility of the reaction between C-S-H and aluminate solution is analyzed thermodynamically, and then experiments are carried out to validate the thermodynamic conclusion. Calcium hydroxide and sodium silicate are used as starting materials to synthesize two kinds of calcium silicate hydrate at 120℃, respectively being CaO·SiO₂·H₂O (signed as CSH1) and 2CaO·SiO₂·1.17H₂O (signed as CSH2). The content of dissociated CaO in both CaO·SiO₂·H₂O and 2CaO·SiO₂·1.17H₂O are measured as respectively 0.3% and 0.7% by glycol-ethanol means^[12]. The loss of alumina in solution, caused by the formation of 3CaO·Al₂O₃·xSiO₂·(6-2x)H₂O due to the reactions of those CaO with aluminate, is not significant (respectively 0.018g/L and 0.4g/L) and can be ignored. The samples are then treated in aluminate solution with the caustic ratio α_k of 1.5(Na₂O/Al₂O₃, molar ratio)at L/S of 10 (mass ratio of liquid to solid), unless especially noted. The specimens are taken out and filtered at designed time to measure the concentration of SiO₂ and Al₂O₃ in filtrate and mineral phase in residue.

3 RESULTS AND DISCUSSION

3.1 Thermodynamic analysis

The high SiO₂ concentration in the solution after the reaction of C-S-H and aluminate solution means that the substitution reaction of Al for Si, only in bridging sites of the dreierkette^[13], may happen as follows:

3[mCaO·nSiO₂·xH₂O]+2mAl(OH)^-₄+

  (m-3x)H₂O=m[3CaO·Al₂O₃·

  SiO₂·4H₂O]+(3n-m)H₂SiO^2-₄+

  (4m-6n)OH^-(5)

The XRD patterns reveal that DSP emerges at the beginning of the reaction of C-S-H and aluminate solution, which means that the formation of DSP may be not only caused by the reaction between H₂SiO^2-₄ from reaction (5) and Al(OH)^-₄ in solution, but also by the following reaction regarding to Ref.[9]:

3[mCaO·nSiO₂·xH₂O]+(2m+3n-m)·

  Al(OH)^-₄+(3n-m)Na⁺=m[3CaO·

  Al₂O₃·SiO₂·4H₂O]+(3n-m)/2·

  [Na₂O·Al₂O₃·2SiO₂·2H₂O]+

  2mOH^-+(3n+3x-2m)H₂O(6)

The calculation results of dependence of Gibbs free energy change on temperature for reactions (5) and (6) have been carried out with the Alumina Thermodynamic Database^[14]. Fig.1 shows that it is possible for both CaO·SiO₂·H₂O and 2CaO·SiO₂·1.17H₂O to react with Al(OH)^-₄ according to reaction (5), but impossible for 6CaO·6SiO₂·H₂O. Meanwhile, all the C-S-H are able to react with aluminate solution on the basis of reaction (6). Thermodynamically speaking, the stability sequence of the calcium silicate hydrates mentioned above is 6CaO·6SiO₂·H₂O>CaO·SiO₂·H₂O>2CaO·SiO₂·1.17H₂O.

]Fig.1  Variation of ΔG versus temperature

3.2 Relationship between types of C-S-H and its stability

Fig.2 demonstrates the loss of alumina due to SiO₂(m_Al2O3) and the dissolution of SiO₂(η_SiO2) versus time in the reaction of the two types of C-S-H with aluminate solution. Fig.2 indicates that the stability of the two types of C-S-H are quit different in aluminate solution. The m_Al2O3 of CaO·SiO₂·H₂O is obviously less than that of 2CaO·SiO₂·1.17H₂O and the η_SiO2 of the later is more than that of the former. Thus it can be concluded that CaO·SiO₂·H₂O is more stable than 2CaO·SiO₂·1.17H₂O, being consistent with the thermodynamic calculation. This can be explained as that there are silicate anion chains in all C-S-H and these chains become progressively longer as the Ca/Si ratio decreases, which causes the enhancement of the aggregation degree of Si and thus the increment of stability of C-S-H^[15].

3.3 Influence of temperature on stability of C-S-H

Fig.3 indicates the effect of temperature on η_{SiO2 in the case of the same aluminate solution and tells that the stability of C-S-H depends on temperature. The change of Gibbs free energy for the reactions increases with the elevation of temperature, resulting in the decrease of ηSiO2} and thus C-S-H becomes more stable in aluminate solution at higher temperature. 3.4 Influence of composition of aluminate solution on stability of C-S-H

3.4.1 Caustic(Na₂O_k) concentration

Fig.4 depicts the results of the reaction between CaO·SiO₂·H₂O and aluminate solution with different Na₂O_k concentrations. The concentration of SiO₂ entering the solution remarkably increases with the augment of Na₂O_k concentration and has a maximum value. The high Na₂O_k concentration represents a high Al(OH)^-₄ concentration at a certain α_k which is beneficial to SiO₂ in C-S-H entering the solution according to reaction (5). Subsequently, after the SiO₂ exceeds its equilibrium, the concentration of SiO₂ in the solution will drop due to the formation of DSP produced by the reaction of H₂SiO^2-₄ and the components of solution. This process, together with reactions (5) and (6), will contribute to the loss of alumina, m_Al2O3. On the contrary, there is a low concentration of Al(OH)^-₄ in the system with low Na₂O_k concentration (50g/L) and thus a limited reaction degree, which results in a less and invariable SiO₂ concentration in the solution at any time.

Fig.2  Loss of alumina (a) and dissolution of SiO₂(b) versus time

Fig.3  Dissolution of SiO₂ versus time

Fig.4  SiO₂ concentration in solution(a) and loss of alumina(b) versus time

3.4.2 Concentration of Na₂CO₃

Fig.5 shows the result of the reaction of two kinds of C-S-H and aluminate solution in different Na₂CO₃ concentrations. We can conclude from the figure that the dependence of η_SiO2 on Na₂CO₃ concentration for C-S-H is not significant, indicating a less effect of Na₂CO₃ concentration on the stability of C-S-H.

Fig.5  Dissolution of SiO₂ versus time

3.4.3 Caustic ratio(α_k)

From Fig.6 we can know that the higher the α_k of aluminate solution is, the more the SiO₂ in C-S-H dissolves. Obviously, the free NaOH concentration in the aluminate solution with α_k=3.0 is higher than that in the aluminate solution with α_k=1.5 in a certain caustic concentration. The concentration of SiO₂ entering the solution in the former solution should be more than that in the later, which is not identical with the fact, if the decomposition of C-S-H is mainly through reaction (2). Combining what mentioned above and Fig.5, the conclusion that reactions (1) and (2) contribute less than reaction (5) to the higher SiO₂ concentration in the sinter leaching operation in the sintering process, can be drawn.

Fig.6  SiO₂ concentration in solution versus time

4 CONCLUSIONS

1) Thermodynamically, CaO·SiO₂·H₂O is more stable than 2CaO·SiO₂·1.17H₂O in aluminate solution and the stability of C-S-H increases with the elevation of temperature, which has been verified by experiments.

2) The concentrations of Na₂CO₃ and NaOH have less influence on the stability of calcium silicate hydrates; meanwhile the Al(OH)^-₄ concentration in aluminate solution affects those remarkably through the formation of hydro-garnet and sodium alumino-silicate hydrate.

3) Al(OH)^-₄ contributes more significantly than NaOH and Na₂CO₃ on the high SiO₂ concentration in the solution after the reaction of calcium silicate hydrates with aluminate solution.

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Foundation item: Project(50274076) supported by the National Natural Science Foundation of China

Received date: 2005-01-27; Accepted date:2005-07-07

Correspondence: LI Xiao-bin, Professor, PhD; Tel: +86-731-8830453; E-mail: X.B.Li@mail.csu.edu.cn

(Edited by YANG Bing)